Cell Communication Campbell Biology PDF

11 Cell Communication KEY CONCEPTS 11.1 External signals are converted to responses within the cell p. 213 11.2 Signal reception: A signaling molecule binds to a receptor, causing it to change shape p. 217 11.3 Signal transduction: Cascades of molecular interactions transmit signals from rec...

11 Cell Communication KEY CONCEPTS 11.1 External signals are converted to responses within the cell p. 213 11.2 Signal reception: A signaling molecule binds to a receptor, causing it to change shape p. 217 11.3 Signal transduction: Cascades of molecular interactions transmit signals from receptors to relay molecules in the cell p. 221 11.4 Cellular response: Cell signaling leads to regulation of transcription or cytoplasmic activities p. 226 11.5 Apoptosis requires integration of multiple cell-signaling pathways p. 229 Study Tip Make a table: As you read about examples of cell signaling in this chapter, make a table and classify the events of each example into three stages: signal reception, signal transduction, and cellular response. Example of Cell Signaling Signal Reception Epinephrine Epinephrine binds cellsurface receptor. Signal Transduction Cellular Response Relay molecules each activate the next molecule. An enzyme is activated that breaks down glycogen into glucose for energy to fight or flee. Figure 11.1 This impala is fleeing for its life, racing to escape the predatory cheetah nipping at its heels. The impala is breathing rapidly, its heart pounding and its legs pumping furiously. These physiological functions are all part of the impala’s “fightor-flight” response, driven by hormones released from its adrenal glands at times of stress—in this case, upon sensing the cheetah. How does cell signaling fuel the desperate flight of an impala? The impala senses a cheetah. Its brain signals the adrenal glands to release epinephrine into the blood. Signal reception Go to Mastering Biology For Students (in eText and Study Area) • Get Ready for Chapter 11 • Animation: Overview of Cell Signaling • Animation: Signal Transduction Pathways For Instructors to Assign (in Item Library) • Problem-Solving Exercise: Can a Skin Wound Turn Deadly? • Tutorial: Cell-Signaling: Transduction and Response An epinephrine molecule binds to a receptor on a muscle cell. The enzyme breaks down glycogen, releasing glucose that fuels the leg muscles. Relay molecules transmit the signal, ultimately activating an enzyme. Epinephrine Glycogen activates Receptor activates Pathway of relay molecules Muscle cell in leg 212 Cellular response Signal transduction Enzyme Glucose CONCEPT 11.1 External signals are converted to responses within the cell Scientists think that signaling mechanisms first evolved hundreds of millions of years ago in ancient prokaryotes and singlecelled eukaryotes and then were adapted for new uses in their multicellular descendants. So let’s begin by considering signaling in some examples of single-celled organisms: bacteria and yeasts. Evolution of Cell Signaling EVOLUTION Research during the 1970s suggested that bacterial cells—somewhat surprisingly, since they are single-celled organisms—were capable of signaling to each other. Since then, we have come to understand that cell signaling is critical among prokaryotes. Bacterial cells secrete molecules that can be detected by other bacterial cells (Figure 11.2). Sensing the concentration of such signaling molecules allows bacteria to monitor their own local cell density, a phenomenon called quorum sensing. Quorum sensing allows bacterial populations to coordinate the behavior of all cells in a population in activities that require a given density of cells acting at the same time. One example is formation of a biofilm, an aggregation of bacterial cells attached to a surface by molecules secreted by the cells, but only after the . Figure 11.2 Communication among bacteria. Soil-dwelling bacteria called myxobacteria (“slime bacteria”) use chemical signals to share information about nutrient availability. When food is scarce, starving cells secrete a signaling molecule that stimulates neighboring cells to aggregate. The cells form a structure called a fruiting body that produces spores, thick-walled cells that can survive until the environment improves. The myxobacteria shown here are the species Myxococcus xanthus (steps 1–3, SEMs; lower photo, LM). 1 Individual rod-shaped cells cells have reached a certain density. The biofilm protects the cells in it, and they often derive nutrition from the surface they are on. Biofilms are believed to be involved in up to 80% of all human bacterial infections. You have probably encountered biofilms many times, perhaps without realizing it. The slimy coating on a fallen log or on leaves lying on a forest path, and even the film on your teeth each morning, are examples of bacterial biofilms. In fact, tooth-brushing and flossing disrupt biofilms that would otherwise cause cavities and gum disease. Another example of bacterial behavior coordinated by quorum sensing is the secretion of toxins by infectious bacteria, which has serious medical implications. Sometimes treatment by antibiotics doesn’t work with such infections because of antibiotic resistance that has evolved in a particular strain of bacteria. A promising alternative treatment would be to disrupt toxin production by interfering with the signaling pathways used in quorum sensing. In the Problem-Solving Exercise, you can participate in the process of scientific thinking involved in this novel approach. Now let’s look at an example of cell signaling in yeasts (single-celled fungi). Cells of the yeast Saccharomyces cerevisiae—which are used to make bread, wine, and beer— identify their sexual mates by chemical signaling when they reproduce sexually. There are two sexes, or mating types, called a and a (Figure 11.3). Each type secretes a specific . Figure 11.3 Communication between mating yeast cells. Saccharomyces cerevisiae cells use chemical signaling to identify cells of the opposite mating type and initiate the mating process. The two mating types and their corresponding chemical signaling molecules, or mating factors, are called a and a. c factor Receptor 1 Exchange of mating factors. Each mating cell type secretes a mating factor that binds to receptors on the other mating type. c a Yeast cell, mating type a a factor Yeast cell, mating type c 0.5 mm 2 Mating. Binding of 2 Aggregation in progress 2.5 mm 3 Spore-forming structure (fruiting body) Fruiting bodies the factors to receptors induces changes in the cells that lead to their fusion. 3 New a/c cell. The nucleus of the fused cell includes all the genes from the a and c cells. CHAPTER 11 c a a/c Cell Communication 213 PROBLEM-SOLVING EXERCISE rn deadly? Can a skin wound tu Cells sense their own population density by quorum sensing, and at a certain density they start to secrete toxin. In this exercise, you will analyze whether blocking quorum sensing can stop S. aureus from producing toxin. Your Approach In S. aureus, quorum sensing involves two separate signal trans- Instructors: A version of this Problem-Solving Exercise can be assigned in Mastering Biology. Or a more extensive investigation called “Solve It: Is It Possible to Treat Bacterial Infections Without Traditional Antibiotics?” can be assigned. Your Data Concentration of toxin in culture (μmoles/mL) n s (S. aureus) is a commo Staphylococcus aureu of e fac on the sur bacterial species found turn into a serious can t tha n ski hy alt he a d into tissue through pathogen if introduce the ter en ls cel s eu aur S. cut or abrasion. Once rete tain density, they sec body and reach a cer s ute trib con d an ls cel a toxin that kills body . ge ma da d mation an significantly to inflam strain a ry car le op pe 0 10 Because about one in tibiistant to common an of S. aureus that is res tly en an rm pe n tur can otics, a minor infection ly. harmful or even dead duction pathways. Two candidate synthetic peptides (short proteins), called peptides 1 and 2, have been proposed to interfere with S. aureus quorum-sensing pathways. Your job is to test each potential inhibitor of quorum sensing to see if it blocks either or both of the pathways that lead to toxin production. For your experiment, you grow four cultures of S. aureus to a standardized high density and measure the concentration of toxin in the culture. The control culture contains no peptide. The other cultures have one or both candidate inhibitory peptides mixed into the growth medium before starting the cultures. The Cell 1.5 1.0 0.5 0 Control Peptide 1 Peptide 2 Peptides 1 + 2 Your Analysis 1. Rank the cultures according to toxin production, from most to least. 2. Which, if any, of the cultures with peptide(s) resulted in a toxin concentration similar to the control culture? What is your evidence for this? 3. Was there an additive effect on toxin production when peptides 1 and 2 were both present in the growth medium? What is your evidence for this? 4. Based on these data, would you hypothesize that peptides 1 and 2 act on the same quorum-sensing pathway leading to toxin production or on two different pathways? What is your reasoning? 5. Do these data suggest a possible treatment for antibiotic-resistant S. aureus infections? What else would you want to know to investigate this further? factor that binds only to receptors on the other type of cell. When exposed to each other’s mating factors, a pair of cells of opposite type change shape, grow toward each other, and fuse (mate). The new a/a cell contains all the genes of both original cells, providing advantages to the cell’s descendants, which arise by subsequent cell divisions. The unique match between mating factor and receptor is key to ensuring mating only between cells of the same species of yeast. How does the binding of a mating factor by the yeast cell-surface receptor initiate a signal that brings about UNIT TWO 2.0 Data from N. Balaban et al., Treatment of Staphylococcus aureus biofilm infection by the quorumsensing inhibitor RIP, Antimicrobial Agents and Chemotherapy 51(6):2226–2229 (2007). Mastering Biology HHMI Video: Interview with Bonnie Bassler BBC Video: Brushing Your Teeth Can Save Your Life 214 2.5 Mastering Biology Interview with Bonnie Bassler: Exploring how bacteria communicate with each other the cellular response of mating? This occurs in a series of three major steps—signal reception, signal transduction, and cellular response—called a signal transduction pathway. Many such pathways exist in the cells of both unicellular and multicellular organisms. In fact, the molecular details of signal transduction in yeasts and mammals are strikingly similar, even though it’s been over a billion years since they shared a common ancestor. This similarity suggests that early versions of cell-signaling mechanisms evolved well before the first multicellular organisms appeared on Earth. Local and Long-Distance Signaling . Figure 11.4 Communication requiring contact between cells. Like bacteria or yeast cells, cells in a multicellular organism communicate by way of a signaling molecules targeted for cells that may or may not be immediately adjacent. As we saw in Concepts 6.7 and 7.1, eukaryotic cells may communicate by direct contact, which is one type of local signaling. Many animal and plant cells have cell junctions that directly connect the cytoplasms of adjacent cells (Figure 11.4a). In these cases, signaling substances dissolved in the cytosol can pass between neighboring cells. Moreover, some animal cells may communicate by direct contact between cell-surface molecules, as shown in Figure 11.4b. This type of local signaling is especially important in embryonic development, the immune response, and in maintaining adult stem cell populations. In many other cases of local signaling, signaling molecules are secreted by the signaling cell. Some molecules travel only short distances; such local regulators influence cells that are nearby. This type of local signaling in animals is called paracrine signaling (Figure 11.5a). One class of local regulators in animals, growth factors, consists of compounds that stimulate nearby target cells to grow and divide. Numerous cells can simultaneously receive and respond to the growth factors produced by a single cell in their vicinity. A highly specialized type of local signaling called synaptic signaling occurs in the animal nervous system Figure 11.5b; see Concept 48.4. An electrical signal along a nerve cell triggers the secretion of neurotransmitter molecules. These molecules act as chemical signals, diffusing across the synapse— the narrow space between the nerve cell and its target cell—triggering a response in the target cell. Drugs to treat Plasma membranes Gap junctions between animal cells Cell wall Plasmodesmata between plant cells (a) Cell junctions. Both animals and plants have cell junctions that allow molecules, including signaling molecules, to pass readily between adjacent cells without crossing plasma membranes. (b) Cell-surface molecules. In many animal cells, cell-surface molecules on adjacent cells interact with each other, resulting in a signal passing between the cells. depression, anxiety, and post-traumatic stress disorder (PTSD) affect this signaling process. Both animals and plants use molecules called hormones for long-distance signaling. In hormonal signaling in animals, also known as endocrine signaling, specialized cells release hormones, which travel through the circulatory system to other parts of the body, where they reach target cells that can recognize and respond to them (Figure 11.5c). Many . Figure 11.5 Local and long-distance cell signaling by secreted molecules in animals. In both local and long-distance signaling, only specific target cells that can recognize a given signaling molecule will respond to it. Local signaling: up to a few cells‘ distance Target cells Long-distance signaling: up to body-length distance Electrical signal triggers release of neurotransmitter molecules. Endocrine cell Target cell specifically binds hormones. Neurotransmitters diffuse across synapse. Signaling cell Hormones travel in bloodstream. Secretory vesicles Local regulator (a) Paracrine signaling. A signaling cell acts on nearby target cells by secreting molecules of a local regulator (a growth factor, for example). Blood vessel Target cell (b) Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell, such as a muscle or another nerve cell. (c) Endocrine (hormonal) signaling. Specialized endocrine cells secrete hormones into body fluids, often blood. Hormones reach most body cells, but are bound by and affect only some cells. CHAPTER 11 Cell Communication 215 plant hormones reach distant targets by traveling through cells (see Concept 39.2). Like local regulators, plant hormones vary widely in size and type. For instance, the plant hormone ethylene, which promotes fruit ripening and helps regulate growth, is a hydrocarbon of only six atoms (C2H4), small enough to diffuse through the air and pass through cell walls. (Small signaling molecules probably evolved early on among single-celled organisms.) In contrast, the mammalian hormone insulin, which regulates sugar levels in the blood, is a protein with thousands of atoms. What happens when a potential target cell is exposed to a secreted signaling molecule? The ability of a cell to respond is determined by whether it has a specific receptor molecule that can bind to the signaling molecule. The information conveyed by this binding, the signal, must then be changed into another form—transduced—inside the cell before the cell can respond. The remainder of the chapter discusses this process, primarily as it occurs in animal cells. (Two signaling pathway proteins are shown in their cellular context in Figure 6.32a.) stripped of phosphate and released from the liver cell into the blood as glucose, which can fuel cells throughout the body. But how, exactly, does epinephrine mobilize glucose for use? Sutherland’s research team discovered that epinephrine outside the cell stimulates glycogen breakdown by somehow activating an enzyme, glycogen phosphorylase, inside the cell. However, when epinephrine was added to a solution containing the enzyme and its substrate, glycogen, no breakdown occurred. Glycogen phosphorylase could be activated by epinephrine only when the hormone was added to intact cells. This result told Sutherland two things. First, epinephrine does not interact directly with the enzyme responsible for glycogen breakdown; an intermediate step or series of steps must be occurring in the cell. Second, an intact, membrane-bound cell must be present for transmission of the signal to take place. Sutherland’s work suggested that the process going on at the receiving end of a cellular communication can be dissected into three stages: signal reception, signal transduction, and cellular response (Figure 11.6): The Three Stages of Cell Signaling: A Preview 1 Signal reception. Reception is the target cell’s detection of a signaling molecule coming from outside the cell. A chemical signal is “detected” when the signaling molecule binds to a receptor protein located at the cell’s surface (or inside the cell, to be discussed later). Our current understanding of how signaling molecules act through signal transduction pathways had its origins in the pioneering work of Earl W. Sutherland, whose research led to a Nobel Prize in 1971. Sutherland and his colleagues at Vanderbilt University were investigating how the animal hormone epinephrine (also called adrenaline) triggers the “fightor-flight” response in animals like the impala in Figure 11.1. One effect of epinephrine is to mobilize fuel reserves, which can be used by the animal to either defend itself (fight) or try to escape (flight). Epinephrine stimulates the breakdown of the storage polysaccharide glycogen within liver cells and skeletal muscle cells. The breakdown of glycogen releases the sugar glucose 1-phosphate, which the cell converts to glucose 6-phosphate. The liver or muscle cell can then use this compound, an early intermediate in glycolysis, for energy production (see Figure 9.8). Alternatively, the compound can be c Figure 11.6 Overview of cell signaling. From the perspective of the cell receiving the message, cell signaling can be divided into three stages: signal reception, signal transduction, and cellular response. When reception occurs at the plasma membrane, as shown here, the transduction stage is usually a pathway of several steps (three are shown as an example), with each specific relay molecule in the pathway bringing about a change in the next molecule. The final molecule in the pathway triggers the cell’s response. VISUAL SKILLS Where would the epinephrine in Sutherland’s experiment fit into this diagram of cell signaling? EXTRACELLULAR FLUID 2 Signal transduction. The binding of the signaling mol- ecule changes the receptor protein in some way, initiating the process of transduction. The transduction stage converts the signal to a form that can bring about a specific cellular response. In Sutherland’s system, the binding of epinephrine to a receptor protein in a liver cell’s plasma membrane leads to activation of glycogen phosphorylase in the cytosol. Transduction sometimes occurs in a single step but more often requires a sequence of changes in a series of different molecules—a signal transduction pathway. The molecules in the pathway are often called relay molecules; three are shown as an example. CYTOPLASM Plasma membrane 1 Signal reception 2 Signal transduction 3 Cellular response Receptor 1 2 3 Three relay molecules in a signal transduction pathway Signaling molecule Activation of cellular response, such as release of glucose from glycogen Mastering Biology Animation: Overview of Cell Signaling 216 UNIT TWO The Cell 3 Cellular response. The transduced signal finally triggers a specific cellular response. The response may be almost any imaginable cellular activity—such as catalysis by an enzyme (for example, glycogen phosphorylase), rearrangement of the cytoskeleton, or activation of specific genes in the nucleus. The cell-signaling process helps ensure that crucial activities like these occur in the right cells, at the right time, and in proper coordination with the activities of other cells of the organism. We’ll now explore the mechanisms of cell signaling in more detail, including a discussion of regulation and termination of the process. CONCEPT CHECK 11.1 1. Explain how signaling is involved in ensuring that yeast cells fuse only with cells of the opposite mating type. 2. In liver cells, glycogen phosphorylase acts in which of the three stages of the signaling pathway associated with an epinephrine-initiated signal? 3. WHAT IF? If epinephrine were mixed with glycogen phosphorylase and glycogen in a cell-free mixture in a test tube, would glucose 1-phosphate be generated? Why or why not? For suggested answers, see Appendix A. CONCEPT 11.2 Signal reception: A signaling molecule binds to a receptor, causing it to change shape A wireless router may broadcast its network signal indiscriminately, but only computers with the correct password can connect to it: Reception of the signal depends on the receiver. Similarly, the signals emitted by an a mating type yeast cell are “heard” only by its prospective mates, a cells. In the case of the epinephrine circulating throughout the bloodstream of the impala in Figure 11.1, the hormone encounters many types of cells, but only certain target cells, those with the corresponding receptor protein, detect and respond to the epinephrine molecule. The signaling molecule is complementary in shape to a specific site on the receptor and attaches there, like a hand in a glove. The signaling molecule acts as a ligand, the term for a molecule that specifically binds to another (often larger) molecule. Ligand binding generally causes a receptor protein to undergo a change in shape. For many receptors, this shape change directly activates the receptor, enabling it to interact with other molecules in or on the cell. For other receptors, the immediate effect of ligand binding is to cause the aggregation of two or more receptor proteins, which leads to further molecular events inside the cell. Most signal receptors are plasma membrane proteins, but others are located inside the cell. We’ll look at both of these types next. Receptors in the Plasma Membrane Cell-surface transmembrane receptors play crucial roles in the biological systems of animals. The largest family of human cell-surface receptors is that of the G protein-coupled receptors (GPCRs). There are more than 800 GPCRs; an example is shown in Figure 11.7. Another example is the co-receptor hijacked by HIV to enter immune cells (see Figure 7.8); this GPCR is the target of the drug maraviroc, which has shown some success at treating AIDS. Most water-soluble signaling molecules bind to specific sites on transmembrane receptor proteins that transmit information from the extracellular environment to the inside of the cell. We can see how cell-surface transmembrane receptors work by looking at three major types: G protein-coupled receptors (GPCRs), receptor tyrosine kinases, and ion channel receptors. These receptors are discussed and illustrated in Figure 11.8; study this figure before going on. Given the many important functions of cell-surface receptors, it is not surprising that their malfunctions are associated with many human diseases, including cancer, heart disease, and asthma. To better understand and treat these conditions, a major focus of both university research teams and the pharmaceutical industry has been to analyze the structure of these receptors. Although cell-surface receptors (half of which are GPCRs) represent 30% of all human proteins, determining their structures by X-ray crystallography (see Figure 5.21) has proved challenging. For one thing, cell-surface receptors tend to be flexible and inherently unstable, thus difficult to crystallize. It took years of persistent efforts for researchers to determine the first few of these structures, such as the GPCR . Figure 11.7 The structure of a G protein-coupled receptor (GPCR). This is a model of the human b2-adrenergic receptor, which binds epinephrine (adrenaline). The receptor was crystallized (discussed later in this section) in the presence of both a molecule that mimics epinephrine (green in the model) and cholesterol in the membrane (orange). Two receptors (blue) are shown as ribbon models in a side view. Caffeine can also bind to this receptor; see question 10 at the end of the chapter. d2-adrenergic receptors Plasma membrane Molecule that mimics ligand Cholesterol Mastering Biology Animation: Reception CHAPTER 11 Cell Communication 217 ▼ Figure 11.8 Exploring Cell-Surface Transmembrane Receptors G Protein-Coupled Receptors Signaling molecule binding site Segment that interacts with G proteins inside the cell G protein-coupled receptor G protein-coupled receptor A G protein-coupled receptor (GPCR) is a cell-surface transmembrane receptor that works with the help of a G protein, a protein that binds the energy-rich molecule GTP. Many different signaling molecules—including yeast mating factors, neurotransmitters, and epinephrine (adrenaline) and many other hormones—use GPCRs. G protein-coupled receptors vary in the binding sites for their ligands and also for different types of G proteins inside the cell. Nevertheless, GPCR proteins are all remarkably similar in structure. In fact, they make up a large family of eukaryotic receptor proteins with a secondary structure in which the single polypeptide, represented here in a ribbon model, has seven transmembrane c helices (outlined with cylinders and depicted in a row for clarity). Specific loops between the helices (here, the loops on the right) form binding Plasma membrane Activated receptor GTP Signaling molecule EXTRACELLULAR FLUID GTP GDP CYTOPLASM sites for signaling molecules (outside the cell) and G proteins (on the cytoplasmic side). GPCR-based signaling systems are extremely widespread and diverse in their functions, including roles in embryonic development and sensory reception. In humans, for example, vision, smell, and taste depend on GPCRs (see Concept 50.4). Similarities in the structures of G proteins and GPCRs in diverse organisms suggest that G proteins and their associated receptors evolved very early among eukaryotes. Malfunctions of the associated G proteins themselves are involved in many human diseases, including bacterial infections. The bacteria that cause cholera, pertussis (whooping cough), and botulism, among others, make their victims ill by producing toxins that interfere with G protein function. Up to 60% of all medicines used today exert their effects by influencing G protein pathways. G protein (inactive) Enzyme 1 Attached but able to move along the cytoplasmic side of the membrane, a G protein functions as a molecular switch that is either on or off, depending on whether GDP or GTP is attached —hence the term G protein. (GTP, or guanosine triphosphate, is similar to ATP.) When GDP is bound to the G protein, as shown above, the G protein is inactive. The receptor and G protein work together with another protein, usually an enzyme. Inactive enzyme GDP 2 When the appropriate signaling molecule binds to the extracellular side of the receptor, the receptor is activated and changes shape. Its cytoplasmic side then binds an inactive G protein, causing a GTP to displace the GDP. This activates the G protein. Activated enzyme GTP EXTRACELLULAR FLUID GDP Pi Cellular response CYTOPLASM 3 The activated G protein dissociates from the receptor, diffuses along the membrane, and then binds to an enzyme, altering the enzyme’s shape and activity. Once activated, the enzyme can trigger the next step leading to a cellular response. Binding of signaling molecules is reversible: Like other ligands, they bind and dissociate many times. The ligand concentration outside the cell determines how often a ligand is bound and initiates signaling. 218 UNIT TWO The Cell 4 The changes in the enzyme and G protein are only temporary: The G protein also acts as a GTPase, hydrolyzing its bound GTP to GDP and P i ; as a result, it can no longer activate the enzyme. The G protein leaves the enzyme, which returns to its inactive state. The G protein is now available for reuse. Its GTPase function allows the pathway shut down rapidly when the signaling molecule is no longer present. Receptor Tyrosine Kinases Receptor tyrosine kinases (RTKs) belong to a major class of plasma membrane receptors characterized by having enzymatic activity. An RTK is a protein kinase—an enzyme that catalyzes the transfer of phosphate groups from ATP to another protein. The part of the receptor protein extending into the cytoplasm functions more specifically as a tyrosine kinase, an enzyme that catalyzes the transfer of a phosphate group from ATP to the amino acid tyrosine of a substrate protein. Thus, RTKs are membrane receptors that attach phosphates to tyrosines. Signaling molecule (ligand) Upon binding a ligand such as a growth factor, one RTK may activate ten or more different transduction pathways and cellular responses. Often, more than one signal transduction pathway can be triggered at once, helping the cell regulate and coordinate many aspects of cell growth and cell reproduction. The ability of a single ligand-binding event to trigger so many pathways is a key difference between RTKs and GPCRs; GPCRs generally activate a single transduction pathway. Abnormal RTKs that function even in the absence of signaling molecules are associated with many kinds of cancer. EXTRACELLULAR FLUID Ligand-binding site c helix in the membrane Signaling molecule Tyrosines Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Receptor tyrosine kinase proteins (inactive monomers) CYTOPLASM Dimer 1 Many receptor tyrosine kinases have the structure depicted schematically here. Before the signaling molecule binds, the receptors exist as individual units referred to as monomers. Notice that each monomer has an extracellular ligand-binding site, an c helix spanning the membrane, and an intracellular tail containing multiple tyrosines. 2 The binding of a signaling molecule (such as a growth factor) causes two receptor monomers to associate closely with each other, forming a complex known as a dimer, a process called dimerization. (In some cases, larger clusters form. The details of monomer association are a focus of current research.) Activated relay proteins Tyr Tyr Tyr Tyr Tyr Tyr 6 Activated tyrosine kinase regions (unphosphorylated dimer) ATP 6 ADP P P P Tyr Tyr Tyr Tyr Tyr Tyr P P P Fully activated receptor tyrosine kinase (phosphorylated dimer) CYTOPLASM 3 Dimerization activates the tyrosine kinase region of each monomer; each tyrosine kinase adds a phosphate from an ATP molecule to a tyrosine that is part of the tail of the other monomer. P P P Tyr Tyr Tyr Tyr Tyr Tyr EXTRACELLULAR FLUID Cellular response 1 P P P Cellular response 2 Inactive relay proteins 4 Now that the receptor is fully activated, it is recognized by specific relay proteins inside the cell. Each such protein binds to a specific phosphorylated tyrosine, undergoing a resulting structural change that activates the bound relay protein. Each activated protein triggers a transduction pathway, leading to a cellular response. Continued on next page CHAPTER 11 Cell Communication 219 ▼ Figure 11.8 (continued) Ion Channel Receptors A ligand-gated ion channel is a type of membrane channel receptor containing a region that can act as a “gate,” opening or closing the channel when the receptor changes shape. When a signaling molecule binds as a ligand to the channel receptor, the channel opens or closes, allowing or blocking the flow of specific ions, such as Na+ or Ca2+. Like the other receptors we have discussed, these proteins bind the ligand at a specific site on their extracellular sides. 1 Here we show a ligand-gated ion channel receptor in which the channel remains closed until a ligand binds to the receptor. Signaling molecule (ligand) Channel closed Ligand-gated ion channel receptor 2 When the ligand binds to the receptor and the channel opens, specific ions can flow through the channel and rapidly change the local concentration of that ion inside the cell. This change may directly affect the activity of the cell in some way. 3 When the ligand dissociates from this receptor, the channel closes and ions no longer enter the cell. Ions Plasma membrane Channel open Cellular response Channel closed Ligand-gated ion channels are very important in the nervous system. For example, the neurotransmitter molecules released at a synapse between two nerve cells (see Figure 11.5b) bind as ligands to some ion channels on the receiving cell, causing the channels to open. Ions flow in (or, in some cases, out), triggering an electrical signal that propagates down the length of the receiving cell. Some of the gated ion channels are controlled by electrical signals instead of ligands; these voltage-gated ion channels are crucial to the functioning of the nervous system, as you’ll see in Chapter 48. Some ion channels are present on membranes of organelles, such as the ER. MAKE CONNECTIONS Is the flow of ions through a ligand-gated channel an example of active or passive transport? (Review Concepts 7.3 and 7.4.) Mastering Biology Animation: Acetylcholine Receptor 220 UNIT TWO The Cell shown in Figure 11.7. In that case, the b-adrenergic receptor was stable enough to be crystallized only while it was among membrane molecules and in the presence of a molecule mimicking its ligand. Newer techniques that do not require crystallization, like cryo-EM (see Figure 6.3), have shown promise in determining the structures of cell-surface receptors. Abnormal functioning of receptor tyrosine kinases (RTKs) is associated with many types of cancers. For example, some breast cancer patients have tumor cells with excessive levels of a receptor tyrosine kinase called HER2 (see Concept 12.3 and Figure 18.27). Using molecular biological techniques, researchers have developed a protein called Herceptin that binds to HER2 on cells and inhibits cell division, thus thwarting further tumor development. In some clinical studies, treatment with Herceptin improved patient survival rates by more than one-third. One goal of ongoing research into these cellsurface receptors and other cell-signaling proteins is development of additional successful treatments. Mastering Biology Interview with Diana Bautista: How cell signaling leads to itch and pain (see the interview before Chapter 6) Intracellular Receptors Intracellular receptor proteins are found in either the cytoplasm or nucleus of target cells. To reach such a receptor, a signaling molecule passes through the target cell’s plasma membrane. A number of important signaling molecules can do this because they are either hydrophobic enough or small enough to cross the hydrophobic interior of the membrane (see Concept 7.1). Hydrophobic signaling molecules include steroid hormones and thyroid hormones of animals. Another chemical signaling molecule that possesses an intracellular receptor is nitric oxide (NO), a gas; this very small molecule readily passes between the membrane phospholipids. Once a hormone or other signaling molecule has entered a cell, its binding to an intracellular receptor changes the receptor into a hormone-receptor complex that is able to cause a response—in many cases, the turning on or off of particular genes. The behavior of aldosterone is a representative example of how steroid hormones work. This hormone is secreted by cells of the adrenal gland (a gland that lies above the kidney), then travels through the blood and enters cells all over the body. However, a response occurs only in kidney cells because they alone contain receptors for aldosterone. In these cells, the hormone binds to and activates the receptor protein. With aldosterone attached, the active form of the receptor protein then enters the nucleus and turns on specific genes that control water and sodium flow in kidney cells, ultimately affecting blood volume (Figure 11.9). How does the activated hormone-receptor complex turn on genes? Recall that the genes in a cell’s DNA . Figure 11.9 Steroid hormone interacting with an intracellular receptor. Hormone (aldosterone) EXTRACELLULAR FLUID Plasma membrane Receptor protein Hormonereceptor complex 1 The steroid hormone aldosterone passes through the plasma membrane. 2 Aldosterone binds to a receptor protein in the cytoplasm, activating it. DNA NUCLEUS 4 The bound protein CYTOPLASM New protein 1. Nerve growth factor (NGF) is a water-soluble signaling molecule. Would you expect the receptor for NGF to be intracellular or in the plasma membrane? Explain. 2. WHAT IF? What would the effect be if a cell made defective receptor tyrosine kinase proteins that were unable to dimerize? 3. MAKE CONNECTIONS How is ligand binding similar to the process of allosteric regulation of enzymes? (See Figure 8.20.) For suggested answers, see Appendix A. CONCEPT 3 The hormonereceptor complex enters the nucleus and binds to specific genes. mRNA CONCEPT CHECK 11.2 acts as a transcription factor, stimulating the transcription of the gene into mRNA. 5 The mRNA is translated into a specific protein. MAKE CONNECTIONS Why is a cell-surface receptor protein not required for this steroid hormone to enter the cell? (See Concept 7.2.) Mastering Biology Animation: Steroid Hormone Pathway function by being transcribed and processed into messenger RNA (mRNA), which leaves the nucleus and is translated into a specific protein by ribosomes in the cytoplasm (see Figure 5.22). Special proteins called transcription factors control which genes are turned on—that is, which genes are transcribed into mRNA—in a particular cell at a particular time. When the aldosterone receptor is activated, it acts as a transcription factor that turns on specific genes. (You’ll learn more about transcription factors in Chapters 17 and 18.) By acting as a transcription factor, the aldosterone receptor itself carries out two parts of the signaling pathway, as receptor and transducer. Most other intracellular receptors function in the same way, although many of them, such as the thyroid hormone receptor, are already in the nucleus before the signaling molecule reaches them. Interestingly, many of these intracellular receptor proteins are structurally similar, suggesting an evolutionary kinship. 11.3 Signal transduction: Cascades of molecular interactions transmit signals from receptors to relay molecules in the cell When receptors for signaling molecules are plasma membrane proteins, like most of those we have discussed, the transduction stage of cell signaling is usually a multistep pathway involving many molecules. Steps often include activation of proteins by addition or removal of phosphate groups or release of other small molecules or ions that act as signaling molecules. One benefit of using multiple steps is that a signal caused by a small number of signaling molecules can be greatly amplified. If each molecule transmits the signal to numerous molecules at the next step in the series, the result is a geometric increase in the number of activated molecules by the end (see Figure 11.16). A second benefit of using multistep pathways is that they provide more opportunities for coordination and control than do simpler systems. This allows regulation of the response, as we’ll see later in the chapter. Signal Transduction Pathways The binding of a specific signaling molecule to a receptor in the plasma membrane triggers the first step in the signal transduction pathway—the chain of molecular interactions that leads to a particular response within the cell. Like falling dominoes, the signal-activated receptor activates another molecule, which activates yet another molecule, and so on, until the protein that produces the final cellular response is activated. The molecules that relay a signal from receptor to response, which we call relay molecules in this book, are often proteins. Protein-protein interactions are a major theme of cell signaling—indeed, a unifying theme of all cellular activities. Mastering Biology Animation: Signal Transduction Pathways CHAPTER 11 Cell Communication 221 rather than tyrosine. Serine/threonine kinases are widely involved in signaling pathways in animals, plants, and fungi. Many signal transduction pathways use relay molecules that are protein kinases, and they often act on other protein kinases in the pathway. Figure 11.10 depicts a hypothetical pathway containing two different protein kinases that create a phosphorylation cascade. The sequence of steps shown in the figure is similar to many known pathways, including those triggered in yeast by mating factors and in animal cells by many growth factors. The signal is transmitted by a cascade of protein phosphorylations, each causing a shape change in the phosphorylated protein. The shape change results from the interaction of the newly added phosphate groups with charged or polar amino acids on the protein being phosphorylated (see Figure 5.14). The shape change in turn alters the function of the protein, most often activating it. (In some cases, though, phosphorylation instead decreases the activity of the protein.) A significant percentage of our own genes—about 2%—is thought to code for protein kinases. A single cell may have hundreds of different kinds, each specific for a different substrate protein. Together, protein kinases probably regulate the activity of a large proportion of the thousands of proteins in a cell. Among these are most of the proteins that, in turn, regulate cell division. Abnormal activity of such a Keep in mind that the original signaling molecule is not physically passed along a signaling pathway; in most cases, it never even enters the cell. When we say that the signal is relayed along a pathway, we mean that certain information is passed on. At each step, the signal is transduced into a different form, commonly a shape change in the next protein. Very often, the shape change is brought about by phosphorylation. Protein Phosphorylation and Dephosphorylation Previous chapters introduced the concept of activating a protein by adding one or more phosphate groups to it (see Figure 8.11a). In Figure 11.8, you have already seen how phosphorylation is involved in the activation of receptor tyrosine kinases. In fact, phosphorylation of proteins and its reverse, dephosphorylation, are commonly used in cells to regulate protein activity. An enzyme that transfers phosphate groups from ATP to a protein is generally known as a protein kinase. Recall that a receptor tyrosine kinase (RTK) is a specific kind of protein kinase that phosphorylates tyrosines on the other RTK in a dimer. Most cytoplasmic protein kinases, however, act on proteins different from themselves. Another distinction is that most cytoplasmic protein kinases phosphorylate either of two other amino acids, serine or threonine, c Figure 11.10 A phosphorylation cascade. In a phosphorylation cascade, a series of different proteins in a pathway are phosphorylated in turn, each protein adding a phosphate group to the next one in line. Here, phosphorylation activates each protein, and dephosphorylation returns it to its inactive form. The active and inactive forms of each protein are represented by different shapes to remind you that activation is usually associated with a change in molecular shape. Signaling molecule Activated relay molecule Receptor 1 A relay molecule activates protein kinase 1. Inactive protein kinase 1 n io ATP PP Inactive protein ATP UNIT TWO The Cell P 3 Active protein kinase 2 phosphorylates a protein (purple) that brings about the cell‘s response to the signal. ADP PP de Active protein kinase 2 a sc ADP Pi 222 ca Pi lat 4 Protein phosphatases (PP) catalyze the removal of the phosphate groups from the proteins, making the proteins inactive again. ry Inactive protein kinase 2 2 Active protein kinase 1 activates protein kinase 2. ho in protein kinase 2 made it incapable of being phosphorylated? p os Ph Active protein kinase 1 WHAT IF? What would happen if a mutation P Active protein Cellular response kinase can cause abnormal cell division and contribute to the development of cancer. Equally important in the phosphorylation cascade are the protein phosphatases, enzymes that can rapidly remove phosphate groups from proteins, a process called dephosphorylation. By dephosphorylating and thus inactivating protein kinases, phosphatases provide the mechanism for turning off the signal transduction pathway when the initial signal is no longer present. Phosphatases also make the protein kinases available for reuse, enabling the cell to respond again to an extracellular signal. The phosphorylation-dephosphorylation system acts as a molecular switch in the cell, turning activities on or off, or up or down, as required. At any given moment, the activity of a protein regulated by phosphorylation depends on the balance in the cell between active kinase molecules and active phosphatase molecules. Small Molecules and Ions as Second Messengers Not all components of signal transduction pathways are proteins. Many signaling pathways also involve small, nonprotein, water-soluble molecules or ions called second messengers. (The pathway’s “first messenger” is considered to be the extracellular signaling molecule—the ligand—that binds to the membrane receptor.) Because they are small and water-soluble, they can readily spread through regions of the cell by diffusion. For example, as we’ll see shortly, a second messenger called cyclic AMP carries the signal initiated by epinephrine from the plasma membrane of a liver or muscle cell into the cell’s interior, where the signal eventually brings about glycogen breakdown. Second messengers participate in pathways that are initiated by both G protein-coupled receptors and receptor tyrosine kinases. The two most common second messengers are cyclic AMP and calcium ions, Ca2 +. A large variety of relay proteins respond to changes in the cytosolic concentration of one or the other of these second messengers. Cyclic AMP Having discovered that epinephrine somehow causes glycogen breakdown within cells, Earl Sutherland next looked for a second messenger that transmits the signal from the plasma membrane to the metabolic machinery in the cytoplasm. He found that binding of epinephrine to the G protein-coupled receptor (GPCR) in the plasma membrane results in a rise in the cytosolic concentration of cyclic AMP (cAMP; cyclic adenosine monophosphate), a small molecule produced from ATP. As shown in Figure 11.11, an enzyme embedded in the plasma membrane, adenylyl cyclase (also known as adenylate cyclase), converts ATP to cAMP in response to an extracellular signal—in this case, provided by epinephrine. When epinephrine outside the cell binds to a GPCR, it activates a G protein that in turn activates adenylyl cyclase. Adenylyl cyclase can then catalyze the synthesis of many molecules of cAMP. In this way, the normal cellular concentration of cAMP can be boosted 20-fold in a matter of seconds. The cAMP broadcasts the signal to the cytoplasm. It does not persist for long in the absence of the hormone because a different enzyme, called phosphodiesterase, converts cAMP to AMP. Another surge of epinephrine is needed to boost the cytosolic concentration of cAMP again. Subsequent research has revealed that epinephrine is only one of many hormones and many other signaling molecules that lead to activation of adenylyl cyclase by G proteins and formation of cAMP (Figure 11.12). The immediate effect of an elevation in cAMP level is usually the activation of a serine/threonine kinase called protein kinase A. The activated protein kinase A then phosphorylates various other proteins, depending on the cell type. (The complete pathway for epinephrine’s stimulation of glycogen breakdown is shown later, in Figure 11.16.) . Figure 11.11 Cyclic AMP. The second messenger cyclic AMP (cAMP) is made from ATP by adenylyl cyclase, an enzyme embedded in the plasma membrane. The phosphate group in cAMP is attached to both the 5¿ and the 3¿ carbons; cAMP’s cyclic arrangement is the basis for its name. cAMP is inactivated by phosphodiesterase, an enzyme that converts it to AMP. P P Adenylyl cyclase P 5¿C Phosphodiesterase P P ATP Pyrophosphate P 3¿C Pi cAMP H2O AMP WHAT IF? What would happen if a molecule that inactivated phosphodiesterase were introduced into the cell? CHAPTER 11 Cell Communication 223 . Figure 11.12 cAMP as a second messenger in a G protein signaling pathway. First messenger (signaling molecule such as epinephrine) G protein-coupled receptor (GPCR) 1 Adenylyl cyclase G protein 2 3 GDP GTP 1 First messenger binds to GPCR, activating it. ATP 2 Activated GPCR binds to G protein, which is then bound by GTP, activating the G protein. 4 cAMP 5 3 Activated G protein/ GTP binds to adenylyl cyclase. GTP is hydrolyzed, activating adenylyl cyclase. 4 Activated adenylyl cyclase converts ATP to cAMP. Second messenger Protein kinase A Cellular responses 5 cAMP, a second messenger, activates another protein, leading to cellular responses. DRAW IT The bacterium that causes the disease cholera produces a toxin that locks the G protein in its activated state. Review Figure 11.8, then draw this figure as it would be if cholera toxin were present. (You do not need to draw the cholera toxin molecule.) Further regulation of cell metabolism is provided by other G protein systems that inhibit adenylyl cyclase. In these systems, a different signaling molecule activates a different receptor, which in turn activates an inhibitory G protein that blocks activation of adenylyl cyclase. Cell activities can be fine-tuned by the balance between these systems. Now that we know about the role of cAMP in G protein signaling pathways, we can explain in molecular detail how certain microorganisms cause disease. Consider cholera, a disease that is frequently epidemic in places where the water supply is contaminated with human feces. People acquire the cholera bacterium, Vibrio cholerae, by drinking contaminated water. The bacteria form a biofilm on the lining of the small intestine and produce a toxin. The cholera toxin is an enzyme that chemically modifies a G protein involved in regulating salt and water secretion. Because the modified G protein is unable to hydrolyze GTP to GDP, it remains stuck in its active form, continuously stimulating adenylyl cyclase to make cAMP (see the question with Figure 11.12). The resulting high concentration of cAMP causes the intestinal cells to secrete large amounts of salts into the intestines, with water following by osmosis. An infected person quickly develops profuse diarrhea and if left untreated can soon die from the loss of water and salts. 224 UNIT TWO The Cell Our understanding of signaling pathways involving cyclic AMP or related messengers has allowed us to develop treatments for certain conditions in humans such as erectile dysfunction. In one pathway, the gas nitric oxide (NO) is released by a cell and enters a neighboring muscle cell, where it causes production of a molecule similar to cAMP called cyclic GMP (cGMP). The cGMP then acts as a second messenger that causes relaxation of muscles, such as those in the walls of arteries. A compound that prolongs the signal (by inhibiting the hydrolysis of cGMP to GMP) was originally prescribed for chest pains because it relaxed blood vessels and increased blood flow to the heart muscle. Under the trade name Viagra, this compound is now widely used as a treatment for erectile dysfunction in human males. Because Viagra leads to dilation of blood vessels, it also allows increased blood flow to the penis, optimizing physiological conditions for penile erections. Calcium Ions and Inositol Trisphosphate (IP3) Many of the signaling molecules that function in animals— including neurotransmitters, growth factors, and some hormones—induce responses in their target cells through signal transduction pathways that increase the cytosolic concentration of calcium ions (Ca2 + ). Calcium is even more widely used than cAMP as a second messenger. Increasing the local cytosolic concentration of Ca2 + causes many responses in animal cells, including muscle cell contraction, exocytosis of molecules (secretion), and cell division. In plant cells, a wide range of hormonal and environmental stimuli can cause brief increases in cytosolic Ca2 + concentration, triggering various signaling pathways, such as the pathway for greening in response to light (see Figure 39.4). Cells use Ca2 + as a second messenger in pathways triggered by both G protein-coupled receptors and receptor tyrosine kinases. Although cells always contain some Ca2 +, this ion can function as a second messenger because its concentration in the cytosol is normally much lower than the concentration outside the cell (Figure 11.13). In fact, the level of Ca2 + in the blood and extracellular fluid of an animal is often more than 10,000 times higher than that in the cytosol. Calcium ions are actively transported out of the cell and are actively imported from the cytosol into the endoplasmic reticulum (and, under some conditions, into mitochondria and chloroplasts) by various protein pumps. As a result, the calcium concentration in the ER is usually much higher than that in the cytosol. Because the cytosolic calcium level is low, a small change in absolute numbers of ions represents a relatively large percentage change in local calcium concentration. In response to a signal relayed by a signal transduction pathway, the cytosolic calcium level may rise, usually by a . Figure 11.13 The maintenance of calcium ion concentrations in an animal cell. The Ca2 + concentration in the cytosol is usually much lower (light green) than in the extracellular fluid and ER (dark green). Protein pumps in the plasma membrane and the ER membrane, driven by ATP, move Ca2 + from the cytosol into the extracellular fluid and into the lumen of the ER. Mitochondrial pumps, driven by chemiosmosis (see Concept 9.4), move Ca2 + into mitochondria when the calcium level in the cytosol rises significantly. Key High [Ca2+] Low [Ca2+] Endoplasmic reticulum (ER) Plasma membrane ATP Mitochondrion mechanism that releases Ca2 + from the cell’s ER. The pathways leading to calcium release involve two other second messengers, inositol trisphosphate (IP3) and diacylglycerol (DAG). These two messengers are produced by cleavage of a certain kind of phospholipid in the plasma membrane. Figure 11.14 shows the complete picture of how a signal causes IP3 to stimulate the release of calcium from the ER. Because IP3 acts before calcium in these pathways, calcium could be considered a “third messenger.” However, scientists use the term second messenger for all small, nonprotein components of signal transduction pathways. CONCEPT CHECK 11.3 1. What is a protein kinase, and what is its role in a signal transduction pathway? 2. When a signal transduction pathway involves a phosphorylation cascade, how does the cell’s response get turned off? Nucleus 3. What is the actual “signal” that is being transduced in any signal transduction pathway, such as those shown in Figures 11.6 and 11.10? In what way is this information being passed from the exterior to the interior of the cell? Ca2+ pump CYTOSOL 4. WHAT IF? If you exposed a cell to a ligand that binds to a receptor and activates phospholipase C, predict the effect the IP3-gated calcium channel would have on Ca2 + concentration in the cytosol. ATP EXTRACELLULAR FLUID c Figure 11.14 Calcium and IP3 in signaling pathways. Calcium ions (Ca2 + ) and inositol trisphosphate (IP3) function as second messengers in many signal transduction pathways. In this figure, the process is initiated by the binding of a signaling molecule to a G protein-coupled receptor. A receptor tyrosine kinase could also initiate this pathway by activating phospholipase C. MAKE CONNECTIONS Explain the difference in function (in terms of ion transport) between the Ca2 + pump in Figure 11.13 and the Ca2 + channel protein shown here. (See Figure 7.17.) For suggested answers, see Appendix A. 1 A signaling molecule binds to a receptor, leading to activation of phospholipase C. EXTRACELLULAR FLUID CYTOSOL Endoplasmic reticulum (ER) lumen 2 Phospholipase C cleaves a plasma membrane phospholipid called PIP2 into DAG and IP3. 3 DAG functions as a second messenger in other pathways. Signaling molecule (first messenger) G protein G protein-coupled receptor GTP GDP DAG Phospholipase C PIP2 IP3-gated calcium channel IP3 (second messenger) Ca2+ Various proteins activated Ca2+ (second messenger) 4 IP3 quickly diffuses through the cytosol and binds to an IP3gated calcium channel in the ER membrane, causing it to open. 45 Calcium ions flow out of the ER (down their concentration gradient), raising the Ca2+ level in the cytosol. CHAPTER 11 Cellular responses 46 The calcium ions activate the next protein in one or more signaling pathways. Cell Communication 225 CONCEPT 11.4 Cellular response: Cell signaling leads to regulation of transcription or cytoplasmic activities We now take a closer look at the cell’s subsequent response to an extracellular signal—what some researchers call the “output response.” What is the nature of the final step in a signaling pathway? . Figure 11.15 Nuclear responses to a signal: the activation of a specific gene by a growth factor. This diagram shows a typical signaling pathway that leads to regulation of gene activity in the cell nucleus. The initial signaling molecule, in this case a growth factor, triggers a phosphorylation cascade, as in Figure 11.10. (The ATP molecules and phosphate groups are not shown.) Once phosphorylated, the last kinase in the sequence enters the nucleus and activates a transcription factor, which stimulates transcription of a specific gene (or genes). The resulting mRNAs then direct the synthesis of a particular protein. Growth factor Signal reception Receptor Nuclear and Cytoplasmic Responses Ultimately, a signal transduction pathway leads to the regulation of one or more cellular activities. The response may occur in the nucleus or in the cytoplasm of the cell. Many signaling pathways ultimately regulate protein synthesis, usually by turning specific genes on or off in the nucleus. Like an activated steroid receptor (see Figure 11.9), the final activated molecule in a signaling pathway may function as a transcription factor. Figure 11.15 shows an example in which a signaling pathway activates a transcription factor that turns a gene on: The response to this growth factor signal is transcription, the synthesis of one or more specific mRNAs, which will be translated in the cytoplasm into specific proteins. In other cases, the transcription factor might regulate a gene by turning it off. Often, a transcription factor regulates several different genes. Sometimes a signaling pathway may regulate the activity of proteins rather than causing their synthesis by activating gene expression. This directly affects proteins that function outside the nucleus.