Heterotrimeric G protein

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This heterotrimeric G protein is illustrated with its theoretical lipid anchors. GDP is black. Alpha chain is yellow. Beta and gamma chains are blue.

3D structure of a heterotrimeric G protein

An heterotrimeric G protein. GDP is in purple. Alpha chain in orange. Beta chain in blue. Gamma chain in green. An important loop for signal transduction is shown in red (PDB code=1gg2) (more details...)

"G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes referred to as the "large" G proteins. These proteins are activated by G protein-coupled receptors and are made up of alpha (α), beta (β) and gamma (γ) subunits.

[edit] Alpha subunits

G_α subunits consist of two domains, the GTPase domain, and the alpha-helical domain. There exist at least 20 different G_α subunits, which are separated into several main families:

G_αs or simply G_s (stimulatory) - activates adenylate cyclase to increase cAMP synthesis
G_αi or simply G_i (inhibitory) - inhibits adenylate cyclase
G_olf (olfactory) - couples to olfactory receptors
G_t (transducin) - transduces visual signals in conjunction with rhodopsin in the retina
G_q - stimulates phospholipase C
The G_12/13 family - important for regulating (via guanine nucleotide exchange factors) the cytoskeleton, cell junctions, and other processes related to movements

[edit] Beta-gamma complex

The β and γ subunits are closely bound to one another and are referred to as the beta-gamma complex. The G_βγ complex is released from the G_α subunit after its GDP-GTP exchange.

The free G_βγ complex can act as a signaling molecule itself, by activating other second messengers or by gating ion channels directly.

For example, the G_βγ complex, when bound to histamine receptors, can activate phospholipase A₂. G_βγ complexes bound to muscarinic acetylcholine receptors, on the other hand, directly open G-protein coupled inward rectifying potassium channels (GIRKs).