Heterotrimeric G protein

Heterotrimeric G-protein GTPase
EC number
CAS number 9059-32-9
IntEnz IntEnz view
ExPASy NiceZyme view
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum
Gene Ontology AmiGO / EGO
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

"G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes referred to as the "large" G proteins (as opposed to the subclass of smaller, monomeric small GTPases) . These proteins are activated by G protein-coupled receptors and are made up of alpha (α), beta (β) and gamma (γ) subunits,[1] the latter two referred to as the beta-gamma complex.

There are four main families of G proteins: Gi/Go, Gq, Gs, and G12.[2]

Alpha subunits

Main article: G alpha subunit

Reconstitution experiments carried out in the early 1980s showed that purified Gα subunits can directly activate effector enzymes. The GTP form of the α subunit of transducin (Gt) activates the cyclic GMP phosphodiesterase from retinal rod outer segments,[3] and the GTP form of the α subunit of the stimulatory G protein (Gs) activates hormone-sensitive adenylate cyclase.[4][5]

Gα subunits consist of two domains, the GTPase domain, and the alpha-helical domain.

There exist at least 20 different Gα subunits, which are separated into four main families. This nomenclature is based on their sequence homologies:[6]

G-protein-family α-subunit Gene Signal transduction Use/Receptors (examples) Effects (examples)
Gi/o αi, αo GNAO1, GNAI1, GNAI2, GNAI3 Inhibition of adenylate cyclase, opens K+-channels (via β/γ subunits), closes Ca2+-channels Muscarinic M2 and M4,[7] chemokine receptors, α2-Adrenoreceptors, Serotonin 5-HT1 receptors, Histamine H3 and H4, Dopamine D2-like receptors Smooth muscle contraction, depress neuronal activity
Gt αt (Transducin) GNAT1, GNAT2 Activation of phosphodiesterase 6 Rhodopsin Vision
Ggust αgust (Gustducin) GNAT3 Activation of phosphodiesterase 6 Taste receptors Taste
Gz αz GNAZ Inhibition of adenylate cyclase ? Maintaining the ionic balance of perilymphatic and endolymphatic cochlear fluids.
Gs αs GNAS Activation of adenylate cyclase Beta-adrenoreceptors; Serotonin 5-HT4, 5-HT6 and 5-HT7; Dopamine D1-like receptors, Histamine H2 Increase heart rate, Smooth muscle relaxation, stimulate neuronal activity
Golf αolf GNAL Activation of adenylate cyclase olfactory receptors Smell
Gq αq, α11, α14, α15, α16 GNAQ, GNA11, GNA14, GNA15 Activation of phospholipase C α1-Adrenoreceptors, Muscarinic M1, M3, and M5,[7] Histamine H1, Serotonin 5-HT2 receptors Smooth muscle contraction, Ca2+ flux
G12/13 α12, α13 GNA12, GNA13 Activation of the Rho family of GTPases Cytoskeletal functions, Smooth muscle contraction

Beta-gamma complex

Main article: Beta-gamma complex

The β and γ subunits are closely bound to one another and are referred to as the beta-gamma complex. Upon activation of the GPCR, the Gβγ complex is released from the Gα subunit after its GDP-GTP exchange.


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

For example, the Gβγ complex, when bound to histamine receptors, can activate phospholipase A2. Gβγ complexes bound to muscarinic acetylcholine receptors, on the other hand, directly open G protein-coupled inward rectifying potassium channels (GIRKs). They can also activate L-type calcium channels, as in H3 receptor pharmacology.


  1. Hurowitz EH, Melnyk JM, Chen YJ, Kouros-Mehr H, Simon MI, Shizuya H; Melnyk; Chen; Kouros-Mehr; Simon; Shizuya (2000). "Genomic characterization of the human heterotrimeric G protein alpha, beta, and gamma subunit genes". DNA Res. 7 (2): 111–20. doi:10.1093/dnares/7.2.111. PMID 10819326.
  2. Ellis, Claire (Jul 2004). "The state of GPCR research in 2004". Nature Reviews Drug Discovery (3 ed.). 3 (7): 577–626. doi:10.1038/nrd1458.
  3. Fung, BKK; Hurley, JB; Stryer, L (1981). "Flow of information in the light-triggered cyclic nucleotide cascade of vision". Proc. Natl. Acad. Sci. USA. 78 (1): 152–156. doi:10.1073/pnas.78.1.152. PMC 319009Freely accessible. PMID 6264430.
  4. Cerione, RA; Sibley, DR; Codina, J; Benovic, JL; Winslow, J; Neer, EJ; Birnbaumer, L; Caron, MG; Lefkowitz, RJ; et al. (1984). "Reconstitution of a hormone-sensitive adenylate cyclase system. The pure beta-adrenergic receptor and guanine nucleotide regulatory protein confer hormone responsiveness on the resolved catalytic unit". J. Biol. Chem. 259 (16): 9979–9982. PMID 6088509.
  5. May, DC; Ross, EM; Gilman, AG; Smigel, MD (1985). "Reconstitution of catecholamine-stimulated adenylate cyclase activity using three purified proteins". J. Biol. Chem. 260 (29): 15829–15833. PMID 2999139.
  6. Strathmann MP, Simon MI; Simon (1991). "G alpha 12 and G alpha 13 subunits define a fourth class of G protein alpha subunits". Proc. Natl. Acad. Sci. U.S.A. 88 (13): 5582–6. doi:10.1073/pnas.88.13.5582. PMC 51921Freely accessible. PMID 1905812.
  7. 1 2 Kou Qin, Chunmin Dong, Guangyu Wu & Nevin A Lambert; Dong; Wu; Lambert (August 2011). "Inactive-state preassembly of Gq-coupled receptors and Gq heterotrimers". Nature Chemical Biology. 7 (11): 740–747. doi:10.1038/nchembio.642. PMC 3177959Freely accessible. PMID 21873996.
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