11.2 Membrane receptors and signal transduction

Membrane receptors are cellular gatekeepers, translating external signals into internal responses. They come in various types, each with unique structures and functions, allowing cells to respond to a diverse array of stimuli.

Signal transduction is the cellular communication highway, converting external cues into internal actions. This process involves a series of molecular events, including protein modifications like phosphorylation, which amplify and regulate the signal's journey through the cell.

Types and Functions of Membrane Receptors

Types of membrane receptors

• G protein-coupled receptors (GPCRs)
  • Largest family of membrane receptors includes rhodopsin and $\beta$-adrenergic receptors
  • Characterized by seven transmembrane domains that span the plasma membrane
  • Coupled to heterotrimeric G proteins on the intracellular side of the membrane
• Receptor tyrosine kinases (RTKs)
  • Single-pass transmembrane proteins that dimerize upon ligand binding
  • Possess intrinsic tyrosine kinase activity in their cytoplasmic domain which is activated by dimerization
  • Bind growth factors (epidermal growth factor), hormones (insulin), and cytokines (interferons)
• Ion channel receptors
  • Transmembrane proteins that form ion channels allowing ions to pass through the membrane
  • Can be ligand-gated (acetylcholine receptor) or voltage-gated (sodium channels)
  • Allow rapid ion flux across the membrane in response to specific stimuli like neurotransmitters or changes in membrane potential

Structure and function of receptors

• G protein-coupled receptors (GPCRs)
  • Structure: Seven transmembrane $\alpha$-helical domains, with an extracellular N-terminus that binds ligands and an intracellular C-terminus that interacts with G proteins
  • Function: Bind extracellular ligands (hormones, neurotransmitters, odorants) and activate intracellular G proteins, which then modulate the activity of effector proteins like enzymes (adenylyl cyclase) or ion channels (potassium channels)
• Receptor tyrosine kinases (RTKs)
  • Structure: Extracellular ligand-binding domain, single transmembrane domain, and cytoplasmic tyrosine kinase domain with multiple tyrosine residues
  • Function: Bind growth factors and other ligands, leading to receptor dimerization and autophosphorylation of tyrosine residues, which serve as docking sites for signaling proteins containing SH2 or PTB domains
• Ion channel receptors
  • Structure: Transmembrane proteins with a central pore that allows ion passage
    • Ligand-gated: Binding of a specific ligand (GABA, glycine) induces conformational changes that open the channel
    • Voltage-gated: Changes in membrane potential trigger channel opening or closing through the movement of voltage-sensing domains
  • Function: Rapidly change the membrane potential or intracellular ion concentrations in response to specific stimuli, enabling fast synaptic transmission or muscle contraction

Signal Transduction and Intracellular Responses

Process of signal transduction

• Signal transduction is the process by which cells convert extracellular signals (hormones, growth factors) into intracellular responses (changes in metabolism, gene expression)
• Involves a series of molecular events that relay the signal from the cell surface to the interior of the cell
• Key steps in signal transduction:
  1. Reception: Ligand binds to the extracellular domain of the receptor
  2. Transduction: Conformational changes in the receptor lead to activation of intracellular signaling molecules (G proteins, kinases)
  3. Amplification: Signaling cascades amplify the initial signal through sequential activation of enzymes (kinases, GTPases)
  4. Response: Activation of effector proteins (transcription factors, metabolic enzymes) leads to changes in cellular behavior or gene expression
• Allows cells to respond to their environment and communicate with each other

Protein modifications in signaling

• Protein modifications, particularly phosphorylation, play a crucial role in signal transduction by regulating protein activity and interactions
• Phosphorylation is the addition of a phosphate group to serine, threonine, or tyrosine residues catalyzed by protein kinases
• Kinases (receptor tyrosine kinases, MAP kinases) catalyze phosphorylation, while phosphatases (protein tyrosine phosphatases) remove phosphate groups
• Phosphorylation can:
  • Alter protein conformation and activity by inducing structural changes
  • Create binding sites for other signaling proteins containing SH2 or PTB domains
  • Regulate protein localization (nuclear translocation) and interactions (protein complexes)
• Signaling cascades often involve a series of phosphorylation events, allowing for signal amplification and integration of multiple signals
• Dephosphorylation by phosphatases helps terminate signaling and maintains cellular homeostasis by counteracting kinase activity