Cell Communication and Signal Transduction

• Cell communication involves general features, receptor activation, cell surface receptors, intracellular receptors, and signal transduction
• Signal Transduction and Cellular Response can occur via an Enzyme-Linked Receptor or a G-Protein-Coupled Receptor.

Why Cells Need Signals

• Cells require signals to respond to changing environments
• Cells also need signals to communicate with each other.
• Responding to the environment can be critical for survival, such as yeast cells synthesizing more glucose transporters and enzymes when glucose is present.

Types of Cell Signaling

• Direct intercellular signaling involves signals passing through an intercellular channel from the cytosol of one cell to adjacent cells.
• Contact-dependent signaling involves membrane-bound signals binding to receptors on adjacent cells.
• Autocrine signaling involves cells releasing signals that affect themselves and nearby target cells.
• Paracrine signaling involves cells releasing signals that affect nearby target cells.
• Endocrine signaling involves cells releasing signals that travel long distances to affect target cells.

Common Stages of Cell Signaling

• Receptor activation involves a signaling molecule binding to a receptor, causing a conformational change that activates the receptor's function
• Signal transduction involves the activated receptor stimulating a series of proteins, forming a signal transduction pathway.
• Cellular response involves the signal transduction pathway affecting the functions and/or amounts of cellular proteins, leading to a cellular response.
• Phosphorylation/dephosphorylation control protein functions

Receptor Activation

• A ligand, or signal molecule, binds to a receptor with specificity.
• Ligand binding alters the receptor's structure.
• Once the ligand is released, the inactive receptor reverts.

Receptors Defined by Location

• Intracellular receptors lack an extracellular binding site for ligands.
• Examples of ligands of intracellular receptors - steroid hormones and other small lipid-soluble ligands.
• Cell Surface Receptors include Ligand-Gated Ion Channels, Enzyme-Linked Receptors and G-Protein-Coupled Receptors (GPCR)

Cell Surface Receptors

• Ligand gated ion channels open when ligands binding, and permit ions to flow through the membrane
• Enzyme-linked receptors have an extracellular domain that binds the signal and an intracellular domain with a catalytic function, often a kinase that promotes a reaction
• When a signaling molecule binds an enzyme linked receptor, its intracellular catalytic domain becomes active

Signal Transduction and Cellular Response via an-Enzyme Linked Receptor

• Receptor tyrosine kinases (RTKs) make up a category of enzyme-linked receptors found in animals.
• RTKs recognize diverse signaling molecules, influencing the internal insulin response protein.
• The RTK insulin receptor activates by insulin, which allows it to activate these insulin response proteins.
• Protein kinase cascades amplify signals, where this RTK receptor can be linked to the kinase cascade via an activator protein (RAS)

Scaffolding Proteins

• These proteins organize kinase cascades
• Kinases need to act in a specific sequence.
• the cell response is elicited by the protein's specific actions

G-Protein-Coupled Receptors (GPCR)

• Activated receptors bind a G protein.
• G protein releases GDP and binds GTP instead.
• GTP then causes the G protein to dissociate.
• The α subunit and β/γ dimer interact with other proteins in a signaling pathway.

Signal Transduction and Cellular Response via a G-Protein-Coupled Receptor

• Signals binding to the cell surface are considered "first messengers."
• Many signal transduction pathways lead to the production of second messengers.
• Second messengers relay signals inside the cell.
• A common second messenger is cyclic AMP (cAMP).

Epinephrine

• Epinephrine (adrenaline) is a hormone produced in stressful situations
• Epinephrine activates a GPCR
• This causes a G protein to bind GTP and promotes α subunit dissociation from β/γ dimer, which activated adenylyl cyclase
• Adenylyl cyclase promotes cAMP synthesis from ATP
• cAMP binds to the regulatory subunits creating PKA, which releases the catalytic subunits which phosphorylate specific cellular proteins which elicit a cellular response

Cell communication involves general features, receptor activation, cell surface receptors, intracellular receptors, and signal transduction. Signal Transduction and Cellular Response can occur via an Enzyme-Linked Receptor or a G-Protein-Coupled Receptor.