Gproteiineilla

G-proteins are a family of guanine nucleotide–binding proteins that act as molecular switches to regulate intracellular signaling in response to extracellular stimuli. They participate in a wide range of cellular processes, including sensory perception, growth, metabolism, and membrane transport. The term G-proteins commonly refers to two major classes: heterotrimeric G proteins that couple to G-protein-coupled receptors (GPCRs) and the diverse family of small GTPases that control vesicle trafficking, cytoskeletal organization, and gene expression.

Heterotrimeric G proteins consist of three subunits: Gα, Gβ, and Gγ. In the resting state, the Gα subunit holds GDP and associates with Gβγ. Upon GPCR activation, the receptor acts as a guanine nucleotide exchange factor, prompting Gα to release GDP and bind GTP. Gα-GTP and Gβγ then dissociate and regulate multiple effectors. Gα subunits are classified into families such as Gs (activates adenylyl cyclase), Gi/o (inhibits adenylyl cyclase), Gq/11 (activates phospholipase Cβ), and G12/13 (activates Rho family signaling). The Gβγ dimer also modulates effectors, including ion channels and certain enzymes. Termination occurs through GTP hydrolysis by Gα, aided by intrinsic activity and regulators of G protein signaling (RGS proteins).

Small GTPases, including the Ras, Rho, Rab, Arf, and Ran families, act as monomeric switches. They cycle between GDP-bound inactive and GTP-bound active states, controlled by guanine nucleotide exchange factors, GTPase-activating proteins, and guanine nucleotide dissociation inhibitors. These proteins orchestrate growth, cytoskeletal dynamics, membrane trafficking, and nuclear transport.

Disruptions in G-protein signaling are implicated in various diseases and are targets for pharmacological intervention, making G-proteins central to cell biology and medicine.