Biology Textbook PDF - Cell Signaling - Pearson

874 Chapter 15: Cell Signaling EXTRACELLULAR SIGNAL MOLECULE Figure 15–1 A simple intracellular signaling pathway activated by an...

874 Chapter 15: Cell Signaling EXTRACELLULAR SIGNAL MOLECULE Figure 15–1 A simple intracellular signaling pathway activated by an extracellular signal molecule. The signal RECEPTOR PROTEIN molecule usually binds to a receptor protein that is embedded in the plasma plasma membrane of membrane of the target cell. The receptor target cell activates one or more intracellular signaling CYTOSOL pathways, involving a series of signaling proteins and small chemical messengers. Finally, one or more of the intracellular signaling molecules alters the activity of effector proteins and thereby the behavior of the cell. INTRACELLULAR SIGNALING MOLECULES EFFECTOR PROTEINS metabolic transcription cytoskeletal enzyme regulatory protein protein altered altered gene altered cell metabolism expression shape or movement Communication between cells in multicellular organisms is mediated mainly by extracellular signal molecules. Some of these operate over long distances, signaling to cells far away; others signal only to immediate neighbors. Most cells in multicellular organisms both emit and receive signals. Reception of the sig- nals depends on receptor proteins, usually (but not always) at the cell surface, which bind the signal molecule. !e binding activates the receptor, which in turn activates one or more intracellular signaling pathways or systems. !ese systems MBoC7 m15.01/15.01 depend on intracellular signaling proteins, which process the signal inside the receiving cell and distribute it to the appropriate intracellular targets. Some of these proteins produce small chemical messengers called second messengers, which carry the signal to other signaling proteins. !e targets that lie at the end of signaling pathways are generally called e!ector proteins, which are altered in some way by the incoming signal and implement the appropriate change in cell behavior. Depending on the signal and the type and state of the receiving cell, these e"ectors can be transcription regulators, ion channels, components of a metabolic pathway, or parts of the cytoskeleton (Figure 15–1). !e fundamental features of cell signaling have been conserved throughout the evolution of the eukaryotes. In budding yeast, for example, the response to mating factor depends on cell-surface receptor proteins, intracellular GTP-binding proteins, and protein kinases that are clearly related to functionally similar proteins in animal cells. !rough gene duplication and divergence, however, the signaling systems in animals have become much more elaborate than those in yeasts; the human genome, for example, contains more than 1500 genes that encode receptor proteins, and the number of di"erent receptor proteins is further increased by alternative RNA splicing and post-translational modi#cations. Extracellular Signals Can Act Over Short or Long Distances Many extracellular signal molecules remain bound to the surface of the signaling cell and in$uence only cells that contact it (Figure 15–2A). Such contact-dependent signaling is especially important during development and in immune responses. MBOC7_ptr_ch15_873-948.indd 874 08/12/21 12:23 PM  PRINCIPLES OF CELL SIGNALING 875 (A) CONTACT-DEPENDENT (B) PARACRINE Figure 15–2 Four forms of intercellular signaling. (A) Contact-dependent signaling requires cells to be in direct membrane– membrane contact. (B) Paracrine signaling signaling cell target cell signaling cell depends on local mediators that are released into the extracellular space and act on neighboring cells. (C) Synaptic target cells signaling is performed by neurons that transmit signals electrically along their axons and release neurotransmitters at membrane- bound signal local chemical synapses, which are often located molecule mediator far away from the neuronal cell body. (D) Endocrine signaling depends on endocrine cells, which secrete hormones into the bloodstream for distribution (C) SYNAPTIC (D) ENDOCRINE throughout the body. Many of the same types of signaling molecules are used endocrine cell receptor target cell in paracrine, synaptic, and endocrine synapse signaling; the crucial differences lie in the neuron speed and selectivity with which the signals hormone are delivered to their targets. axon target cell cell neurotransmitter body bloodstream target cell Contact-dependent signaling during development can sometimes operate over relatively large distances if the communicating cells extend long, thin processes to make contact with one another. In most cases, however, signaling cells secrete signal molecules into the extra- cellular $uid. Often, the secreted molecules are local mediators, which act only on cells in the local environment of the signaling cell. !is is called paracrine signaling (Figure 15–2B). Usually, the signaling and target cells in paracrine signaling are of di"erent cell types, but cells may also produce signals that they themselves respond to: this is referred to as autocrine signaling. Cancer cells, for example, often produce extracellular signals that stimulate their own survival and proliferation. Large multicellular organisms like us also need long-range signaling mecha- nisms to coordinate the behavior of cells in remote parts of the body. !us, they MBoC7 m15.02/15.02 have evolved cell types specialized for intercellular communication over large distances. !e most sophisticated of these are nerve cells, or neurons, which typi- cally extend long, branching processes (axons) that enable them to contact target cells far away, where the processes terminate at the specialized sites of signal transmission known as chemical synapses. When a neuron is activated by stim- uli from other nerve cells, it sends electrical impulses (action potentials) rapidly along its axon; when the impulse reaches the synapse at the end of the axon, it triggers secretion of a chemical signal that acts as a neurotransmitter. !e tightly organized structure of the synapse ensures that the neurotransmitter is delivered speci#cally to receptors on the postsynaptic target cell (Figure 15–2C). !e details of this synaptic signaling process are discussed in Chapter 11. A quite di"erent strategy for signaling over long distances makes use of endo- crine cells, which secrete their signal molecules, called hormones, into the bloodstream. !e blood carries the molecules far and wide, allowing them to act on target cells that may lie almost anywhere in the body (Figure 15–2D). Extracellular Signal Molecules Bind to Specific Receptors Cells in multicellular animals communicate by means of hundreds of kinds of extracellular signal molecules. !ese include proteins, small peptides, amino acids, nucleotides, steroids, retinoids, fatty acid derivatives, and even dissolved MBOC7_ptr_ch15_873-948.indd 875 08/12/21 12:23 PM  876 Chapter 15: Cell Signaling Figure 15–3 The binding of extracellular signal molecules to either cell-surface or (A) CELL-SURFACE RECEPTORS intracellular receptors. (A) Most signal molecules are hydrophilic and are therefore unable to cross the target cell’s plasma membrane directly; instead, they bind to cell-surface receptors, which in plasma membrane cell-surface turn generate signals inside the target cell (see Figure 15–1). (B) Some small signal molecules, receptor protein by contrast, diffuse across the plasma membrane and bind to receptor proteins inside the target cell—either in the cytosol or in the nucleus (as shown here). Many of these small signal molecules are hydrophobic and poorly soluble in aqueous solutions; they are therefore transported in the bloodstream and other extracellular fluids bound to carrier proteins, from which they dissociate before entering the target cell. hydrophilic signal molecule target cell gases such as nitric oxide and carbon monoxide. Most of these signal molecules (B) INTRACELLULAR RECEPTORS are released into the extracellular space by exocytosis from the signaling cell, as discussed in Chapter 13. Some, however, are emitted by di"usion through the small, hydrophobic signal molecule signaling cell’s plasma membrane, whereas others are displayed on the external surface of the cell and remain attached to it, signaling to target cells only upon target cell contact. Transmembrane signal proteins may operate in this way, although in carrier protein some cases their extracellular domains are released from the signaling cell’s sur- face by proteolytic cleavage and then act at a distance. Regardless of the nature of the signal, the target cell responds by means of a receptor, which binds the signal molecule and then initiates a response in the target cell. !e binding site of the receptor has a complex structure that is shaped nucleus to recognize the signal molecule with high speci#city, helping to ensure that the intracellular receptor protein receptor responds only to the appropriate signal and not to the many other sig- naling molecules the cell is exposed to. Many signal molecules act at very low concentrations (typically 10–8 M), and their receptors usually bind them with high a%nity (dissociation constant Kd 10–8 M; see Figure 3–42). In most cases, receptors are transmembrane proteins on the target-cell sur- face. When these proteins bind an extracellular signal molecule (a ligand), they become activated and generate various intracellular signals that alter the behav- ior of the cell. In other cases, the receptor proteins are inside the target cell, and the signal molecule has to enter the cell to bind to them: this requires that the signal molecule be su%ciently small and hydrophobic to di"use across the target cell’s plasma membrane (Figure 15–3). !is chapter focuses primarily on signal- ing through cell-surface receptors, but we will brie$y describe signaling through intracellular receptors later in the chapter. MBoC7 m15.03/15.03 Each Cell Is Programmed to Respond to Specific Combinations of Extracellular Signals A typical cell in a multicellular organism is exposed to hundreds of di"erent sig- nal molecules in its environment. !e molecules can be soluble, bound to the extracellular matrix, or bound to the surface of a neighboring cell; they can be stimulatory or inhibitory; they can act in innumerable di"erent combinations; and they can in$uence almost any aspect of cell behavior. !e cell responds to this blizzard of signals selectively, in large part by expressing only those receptors and intracellular signaling systems that respond to the signals that are required for the regulation of that cell. Most cells respond to many di"erent signals in the environment, and some of these signals may in$uence the response to other signals. One of the key chal- lenges in cell biology is to determine how a cell integrates all of this signaling information in order to make decisions—to divide, to move, to di"erentiate, and so on. Many cells, for example, require a speci#c combination of extracellular sur- vival factors to allow the cell to continue living; when deprived of these signals, the cell activates a suicide program and kills itself—usually by apoptosis, as dis- cussed in Chapter 18. Cell proliferation often depends on a combination of signals that promote both cell division and survival, as well as signals that stimulate cell growth (Figure 15–4). On the other hand, di"erentiation into a nondividing state (called terminal di!erentiation) frequently requires a di"erent combination of survival and di"erentiation signals that must override any signal to divide. MBOC7_ptr_ch15_873-948.indd 876 08/12/21 12:23 PM  PRINCIPLES OF CELL SIGNALING 877 A Figure 15–4 An animal cell’s dependence on multiple extracellular signal molecules. Each cell type displays B SURVIVE a set of receptors that enables it to respond to a corresponding set of signal molecules C produced by other cells. These signal molecules work in various combinations A to regulate the behavior of the cell. As shown here, an individual cell often requires GROW + DIVIDE multiple signals to survive (blue arrows) B and additional signals to grow and divide (red arrows) or differentiate (green arrows). If deprived of appropriate survival signals, C E a cell will undergo a form of cell suicide D known as apoptosis. The actual situation is A even more complex. Although not shown, some extracellular signal molecules act to inhibit these and other cell behaviors or B DIFFERENTIATE even to induce apoptosis. C G F apoptotic DIE cell A signal molecule often has di"erent e"ects on di"erent types of target cells. !e neurotransmitter acetylcholine (Figure 15–5A), for example, decreases the rate of action potential #ring in heart pacemaker cells (Figure 15–5B) and stimu- lates the production of saliva by salivary gland cells (Figure 15–5C), even though the acetylcholine receptors are the same on both cell types. In skeletal muscle, acetylcholine causes the cells to contract by binding to a di"erent type of acetyl- 15–5D). choline receptor (FigureMBoC7 !e di"erent e"ects of acetylcholine in these m15.04/15.04 cell types result from di"erences in the intracellular signaling proteins, e"ector proteins, and genes that are activated. !us, an extracellular signal itself has little information content; it simply induces the cell to respond according to its pre- determined state, which depends on the cell’s developmental history and the speci#c genes it expresses. (A) acetylcholine (B) heart pacemaker cell (C) salivary gland cell (D) skeletal muscle cell CH3 acetylcholine H3C N+ CH3 receptor protein CH2 CH2 O C O CH3 DECREASED RATE SECRETION CONTRACTION OF FIRING Figure 15–5 Various responses induced by the neurotransmitter acetylcholine. (A) The chemical structure of acetylcholine. (B–D) Different cell types are specialized to respond to acetylcholine in different ways. In some cases (B and C), acetylcholine binds to the same type of acetylcholine receptor (a G-protein-coupled receptor; see Figure 15–6), but the intracellular signals produced are interpreted differently in cells specialized for different functions. In other cases (D), the acetylcholine receptor protein is different (an ion-channel-coupled receptor; see Figure 15–6). MBOC7_ptr_ch15_873-948.indd 877 MBoC7 m15.05/15.05 08/12/21 12:23 PM  878 Chapter 15: Cell Signaling There Are Three Major Classes of Cell-Surface Receptor Proteins Most extracellular signal molecules bind to speci#c receptor proteins on the surface of the target cells they in$uence and do not enter the cytosol or nucleus. !ese cell-surface receptors act as signal transducers by converting an extracel- lular ligand-binding event into intracellular signals that alter the behavior of the target cell. Most cell-surface receptor proteins belong to one of three classes, de#ned by their transduction mechanism. Ion-channel-coupled receptors, also known as transmitter-gated ion channels or ionotropic receptors, are involved in rapid synaptic signaling between nerve cells and other electrically excitable target cells such as muscle cells (Figure 15–6A). !is type of signaling is mediated by a small number of neurotransmitters that transiently open or close an ion channel formed by the protein to which they bind, brie$y changing the ion permeability of the plasma membrane and thereby changing the excitability of the postsyn- aptic target cell. Most ion-channel-coupled receptors belong to a large family of homologous, multipass transmembrane proteins. Because they are discussed in detail in Chapter 11, we will not consider them further here. G-protein-coupled receptors act by indirectly regulating the activity of a separate plasma-membrane-bound target protein, which is generally either an enzyme or an ion channel. A heterotrimeric GTP-binding protein (G protein) mediates the interaction between the activated receptor and this target protein (Figure 15–6B). !e activation of the target protein can change the concentration of one or more small intracellular signaling molecules (if the target protein is an Figure 15–6 Three classes of cell- (A) ION-CHANNEL-COUPLED RECEPTORS surface receptors. (A) Ion-channel- ions coupled receptors (also called transmitter- gated ion channels), (B) G-protein-coupled receptors, and (C) enzyme-coupled signal molecule receptors. Although many enzyme-coupled plasma receptors have intrinsic enzymatic activity, membrane as shown on the left in C, many others rely on associated enzymes, as shown on the right in C. Ligands activate many enzyme- coupled receptors by promoting their dimerization, which results in the interaction (B) G-PROTEIN-COUPLED RECEPTORS and activation of the cytoplasmic domains. signal molecule activated inactive inactive inactive activated enzyme receptor G protein enzyme receptor and activated G protein G protein (C) ENZYME-COUPLED RECEPTORS signal molecule signal molecule in form of a dimer OR inactive catalytic active catalytic activated domain domain associated enzyme MBOC7_ptr_ch15_873-948.indd 878 08/12/21 12:23 PM MBoC7 m15.06/15.06  PRINCIPLES OF CELL SIGNALING 879 enzyme) or it can change the ion permeability of the plasma membrane (if the target protein is an ion channel). !e small intracellular signaling molecules act in turn to alter the behavior of yet other signaling proteins in the cell. Enzyme-coupled receptors either function as enzymes or associate directly with enzymes that they activate (Figure 15–6C). !ey are usually single-pass transmembrane proteins that have their ligand-binding site outside the cell and their catalytic or enzyme-binding site inside. Enzyme-coupled receptors are het- erogeneous in structure compared with the other two classes; the great majority, however, are either protein kinases or associate with protein kinases, which phos- phorylate speci#c sets of proteins in the target cell when activated. !ere are also some types of cell-surface receptors that do not #t easily into any of these classes but have important functions in controlling the specializa- tion of di"erent cell types during development and in tissue renewal and repair in adults. We discuss these in a later section, after we explain how G-protein- coupled receptors and enzyme-coupled receptors operate. First, we continue our general discussion of the principles of signaling via cell-surface receptors. Cell-Surface Receptors Relay Signals Via Intracellular Signaling Molecules Numerous intracellular signaling molecules relay signals received by cell-surface receptors into the cell interior. !e resulting chain of intracellular signaling events ultimately alters e"ector proteins that are responsible for modifying the behavior of the cell (see Figure 15–1). Some intracellular signaling molecules are small chemicals, which are often called second messengers (the “#rst messengers” being the extracellular sig- nals). !ey are generated in large amounts in response to receptor activation and di"use away from their source, spreading the signal to other parts of the cell. Some, such as cyclic AMP and Ca2+, are water soluble and di"use in the cytosol, while others, such as diacylglycerol, are lipid soluble and di"use in the plane of the plasma membrane. In either case, they pass the signal on by binding to and altering the behavior of selected signaling or e"ector proteins. Most intracellular signaling molecules are proteins, which help relay the signal into the cell by either generating second messengers or activating the next signaling or e"ector protein in the pathway. Many of these proteins behave like molecular switches. When they receive a signal, they switch from an inactive to an active state, until another process switches them o", returning them to their inactive state. !e switching o" can be just as important as the switching on. If a signaling pathway is to recover after transmitting a signal so that it can be ready to transmit another, every activated molecule in the pathway must return to its original, unactivated state. !e largest class of molecular switches consists of proteins that are activated or inactivated by phosphorylation (discussed in Chapter 3). For these proteins, the switch is thrown in one direction by a protein kinase, which covalently adds one or more phosphate groups to speci#c amino acids on the signaling protein, and in the other direction by a protein phosphatase, which removes the phosphate groups (Figure 15–7A). !e activity of any protein regulated by phosphorylation depends on the balance between the activities of the kinases that phosphorylate it and of the phosphatases that dephosphorylate it. About 30–50% of human proteins contain covalently attached phosphate, and the human genome encodes about 520 protein kinases and about 150 protein phos- phatases. A typical mammalian cell makes use of hundreds of distinct types of protein kinases at any moment. Protein kinases attach phosphate to the hydroxyl group of speci#c amino acids on the target protein. !ere are two main types of protein kinase in eukary- otic cells. !e great majority are serine/threonine kinases, which phosphorylate the hydroxyl groups of serines and threonines in their targets. Others are tyrosine kinases, which phosphorylate proteins on tyrosines. Tyrosine kinases are found MBOC7_ptr_ch15_873-948.indd 879 08/12/21 12:23 PM  880 Chapter 15: Cell Signaling Figure 15–7 Two types of intracellular OFF OFF signaling proteins that act as SIGNAL SIGNAL molecular switches. (A) A protein kinase IN IN GDP covalently adds a phosphate from ATP P P to the signaling protein, and a protein ATP protein protein GDP GTP GTP phosphatase removes the phosphate. kinase phosphatase binding hydrolysis Although not shown, many signaling ADP GTP proteins are activated by dephosphorylation rather than by phosphorylation. (B) A GTP- binding protein is induced to exchange its ON ON bound GDP for GTP, which activates the P GTP protein; the protein then inactivates itself by SIGNAL SIGNAL hydrolyzing its bound GTP to GDP. OUT OUT (A) SIGNALING BY PHOSPHORYLATION (B) SIGNALING BY GTP BINDING primarily in multicellular animals; these kinases are not present, for example, in yeast. Many intracellular signaling proteins controlled by phosphorylation are them- selves protein kinases, and these are often organized into kinase cascades. In such a cascade, one protein kinase, activated by phosphorylation, phosphorylates the MBoC7 m15.07/15.07 next protein kinase in the sequence, and so on, relaying the signal onward and, in some cases, amplifying it or spreading it to other signaling pathways. Like the protein kinases, the protein phosphatases are categorized by their speci#city for serine/threonine phosphate or tyrosine phosphate. !ere are about 100 protein tyrosine phosphatases encoded in the human genome, including some dual-speci"city phosphatases that also dephosphorylate serines and threonines. !e other important class of molecular switches consists of GTP-binding proteins (discussed in Chapter 3). !ese proteins switch between two distinct structural conformations: an “on” state when GTP is bound and an “o"” state when GDP is bound. In the “on” state, they bind and thereby activate speci#c sig- naling proteins. GTP-binding proteins usually have intrinsic GTPase activity and shut themselves o" by hydrolyzing their bound GTP to GDP (Figure 15–7B). !e INACTIVE inactive protein then returns to the “on” state when GDP dissociates, allowing MONOMERIC GTPase a new GTP to bind. !ere are two major types of GTP-binding proteins. Large, heterotrimeric GTP-binding proteins (also called G proteins) help relay signals OFF from G-protein-coupled receptors that activate them (see Figure 15–6B). Small GDP GDP P monomeric GTPases (also called monomeric GTP-binding proteins) help relay signals from many classes of cell-surface receptors. GEF GAP For most GTP-binding proteins, the inactivation process (GTP hydrolysis to GTP GDP) and the activation process (GDP dissociation) are slow in the absence of other proteins. Inside the cell, regulatory proteins are used to accelerate one or ON the other process, thereby governing the activation state of the GTP-binding pro- GTP tein. GTPase-activating proteins (GAPs) drive the proteins into an “o"” state by increasing the rate of hydrolysis of bound GTP. Conversely, guanine nucleotide ACTIVE exchange factors (GEFs) activate GTP-binding proteins by promoting the release MONOMERIC GTPase of bound GDP, which allows a new GTP to bind. In the case of heterotrimeric G proteins, the activated receptor serves as the GEF. Figure 15–8 illustrates the regu- Figure 15–8 The regulation of a lation of monomeric GTPases. monomeric GTPase. GTPase-activating proteins (GAPs) inactivate the protein by Not all molecular switches in signaling systems depend on phosphorylation stimulating it to hydrolyze its bound GTP or GTP binding. We see later that some signaling proteins are switched on or to GDP, which remains tightly bound to the o" by the binding of another signaling protein or a second messenger, such as inactivated GTPase. Guanine nucleotide cyclic AMP or Ca2+, or by covalent modi#cations other than phosphorylation or exchange factors (GEFs) activate the dephosphorylation, such as ubiquitylation (discussed in Chapter 3). inactive protein by stimulating it to release MBoC7 e16.12/15.08 its GDP; because the concentration of GTP For simplicity, we often portray a signaling pathway as a series of activation in the cytosol is 10 times greater than the steps (see Figure 15–1). It is important to note, however, that most signaling concentration of GDP, the protein rapidly pathways contain inhibitory steps, and a sequence of two inhibitory steps can binds GTP and is thereby activated. MBOC7_ptr_ch15_873-948.indd 880 08/12/21 12:23 PM  PRINCIPLES OF CELL SIGNALING 881 upstream signal Figure 15–9 A sequence of two inhibitory signals produces a positive signal. (A) In inactive this simple signaling system, a transcription transcription regulator is kept in an inactive state by regulator P protein kinase protein kinase a bound inhibitor protein. In response to some upstream signal, a protein kinase inhibitor protein is activated and phosphorylates the inhibitor, causing its dissociation from the transcription regulator, which can now inhibitor protein transcription regulator activate gene expression. (B) The signaling pathway consists of a sequence of four (A) GENE EXPRESSION (B) GENE EXPRESSION steps, including two sequential inhibitory steps that are equivalent to a single activating step. have the same e"ect as one activating step (Figure 15–9). !is activation scheme is very common in signaling systems, as we will see when we describe speci#c pathways later in this chapter. MBoC7 m15.09/15.09 Intracellular Signals Must Be Specific and Robust in a Noisy Cytoplasm In an idealized signaling pathway like that shown in Figure 15–1, each intracel- lular signaling molecule interacts only with the appropriate downstream target. Similarly, the target is activated only by the appropriate upstream signal. In reality, however, the cell is crowded with closely related signaling molecules that control a diverse array of cellular processes. It is inevitable that a signaling molecule will sometimes interact with molecules in other signaling pathways, potentially creat- ing unwanted cross-talk and interference between signaling systems. How does a signal remain strong and speci#c under these noisy conditions? A key to signaling speci#city is the high a%nity and speci#city of the interac- tions between intracellular signaling molecules and their correct partners. !e binding of a signaling molecule to its target is determined by precise and com- plex interactions between complementary surfaces on the two molecules. Some protein kinases, for example, contain active sites that recognize a speci#c amino acid sequence around the phosphorylation site on the correct target protein, and many signaling enzymes employ additional docking sites, outside their active site, that promote a speci#c, high-a%nity interaction with a complementary site on the target. !ese and related mechanisms provide a strong and persistent interac- tion between the correct partners, thereby enhancing the likelihood that a signal is passed to the appropriate target. !e speci#city of signaling systems also depends on noise #lters that reduce or remove undesirable background signals. Consider a signaling pathway, for example, in which a response is triggered by phosphorylation of several sites on a target protein. Inside the cell, we can generally assume that a constant low level of phosphatase activity is present to remove these phosphorylations. As a result, a strong response is possible only if the appropriate protein kinase reaches a high and persistent level of activity that is su%cient to overcome the opposing phos- phatase activity. If by some random accident another protein kinase interacts brie$y with the target protein and catalyzes phosphorylation on one or two sites, these will be removed by the opposing phosphatase and no response will occur. !e weak background signal is thereby ignored. Cells in a population often exhibit random variations in the concentration or activity of their intracellular signaling molecules. Similarly, individual molecules in a large population of molecules vary in their activity or interactions with other molecules. !is signal variability introduces another form of noise that can inter- fere with the precision and e%ciency of signaling. Most signaling systems, however, generate remarkably robust and precise responses even when upstream signals are variable or some components of the system are disabled. In some cases, this robustness depends on the presence of parallel mechanisms; for example, a signal might employ two parallel pathways to activate a single common downstream tar- get protein, allowing the response to occur even if one pathway is crippled. MBOC7_ptr_ch15_873-948.indd 881 08/12/21 12:23 PM  882 Chapter 15: Cell Signaling Intracellular Signaling Complexes Form at Activated Cell-Surface Receptors One simple and e"ective strategy for enhancing the speci#city of interactions between intracellular signaling molecules and reducing background noise is to localize the molecules in the same part of the cell, often within large pro- tein complexes, thereby promoting their interaction with one another and not with inappropriate partners. Such mechanisms often involve sca!old proteins, which bring together groups of interacting signaling proteins into signaling com- plexes, often before a signal has been received (Figure 15–10A). Because the (A) PREFORMED SIGNALING COMPLEX ON A SCAFFOLD PROTEIN inactive receptor signal molecule activated receptor CYTOSOL plasma membrane 1 1 scaffold protein inactive activated 2 intracellular 2 intracellular signaling proteins signaling proteins 3 3 downstream signals (B) ASSEMBLY OF SIGNALING COMPLEX ON AN ACTIVATED RECEPTOR signal molecule inactive receptor 1 P activated intracellular 2 P signaling 1 proteins 3 P activated receptor inactive intracellular signaling 3 proteins 2 downstream Figure 15–10 Three types of intracellular signals signaling complexes. (A) A receptor and some of the intracellular signaling proteins it activates in sequence are preassembled (C) ASSEMBLY OF SIGNALING COMPLEX ON PHOSPHOINOSITIDE DOCKING SITES into a signaling complex on the inactive receptor by a large scaffold protein. (B) A specific phospholipid signal molecule signaling complex assembles transiently inactive molecules activated receptor receptor (phosphoinositides) on a receptor only after the binding of hyperphosphorylated an extracellular signal molecule has phosphoinositides activated the receptor; here, the activated receptor phosphorylates itself at multiple sites, which then act as docking sites for P P P P PP P PP P intracellular signaling proteins. (C) Activation 2 of a receptor leads to the increased 1 phosphorylation of specific phospholipids 2 (phosphoinositides) in the adjacent plasma membrane; these then serve as docking 1 activated intracellular downstream signals sites for specific intracellular signaling inactive intracellular signaling proteins proteins, which can now interact with signaling proteins each other. MBOC7_ptr_ch15_873-948.indd 882 08/12/21 12:23 PM MBoC7 m15.10/15.10  PRINCIPLES OF CELL SIGNALING 883 sca"old holds the proteins in close proximity, they can interact at high local con- centrations and be activated rapidly, e%ciently, and selectively in response to an appropriate extracellular signal, avoiding unwanted cross-talk with other signaling pathways. In other cases, such signaling complexes form transiently in response to an extracellular signal and rapidly disassemble when the signal is gone. !ey often assemble around a cell-surface receptor after an extracellular signal molecule has activated it. In many of these cases, the cytoplasmic tail of an activated enzyme-coupled receptor is phosphorylated during the activation process, and the phosphorylated amino acids then serve as docking sites for the assem- bly of other signaling proteins (Figure 15–10B). In yet other cases, receptor activation leads to the production of modi#ed phospholipid molecules (called phosphoinositides) in the adjacent plasma membrane, which then recruit spe- ci#c intracellular signaling proteins to this region of membrane, where they are activated (Figure 15–10C). Modular Interaction Domains Mediate Interactions Between Intracellular Signaling Proteins Simply bringing intracellular signaling proteins together into close proxim- ity is sometimes su%cient to activate them. !us, induced proximity, where a signal triggers assembly of a signaling complex, is commonly used to relay sig- nals from protein to protein along a signaling pathway. !e assembly of such signaling complexes depends on various highly conserved, small interaction domains, which are found in many intracellular signaling proteins. Each of these compact protein modules binds to a particular structural motif in another protein or lipid. !e recognized motif in the interacting protein can be a short peptide sequence, a covalent modi#cation (such as a phosphorylated amino acid), or another protein domain. !e use of modular interaction domains pre- sumably facilitated the evolution of new signaling pathways. Because it can be inserted at many locations in a protein without disturbing the protein’s folding or function, a new interaction domain can connect the protein to additional signaling pathways. !ere are many types of interaction domains in signaling proteins. Src homol- ogy 2 (SH2) domains and phosphotyrosine-binding (PTB) domains, for example, bind to phosphorylated tyrosines in a particular peptide sequence on activated receptors or intracellular signaling proteins. Src homology 3 (SH3) domains bind to short, proline-rich amino acid sequences. Some pleckstrin homology (PH) domains bind to the charged head groups of speci#c phosphoinositides that are produced in the plasma membrane in response to an extracellular signal; they enable the protein they are part of to dock on the membrane and inter- act with other similarly recruited signaling proteins (see Figure 15–10C). Some signaling proteins consist solely of two or more interaction domains and func- tion only as adaptors to link two other proteins together in a signaling pathway. Some adaptor proteins have multiple interaction domains as well as their own signal-propagation activity (see Figure 15–12). Interaction domains enable signaling proteins to bind to one another in mul- tiple speci#c combinations. Like Lego bricks, the proteins can form linear or branching chains or three-dimensional networks, which determine the route followed by the signaling pathway. As an example, Figure 15–11 illustrates how some interaction domains mediate the formation of a large signaling complex around the receptor for the hormone insulin. Modular interaction domains are generally located in $exible, unstruc- tured regions of signaling proteins, arrayed along the polypeptide-like beads on a string. Proteins with multiple interaction domains can therefore nucle- ate the formation of large, cross-linked protein matrices around clusters of activated receptors (Figure 15–12). !ese protein matrices behave like gels or biomolecular condensates, thereby creating a local microenvironment that is distinct in composition from the surrounding cytosol (discussed in Chapter 3). MBOC7_ptr_ch15_873-948.indd 883 08/12/21 12:23 PM  884 Chapter 15: Cell Signaling extracellular Figure 15–11 A specific signaling signal molecule complex formed using modular phosphoinositide docking sites plasma membrane interaction domains. This example is based on the insulin receptor, which is a dimeric enzyme-coupled receptor (a P P P P CYTOSOL receptor tyrosine kinase, discussed later). P P First, the activated receptor phosphorylates PH itself on tyrosines, and one of the PH phosphotyrosines then recruits an adaptor P P PTB protein called insulin receptor substrate-1 activated proline-rich receptor regions (IRS1) via a PTB domain of IRS1; the PH P P domain of IRS1 also binds to specific SH3 Sos P SH2 phosphoinositides on the inner surface of P P P the plasma membrane. Then, the activated P receptor phosphorylates IRS1 on tyrosines, and one of these phosphotyrosines binds IRS1 (adaptor protein) the SH2 domain of the adaptor protein Grb2. Next, Grb2 uses one of its two SH3 Grb2 (adaptor protein) scaffold protein domains to bind to a proline-rich region of a protein called Sos, which is thereby brought to the membrane to relay the signal downstream by acting as a GEF (see Figure 15–8) to activate a monomeric Concentration of activated receptors and speci#c signaling proteins in these GTPase called Ras (not shown). Sos also matrices is thought to enhance the strength and speci#city of the receptor signal binds to phosphoinositides in the plasma while reducing interference from other pathways. membrane via its PH domain. Grb2 uses its MBoC7 n15.100/15.11 Another way of bringing receptors and intracellular signaling proteins together other SH3 domain to bind to a proline-rich is to locate them in a speci#c region of the cell. An important example is the pri- sequence in a scaffold protein, which binds several other signaling proteins (not shown). mary cilium that projects like an antenna from the surface of most vertebrate cells The other phosphorylated tyrosines on (discussed in Chapter 16). A number of surface receptors and signaling proteins IRS1 recruit additional signaling proteins are concentrated there—particularly the components of the Hedgehog signaling that have SH2 domains (not shown). system, as we discuss later. Light and smell receptors are also concentrated in specialized cilia. extracellular signal molecule plasma membrane activated CYTOSOL proline-rich region receptor P P P P P P P P P P P P SH2 SH3 adaptor proteins Figure 15–12 Formation of large receptor clusters by multivalent interactions among signaling proteins. The system pictured here contains activated receptor tyrosine kinases that are extensively phosphorylated on disordered regions in the receptor tails. The system also includes two adaptor proteins. One adaptor protein (pink) contains one SH2 domain, which binds phosphorylated tyrosines on the receptors, and two SH3 domains. The other adaptor protein (blue) contains three proline-rich regions that can bind to SH3 domains, plus a protein kinase domain. Numerous multivalent binding interactions can occur among the three components in this system, generating a cross-linked protein matrix or condensate in which the protein kinases of the receptor and adaptor protein are concentrated, potentially providing a more effective signal output. The cross-linking of the matrix can be enhanced further by including adaptor proteins with domains that interact with modified phospholipids in the membrane (see Figure 15–11). MBoC7 n15.101/15.12 MBOC7_ptr_ch15_873-948.indd 884 08/12/21 12:23 PM  PRINCIPLES OF CELL SIGNALING 885 The Relationship Between Signal and Response Varies in Different Signaling Pathways !e function of an intracellular signaling system is to detect and measure a spe- ci#c stimulus in one location of a cell and then generate an appropriately timed and measured response, often at another location. !e system accomplishes this task by sending information in the form of molecular “signals” from the receptor to the #nal e"ector proteins, often through a series of intermediaries that do not simply pass the signal along but also process it along the way. Signaling systems work in various ways: each has evolved to produce a response that is appropriate for the cell function the system controls. In the following paragraphs, we list some basic signaling properties and how they vary in di"erent systems, as a foundation for more detailed discussions later. 1. Response timing varies dramatically in di"erent signaling systems, accord- ing to the speed required for the response. In some cases, such as synaptic signaling (see Figure 15–2C), the response can occur within milliseconds. In other cases, as in the control of cell fate by morphogens during develop- ment, a full response can require hours or days. 2. Sensitivity to extracellular signals can vary greatly. Hormones tend to act at very low concentrations on their distant target cells, which are therefore highly sensitive to low concentrations of signal. Neurotransmitters, on the other hand, operate at much higher concentrations at a synapse, reducing the need for high sensitivity in postsynaptic receptors. Sensitivity is often controlled by changes in the number or a%nity of the receptors on the target cell. A particularly important mechanism for increasing sensitivity is signal ampli"cation, whereby a small number of activated cell-surface receptors evokes a large intracellular response by either producing large amounts of a second messenger or by activating many copies of a down- stream signaling protein. 3. Dynamic range of a signaling system is related to its sensitivity. Some systems, like those involved in simple developmental decisions, are responsive over a narrow range of extracellular signal concentrations. Others, like those controlling vision or some metabolic responses to hormones, are highly responsive over a much broader range of signal plasma strengths. We will see that a broad dynamic range is often achieved by A membrane B adaptation mechanisms that adjust responsiveness according to the pre- vailing amount of signal. 4. Persistence of a response can vary greatly. A transient response of less than a second is appropriate in some synaptic responses, for example, while a pro- longed or even permanent response is required in cell-fate decisions during ATP ATP development. Numerous mechanisms, including positive and negative ADP ADP feedback, can be used to alter the duration and reversibility of a response. P Y P 5. Signal processing can convert a simple signal into a complex response. In many systems, for example, a gradual increase in an extracellular signal is converted into an abrupt, switchlike response. In other cases, a sim- ple input signal is converted into an oscillatory response, produced by a repeating series of transient intracellular signals. Feedback usually lies at DOWNSTREAM SIGNALS the heart of biochemical switches and oscillators, as we describe later. Figure 15–13 An example of signal 6. Integration allows a response to be governed by multiple inputs. As dis- integration. Extracellular signals A and cussed earlier, for example, speci#c combinations of extracellular signals B activate different intracellular signaling are generally required to stimulate complex cell behaviors such as cell pathways, each of which leads to the growth, proliferation, and di"erentiation (see Figure 15–4). !e cell there- phosphorylation of protein Y but at fore has to integrate information coming from multiple signals, which often different sites on the protein. Protein Y is activated only when both of these sites are depends on intracellular coincidence detectors; these proteins are equiva- MBoC7 m15.12/15.13 phosphorylated, and therefore it becomes lent to AND gates in the microprocessor of a computer, in that they are only active only when signals A and B are activated if they receive multiple converging signals (Figure 15–13). simultaneously present. MBOC7_ptr_ch15_873-948.indd 885 08/12/21 12:23 PM  886 Chapter 15: Cell Signaling 7. Coordination of multiple responses in one cell can be achieved by a sin- gle extracellular signal. Some extracellular signal molecules, for example, stimulate a cell to both grow and divide. !is coordination generally depends on mechanisms for distributing a signal to multiple e"ectors, by creating branches in the signaling pathway. In some cases, the branching of signaling pathways can allow one extracellular signal to modulate the strength of a response to other extracellular signals. Given the complexity that arises from behaviors like signal integration, coordi- nation, and feedback, it is clear that signaling systems rarely depend on a simple linear sequence of steps but more often operate like a signaling network, in which information $ows in multiple directions, including backwards. A major research challenge is to understand the nature of these networks and how they control complex cell behaviors. The Speed of a Response Depends on the Turnover of Signaling Molecules !e speed of any signaling response depends on the nature of the intracellular signaling molecules that carry out the target cell’s response. When the response requires only changes in proteins already present in the cell, it can occur very rapidly: an allosteric change in a neurotransmitter-gated ion channel (dis- cussed in Chapter 11), for example, can alter the plasma membrane electrical potential in milliseconds, and responses that depend solely on protein phos- phorylation can occur within seconds or minutes. When the response involves changes in gene expression and the synthesis of new proteins, however, it usu- ally requires many minutes or hours, regardless of the mode of signal delivery (Figure 15–14). When thinking about the speed of a response, it is natural to think of signaling systems in terms of the changes produced when the signal is delivered. But it can be just as important to consider what happens when the signal is withdrawn. In many signaling pathways, the response fades when the signal ceases. Often the e"ect is transient because the signal exerts its e"ects by increasing the concen- trations of intracellular molecules that are short-lived (unstable), undergoing continual turnover. !us, when the extracellular signal is removed, degrada- tion of the molecules quickly wipes out all traces of the signal’s action. !e same principle applies to signals that induce protein phosphorylation by activating a extracellular signal molecule Figure 15–14 Slow and rapid responses to an extracellular signal. Certain types of signal-induced cellular responses, such intracellular signaling cell-surface as increased cell growth and division, pathway receptor protein involve changes in gene expression and the synthesis of new proteins; they nucleus therefore occur slowly, often starting an hour or more after the signal is received. Other responses—such as changes in cell movement, secretion, or metabolism— need not involve changes in gene DNA transcription and therefore occur much ALTERED FAST PROTEIN RNA SLOW more quickly, often starting in seconds (less than a FUNCTION (minutes to or minutes; they may involve the rapid second to hours) phosphorylation of effector proteins in minutes) the cytoplasm, for example. Synaptic ALTERED PROTEIN SYNTHESIS responses mediated by changes in plasma membrane potential are even quicker and membrane can occur in milliseconds (not shown). ALTERED CYTOPLASMIC MACHINERY Some signaling systems generate both rapid and slow responses as shown here, allowing the cell to respond quickly to a signal while simultaneously initiating a more ALTERED CELL BEHAVIOR long-term, persistent response. MBOC7_ptr_ch15_873-948.indd 886 08/12/21 12:23 PM  PRINCIPLES OF CELL SIGNALING 887 10 10 1 min 200 min 40 min relative concentration of intracellular molecule 8 8 6 6 10 min 10 min 4 4 40 min 2 2 200 min 1 min 0 0 2 4 6 8 10 2 4 6 8 10 (A) minutes after the synthesis rate has (B) minutes after the synthesis rate has been decreased by a factor of 10 been increased by a factor of 10 Figure 15–15 The importance of rapid turnover. The graphs show the predicted relative rates of change in the intracellular concentrations of molecules with differing turnover times when their synthesis rates are either (A) decreased or (B) increased suddenly by a factor of 10. In both cases, the concentrations of those molecules that are normally degraded rapidly in the cell (red lines) change quickly, whereas the concentrations of those that are normally degraded slowly (green lines) change proportionally more slowly. The numbers (in blue) on the right are the half-lives assumed for each of the different molecules. MBoC7 m15.14/15.15 protein kinase: because most phosphorylation is continually removed by phos- phatases, the e"ects of the increased protein kinase activity are quickly reversed all-or-none when kinase activity declines. It follows that the speed with which a cell responds to removal of a signal depends on the rate of destruction, or turnover, of the mol- ecules or modi#cations that the signal a"ects. lic bo It is also true, although much less obvious, that this turnover rate can determine per hy response the promptness of the response when an extracellular signal arrives. Consider, for l ida example, two intracellular signaling molecules, X and Y, both of which are nor- mo mally maintained at a steady-state concentration of 1000 molecules per cell. !e sig cell synthesizes and degrades molecule Y at a rate of 100 molecules per second, with each molecule having an average lifetime of 10 seconds. Molecule X has a turnover rate that is 10 times slower than that of Y: it is both synthesized and concentration of signal molecule degraded at a rate of 10 molecules per second, so that each molecule has an aver- age lifetime in the cell of 100 seconds. If a signal acting on the cell causes a tenfold Figure 15–16 Signal processing can produce smoothly graded or switchlike increase in the synthesis rates of both X and Y with no change in the molecular responses. Some cell responses lifetimes, at the end of 1 second the concentration of Y will have increased by increase gradually MBoC7asm15.15/15.16 the concentration nearly 900 molecules per cell (10 100 – 100), while the concentration of X will of extracellular signal molecule increases, have increased by only 90 molecules per cell. In fact, after a molecule’s synthe- eventually reaching a plateau as the sis rate has been either increased or decreased abruptly, the time required for signaling pathway is saturated, resulting in a hyperbolic response curve (blue line). In the molecule to shift halfway from its old to its new equilibrium concentration is other cases, the signaling system reduces equal to its half-life; that is, equal to the time that would be required for its con- the response at low signal concentrations centration to fall by half if all synthesis were stopped (Figure 15–15). and then produces a steeper response at some intermediate signal concentration— resulting in a sigmoidal response curve Cells Can Respond Abruptly to a Gradually Increasing Signal (red line). In still other cases, the response is more abrupt and switchlike; the cell Some signaling systems are capable of generating a smoothly graded response switches completely between a low over a wide range of extracellular signal concentrations (Figure 15–16, blue line); and high response, without any stable such systems are useful, for example, in the #ne-tuning of metabolic processes intermediate response (green line). MBOC7_ptr_ch15_873-948.indd 887 08/12/21 12:23 PM  888 Chapter 15: Cell Signaling by some hormones. Other signaling systems generate signi#cant responses % of maximum activation of target protein 100 only when the signal concentration rises beyond some threshold value. !ese abrupt responses are of two types. One is a sigmoidal response, in which low 80 concentrations of stimulus do not have much e"ect, but then the response rises steeply and continuously at intermediate stimulus levels (Figure 15–16, red 60 line). Such systems provide a #lter to reduce inappropriate responses to low- level background signals but respond with high sensitivity when the stimulus 40 rises to physiological signal concentrations. A second type of abrupt response 1 is the discontinuous or all-or-none response, in which the response switches 2 20 8 on completely (and often irreversibly) when the signal reaches some thresh- 16 old concentration (Figure 15–16, green line). Such responses are particularly useful for controlling the choice between two alternative cell states, and they 0 0 0.5 1.0 1.5 2.0 generally involve positive feedback, as we describe in more detail shortly. relative concentration of Cells use a variety of molecular mechanisms to produce a sigmoidal effector molecule response to increasing signal concentrations. In one mechanism, more than one intracellular signaling molecule must bind to its downstream target pro- Figure 15–17 Activation curves for an allosteric protein as a function tein to induce a response. As we discuss later, for example, four molecules of effector molecule concentration. of the second messenger cyclic AMP must bind simultaneously to each mol- The curves show how the sharpness ecule of cyclic-AMP-dependent protein kinase (PKA) to activate the kinase. of the activation response increases A similar sharpening of response is seen when the activation of an intracel- with an increase in the number of effector molecules that must be bound lular signaling protei