- Small GTPase
Small GTPases are a family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate (GTP). They are a form of G-proteins found in the cytosol which are homologous to the alpha subunit of heterotrimeric G-proteins, but unlike the alpha subunit of G proteins, a small GTPase can function independently as a hydrolase enzyme to bind to and hydrolyze a guanosine triphosphate (GTP) to form guanosine diphosphate (GDP). The most well-known members are the Ras GTPases and hence they are sometimes called Ras superfamily GTPases.
A typical G-protein is active when bound to GTP and inactive when bound to GDP (i.e. when the GTP is hydrolyzed to GDP). The GDP can be then replaced by free GTP. Therefore, a G-protein can be switched on and off. GTP hydrolysis is accelerated by GTPase activating proteins (GAPs), while GTP exchange is catalyzed by Guanine nucleotide exchange factors (GEFs). Activation of a GEF typically activates its cognate G-protein, while activation of a GAP results in inactivation of the cognate G-protein. Guanosine nucleotide dissociation inhibitors (GDI) maintain small GTP-ases in the inactive state.
The Ras superfamily
There are more than a hundred proteins in the Ras superfamily. Based on structure, sequence and function, the Ras superfamily is divided into eight main families, each of which is further divided into subfamilies: Ras, Rho, Rab, Rap, Arf, Ran, Rheb, Rad and Rit. Miro is a recent contributor to the superfamily.
Each subfamily shares the common core G domain, which provides essential GTPase and nucleotide exchange activity.
The surrounding sequence helps determine the functional specificity of the small GTPase, for example the 'Insert Loop', common to the Rho subfamily, specifically contributes to binding to effector proteins such as IQGAP and WASP.
The Ras family is generally responsible for cell proliferation, Rho for cell morphology, Ran for nuclear transport and Rab and Arf for vesicle transport.
- ^ Wennerberg K, Rossman KL, Der CJ (March 2005). "The Ras superfamily at a glance". J. Cell. Sci. 118 (Pt 5): 843–6. doi:10.1242/jcs.01660. PMID 15731001.
- ^ Munemitsu S, Innis M, Clark R, McCormick F, Ullrich A, Polakis P. (1990). "Molecular cloning and expression of a G25K cDNA, the human homolog of the yeast cell cycle gene CDC42". Mol Cell Biol 10 (11): 5977–82. ISSN 0270-7306. PMC 361395. PMID 2122236. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=361395.
Hydrolases: acid anhydride hydrolases (EC 3.6) 3.6.1 3.6.2 3.6.3-4: ATPase3.6.3Cu++ (188.8.131.52)Ca+ (184.108.40.206)Na+/K+ (220.127.116.11)H+/K+ (18.104.22.168)ATP4AOther P-type ATPase3.6.4 3.6.5: GTPase22.214.171.124: Heterotrimeric G protein126.96.36.199: Small GTPase > Ras superfamily188.8.131.52: Protein-synthesizing GTPase184.108.40.206-6: Polymerization motors Synaptic vesicleOther COPI COPII RME/ClathrinCLTA · CLTB · CLTC Caveolae Other/ungroupedVesicle formationRetromer · TIP47Small GTPaseOther MAPsee MAP kinase pathway Calcium G proteinAMP-activated protein kinaseMonomeric Cyclin LipidPhosphoinositide phospholipase C • Phospholipase c gamma Other protein kinase
Serine/threonine: Casein kinase (1, 2) • eIF-2 kinase (EIF2AK3) • Glycogen synthase kinase (GSK1, GSK2, GSK-3, GSK3A, GSK3B) • IκB kinase (CHUK, IKK2, IKBKG) • Interleukin-1 receptor-associated kinase (IRAK1, IRAK2, IRAK3, IRAK4) • Lim kinase (LIMK1, LIMK2) • p21 activated kinases (PAK1, PAK2, PAK3, PAK4) • Rho-associated protein kinase (ROCK1, ROCK2) • Ribosomal s6 kinase (RPS6KA1)Dual-specificity kinase
Other phosphoprotein phosphatase Apoptosissee apoptosis signaling pathway GTP-binding protein regulators Other
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