Ras activation cycle.

Trophic signals activate guanine exchange factors (GEFs) such as SOS and CDC25 to accelerate the rate of guanosine diphosphate (GDP) dissociation, stabilize the Ras protein in its nucleotide free state, and facilitate guanosine triphosphate (GTP) binding. GTPase activating proteins (GAP) accelerate GTP hydrolysis thereby inactivating Ras. Sustained Ras activation may occur in mutant Ras proteins in which GTPase response to GAP binding is inactivated, or by mutational inactivation of GAPs.