1. Factors that regulate the activity of GTP-binding proteins of the ras superfamily. In the active GTP-containing state (represented by a triangle) the protein binds to the effector that mediates its physiological action. Binding to the effector markedly increases the latent GTPase activity of the GTP-binding protein which, on hydrolysis of GTP, switches its conformation to the GDP-containing inactive state (represented by an oval). In this case, therefore, the effector also functions as a GAP (a GTPase-activating protein) that terminates the physiological action of the GTP-binding protein, but a separate GAP protein may also carry out this function. Conversion of the inactive GTP-binding protein into the active form involves a guanine nucleotide exchange factor (GEF; rectangle) that stimulates the release of GDP, which leads to its replacement by GTP. The GEF may be located in a specific membrane or subcellular structure and its activation may require a signal from an upstream regulator, such as a heterotrimeric G protein, or a protein kinase. A guanine nucleotide dissociation inhibitor (GDI; trapezoid) may also be an important regulator of the activity of a GTP-binding protein by binding to the GDP-containing form and preventing its recharging with GTP. A GDI has been shown to be capable of removing the GDP-containing (inactive) form of a GTP-binding protein from membranes and to form a complex with it. Therefore, it is possible that for rab proteins involved in vesicular transport, a GDI may play a role in allowing the cyclic function of the GTP-binding protein by transporting it from the acceptor to the donor membrane.
Source: The Biogenesis of Membranes and Organelles, The Online Metabolic and Molecular Bases of Inherited Disease
Citation: Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson K, Mitchell G. The Online Metabolic and Molecular Bases of Inherited Disease; 2014 Available at: Accessed: October 30, 2017
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