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.
is the cellular communication highway, converting external cues into internal actions. This process involves a series of molecular events, including protein modifications like , which amplify and regulate the signal's journey through the cell.
Types and Functions of Membrane Receptors
Types of membrane receptors
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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 α-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 (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 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 (hormones, growth factors) into intracellular responses (changes in metabolism, )
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:
Reception: Ligand binds to the extracellular domain of the receptor
Transduction: Conformational changes in the receptor lead to activation of intracellular signaling molecules (G proteins, kinases)
Amplification: Signaling cascades amplify the initial signal through sequential activation of enzymes (kinases, )
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