[Isoprenylated proteins and cell proliferation: regulators and effectors of Ras proteins].

Ras proteins play a central role in the control of cellular proliferation. They are 189 amino acid monomeric GTP-binding proteins that cycle between an inactive GDP-bound and the active GTP-bound state, and carry a slow intrinsic GTPase activity. Ras proteins are activated by growth promoting signals incoming from receptor tyrosine kinases via SH2 domain and SH3 domain containing adapter proteins and the Ras exchange factor Sos, as well as from serpentine receptors via the beta gamma subunits of heterotrimeric G proteins and the Ras exchange factor Ras-GRF (or Cdc25). Proteins that can stimulate the GTPase activity of Ras (GAPs) ensure that following mitogenic stimulations, they return to their inactive GDP-bound state; amongst these proteins are p120-GAP, neurofibomin (the product of the susceptibility gene to type I neurofibromatosis), as well as the inositol 1,3,4,5-tetrakisphosphate-dependent GAPIP4BF. Several effectors have been identified that mediate the biological effects of Ras. The serine/threonine kinase Raf-1, as well as the closely related protein B-Raf, elicit the ERK cascade of MAP kinases. Phosphatidylinositol-3-OH kinase is involved in the activation of the Rac/Rho family proteins that play a role in the control of actin polymerisation, as well as in growth control, RalGDS, RGL and Rlf, are responsible for the activation of the Ras-related protein Ral. Recent evidence, using effector domain mutants of Ras, demonstrates that these pathways cooperate to elicit the growth promoting effects of Ras proteins.