Guanine Nucleotide Exchange Factor - Function

Function

Guanine Nucleotide Exchange Factors (GEFs) are proteins involved in the activation of small GTPases. Small GTPases act as molecular switches in intracellular signaling pathways and have many downstream targets. The most well known GTPases comprise the Ras superfamily and are involved in essential cell processes such as cell differentiation and proliferation, cytoskeletal organization, vesicle trafficking, and nuclear transport. GTPases are active when bound to GTP and inactive when bound to GDP, allowing their activity to be regulated by GEFs and the opposing GTPase activating proteins (GAPs).

GDP disassociates from inactive GTPases very slowly. The binding of GEFs to their GTPase substrates catalyzes the dissociation of GDP, allowing a GTP molecule to bind in its place. GEFs function to promote the dissociation of GDP. After GDP has disassociated from the GTPase, GTP generally binds in its place, as the cytosolic ratio of GTP is much higher than GDP at 10:1. The binding of GTP to the GTPase results in the release of the GEF, which can then activate a new GTPase. Thus, GEFs both destabilize the GTPase interaction with GDP and stabilize the nucleotide free GTPase until a GTP molecule binds to it. GAPs act antagonistically to inactivate GTPases by increasing their intrinsic rate of GTP hydrolysis. GDP remains bound to the inactive GTPase until a GEF binds and stimulates its release.

The localization of GEFs can determine where in the cell a particular GTPase will be active. For example, the Ran GEF, RCC1, is present in the nucleus while the Ran GAP is present in the cytosol, modulating nuclear import and export of proteins. RCC1 converts RanGDP to RanGTP in the nucleus, activating Ran for the export of proteins. When the Ran GAP catalyzes conversion of RanGTP to RanGDP in the cytosol, the protein cargo is released.