Heterotrimeric G protein
| Heterotrimeric G-protein GTPase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| EC number | 3.6.5.1 | ||||||||
| CAS number | Template:CAS | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
| Gene Ontology | AmiGO / EGO | ||||||||
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"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
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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-family | |||||
| 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-family | |||||
| 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-family | |||||
| 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-family | |||||
| G12/13 | α12, α13 | GNA12, GNA13 | Activation of the Rho family of GTPases | Cytoskeletal functions, Smooth muscle contraction | |
Beta-gamma complex
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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.
Function
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.
References
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External links
- Heterotrimeric G-Proteins at the US National Library of Medicine Medical Subject Headings (MeSH)
- EC 3.6.5.1
