Structural requirements for the interaction of p21ras with GAP, exchange factors, and its biological effector target.

From the multitude of mutations that have been tested in p21ras, a common theme emerges regarding its interaction with the effector target, GAPs, and the newly discovered exchange factors. Many of the mutations that result in dysfunction for all three types of interactions are localized to the switch 1 and switch 2 regions of the p21ras three-dimensional structure (Fig. 2). These two regions change conformation on GTP binding by p21ras and, accordingly, both GAP binding and Ras biological activity are GTP-dependent processes. In addition, certain mutations in the switch 1 and 2 regions alter the affinity of GAP for p21ras, again implicating this region in the binding interaction. On the other hand, the SDC25 exchange factor appears to promote dissociation of both GTP and GDP from p21ras, suggesting that the overall conformation of the switch 1 and 2 regions may not be important for recognition by SDC25. Moreover, none of the switch 1 or 2 mutations that impaired stimulation by SDC25 affected its binding to p21ras. This suggests that these residues are essential for the mechanism of activation by SDC25 but not for its binding to p21ras. Amino acids at positions 73, 75, and possibly 102 and 103 appear also to be involved in the activation of p21ras by exchange factors. Whether these are required for binding to the exchanger has not been reported.