Phosphorylation Protects Oncogenic RAS from LZTR1-Mediated Degradation.

Oncogenic KRAS and NRAS mutations are common in hematologic malignancies, but how they signal is less well characterized than in carcinomas. To uncover novel RAS biology and potential therapeutic vulnerabilities, we employed a multi-omics screening approach in multiple myeloma to identify regulators of RAS activity. We report that the phosphatase PP1C dephosphorylates the conserved T148 residue on RAS, which in turn permits LZTR1-dependent proteasomal degradation. Notably, LZTR1 is ineffective against KRAS A146 gain-of-function mutations, which are adjacent to T148 and prevalent in hematologic cancers. Remarkably, we find that KRAS protein is four-fold less stable in hematologic versus carcinoma cells, offering a unique therapeutic opportunity targeting RAS protein stability mechanisms. The kinases PAK1 and PAK2 shield RAS from LZTR1-dependent degradation by phosphorylating T148, and targeting PAK1/2 activity improves RAS-directed therapy. Collectively, our findings reveal a novel regulatory circuit governing RAS stability that is preferentially active in blood cancers and potentially druggable.