Regulation of ras p21 by GTPase activating proteins.

We propose a model for dual effector functions of the known ras GAPs p120-GAP and NF1-GAP. This model is consistent with known biological and biochemical effects of GAPs in mammalian cells, but it is clearly not a proven hypothesis, and several difficulties remain in making this model convincing. One is the apparent difference between mammalian cells and yeasts, in which GAPs do not have a demonstrable effector function. The other is the difficulty of eliminating the possibility that other effectors exist that do not have GAP activity and do not bind ras p21 sufficiently tightly to allow detection through physical association. We hope that further analysis of GAP function will clarify the roles of these proteins, allowing at least a partial description of ras action in normal and malignant mammalian cells.