|MoBio||Ras and Other Small G Proteins||Chapter 6|
Small G proteins are monomeric G proteins with molecular weight of 20-40 kDa. Like heterotrimeric G proteins, their activity depends on the binding of GTP. More than 100 small G proteins have been identified. They are classified into five families: Ras, Rho, Rab, Ran and Arf (Table).
Ras and cancer
Mutations of the Ras proto-oncogenes (H-Ras, N-Ras, K-Ras) are found in about 25% of all human tumors. Most of these mutations result in the abrogation of the normal GTPase activity of Ras. The Ras mutants can still bind to GAP, but cannot catalyze GTP hydrolysis. As a result, they remain in the active state for a much longer period. Malignant transformation may arise from the unregulated stimulation of Ras signaling pathways, which either stimulate cell growth or inhibit apoptosis.