Structure and Function of the SHOC2 Holophosphatase Complex in RAS-driven Cancer

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Project Summary/Abstract: RAS proteins are found to be frequently mutated in clinical cases of cancer. Effective targeted therapies against RAS and its downstream pathways have been highly sought after. Among the RAS effectors, the RAF/mitogen-activated protein kinase (MAPK) cascade has been shown to be important for propagating oncogenic signaling downstream of RAS. Many promising targeted therapies have been developed that directly inhibit mutant KRAS and MAPK members but have been limited due to resistance mechanisms. SHOC2 has been recently found to be a potent synthetic lethal vulnerability in various RAS mutant contexts, including RAS-driven leukemia/melanoma and facilitates cancer resistance to numerous RAS/MAPK inhibitors. SHOC2 is a scaffold that binds RAS and PP1C to form a ternary holophosphatase complex which dephosphorylates inhibitory ‘S259’ on RAF proteins and enables MAPK signaling. To actualize SHOC2 as a therapeutic target and inform future efforts to rationally design inhibitors against the complex, a deep understanding of SHOC2 structural features that govern complex assembly and function is needed. We have now resolved the SHOC2 complex structure and have profiled the functional consequence of nearly all possible amino acid substitutions at every position of SHOC2 through deep mutational scanning. These findings highlight novel structural features of SHOC2 that may be important to therapeutically target in RAS mutant cancers. The studies outlined in this proposal will build on this work and aim to (1) define and validate the structural features of SHOC2 holophosphatase complex specificity toward RAF substrate, and (2) identify novel post-translational modifications that regulate SHOC2 function. The training I receive in the K99 phase will provide me the necessary expertise in structural biology, proteomics, and ubiquitin biology as well as equip me with teaching/lab management experience so that I can effectively lead my own independent research program. The K99/R00 will be instrumental in helping me achieve my long-term goal, to establish my laboratory focused on studying novel RAS-associated vulnerabilities and drug resistance mechanisms via systematic approaches in cell signaling and protein structure-function.