Abstract
The conformational states of canonical Ras (H-, K-, and N-Ras) GTPases define their nucleotide-exchange and effector binding capabilities. M-Ras, whose mutational variants can cause cancer directly, and indirectly through their complexes, appear to be one exception. Unlike canonical Ras, the GTP-bound M-Ras is mostly in the inactive state. The active state is stabilized in the holophosphatase ternary complex, which includes SHOC2 scaffolding protein and protein phosphatase-1 (PP1). PP1 dephosphorylates Raf's inhibitory site, promoting Raf activation and MAPK signaling and contributing to Noonan syndrome phenotype. Activating Q71R variant (M-RasQ71R) exhibits higher affinity than the wild type to holophosphatase. With allosteric drug discovery benefitting from insight into allosteric mechanisms, which are unsurprisingly distinct between M-Ras and canonical Ras, we explored M-Ras and M-RasQ71R conformational dynamics in GTP- and GDP-bound states by all-atom molecular dynamics simulations. We show that M-Ras and M-RasQ71R exhibit differential GTP/GDP loading. GTP-bound M-Ras and M-RasQ71R display distinct conformational dynamics in their switch regions although both preferentially assume the inactive conformations. The conserved nucleotide-coordinating asparagine residue in M-Ras G4-loop is the weakest link in the nucleotide-binding coordination, offering mechanistic insights into the GDP release mediated by the guanine nucleotide exchange factor of canonical Ras. Pharmacologically, the occlusion of the Switch II pocket due to the highly mobile Switch II region indicates that it may be infeasible to target this pocket. Targeting M-RasQ71R binding interface with optimized K-Ras inhibitor and cyclophilin A appears an alternative approach. Collectively, Ras allosteric mechanistic scenarios shape their personalities, function, and likely drug discovery.