A G protein is a GTP-binding protein that acts as a molecular switch, active when bound to GTP and inactive when it hydrolyzes GTP to GDP, relaying signals from receptors to downstream effectors.

How do receptor tyrosine kinases differ from G-protein-coupled receptors?

Receptor tyrosine kinases have intrinsic enzymatic activity and signal by dimerizing and phosphorylating tyrosines, whereas G-protein-coupled receptors signal indirectly by activating separate G proteins.

Signal Transduction

Signal transduction is the chemistry of converting a signal detected at a receptor into a chain of intracellular molecular events that produce a cellular response.

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Definition

Signal transduction is the process by which receptor activation is relayed through transducer and effector proteins—often via conformational changes, GTP hydrolysis, and protein phosphorylation—to generate an intracellular response.

Scope

This topic covers the architecture of signaling pathways, the major receptor classes including G-protein-coupled receptors and receptor tyrosine kinases, the role of G proteins as molecular switches, phosphorylation cascades, and the mechanisms that terminate and reset signaling.

Core questions

How do G-protein-coupled receptors transmit a signal across the membrane?
How do G proteins act as molecular switches?
How do receptor tyrosine kinases initiate signaling?
What terminates a signaling cascade?

Key theories

G proteins as molecular switches: Gilman and colleagues established that GTP-binding proteins toggle between active GTP-bound and inactive GDP-bound states, transducing signals from receptors to effectors and switching off through intrinsic GTP hydrolysis.

Mechanisms

Ligand binding alters a receptor's conformation; a G-protein-coupled receptor catalyzes GDP-to-GTP exchange on a G protein, whose activated subunit regulates effector enzymes until GTP is hydrolyzed back to GDP. Receptor tyrosine kinases instead dimerize and autophosphorylate, creating docking sites that assemble signaling complexes and launch phosphorylation cascades. Each step can amplify the signal, and GTP hydrolysis, phosphatases, and receptor desensitization return the system to its resting state.

Clinical relevance

Signal transduction illustrates molecular switching and recognition central to chemical biology and provides the conceptual basis for designing molecular probes. The treatment is mechanistic and non-prescriptive.

History

Rodbell proposed the receptor-transducer-effector logic; Gilman identified and characterized the G proteins; and later work, including Lefkowitz's on receptor structure and regulation, detailed how these receptors are activated and desensitized.

Key figures

Alfred Gilman
Martin Rodbell
Robert Lefkowitz

Seminal works

gilman1987
nelson2021

Frequently asked questions

What is a G protein?: A G protein is a GTP-binding protein that acts as a molecular switch, active when bound to GTP and inactive when it hydrolyzes GTP to GDP, relaying signals from receptors to downstream effectors.
How do receptor tyrosine kinases differ from G-protein-coupled receptors?: Receptor tyrosine kinases have intrinsic enzymatic activity and signal by dimerizing and phosphorylating tyrosines, whereas G-protein-coupled receptors signal indirectly by activating separate G proteins.