The cytoplasmic protein GAP is implicated as the target for regulation by the ras gene product.

About 30% of human tumours contain a mutation in one of the three ras genes leading to the production of p21ras oncoproteins that are thought to make a major contribution to the transformed phenotype of the tumour. The biochemical mode of action of the ras proteins is unknown but as they bind GTP and GDP and have an intrinsic GTPase activity, they may function like regulatory G proteins and control cell proliferation by regulating signal transduction pathways at the plasma membrane. It is assumed that an external signal is detected by a membrane molecule (or detector) that stimulates the conversion of p21.GDP to p21.GTP which then interacts with a target molecule (or effector) to generate an internal signal. Recently a cytoplasmic protein, GAP, has been identified that interacts with the ras proteins, dramatically increasing the GTPase activity of normal p21 but not of the oncoproteins. We report here that GAP appears to interact with p21ras at a site previously identified as the 'effector' site, strongly implicating GAP as the biological target for regulation by p21.