Ras, a membrane-bound GTPase, exists in inactive GDP-bound and active GTP-bound states. Sos, a Ras-specific guanine nucleotide exchange factor couples cell-surface receptors to Ras activation by mediating a signal-dependent nucleotide exchange on Ras. Once activated, Ras stimulates downstream effector pathways that control cell survival, proliferation and differentiation. In basal conditions, the activity of Sos is constrained by several well-defined intramolecular interactions that maintain the protein in an autoinhibited conformation. The main goal of the studies described in this dissertation is to identify mechanisms by which the autoinhibitory state is released. We have found that the Histone Fold (HF) domain of Sos associates with specific membrane lipids in response to growth factor stimulation. This association promotes the catalytic activity of Sos by forcing the domain to adopt a conformation that destabilizes the autoinhibitory state of Sos. The significance of this novel regulatory mechanism is underscored by the findings that gain-of-function Sos variants associated with Noonan syndrome display mutations in the lipid-binding pocket of Sos-HF and these mutations increase membrane association.

A second aspect of the work described in this thesis involves the design of an inhibitor of Ras-Sos interactions using secondary structure mimetics derived from high-resolution structures of Ras-Sos complexes. We demonstrate that a short artificial α-helix, which adopts a conformation that mimics a catalytically relevant fragment of Sos, can disrupt Ras-Sos interactions in vitro and in cell-based assays. These findings indicate the potential of this approach as a targeting strategy for inhibiting the activation of Ras by Sos.