The Ras-specific nucleotide exchange factor Son of sevenless (Sos) is inactive without Ras bound to a distal allosteric site. In contrast, the catalytic domain of Ras guanine nucleotide releasing factor 1 (RasGRF1) is active intrinsically. By substituting residues from RasGRF1 into Sos, we have generated mutants of Sos with basal activity, partially relieved of their dependence on allosteric activation. We have performed molecular dynamics simulations showing how Ras binding to the allosteric site leads to a bias toward the active conformation of Sos. The trajectories show that Sos fluctuates between active and inactive conformations in the absence of Ras and that the activating mutations favor conformations of Sos that are more permissive to Ras binding at the catalytic site. In contrast, unliganded RasGRF1 fluctuates primarily among active conformations. Our results support the premise that the catalytic domain of Sos has evolved an allosteric activation mechanism that extends beyond the simple process of membrane recruitment.
Bibliographical noteFunding Information:
We thank Doug Lowy for RasGRF1 cDNA, Jodi Gureasko, Nick Levinson, and Xuewu Zhang for interesting discussions, and David King for mass spectrometry. We thank Susan Marqusee and Dafna Bar-Sagi for guidance and discussions. H.S. was supported by the Leukemia and Lymphoma Society. G.D.F. is supported by the NSFGRFP, and T.K. by the Sloan Foundation. J.K. is supported by the NCI (R01 CA096504-02).