Phosphorylation Protects Oncogenic RAS from LZTR1-Mediated Degradation.

Oncogenic KRAS and NRAS mutations are common in hematologic malignancies, but their signaling in this context remains less well characterized than in carcinomas. Using multi-omics screens in multiple myeloma, we sought to identify regulators of RAS activity. We found that the phosphatase PP1C dephosphorylated conserved RAS residue T148, permitting LZTR1-dependent proteasomal degradation. LZTR1 was ineffective against KRAS A146 gain-of-function mutations, which lie adjacent to T148 and are enriched in hematologic cancers, such as diffuse large B cell lymphoma and acute myeloid leukemia. Remarkably, KRAS protein stability was four-fold lower in hematologic versus carcinoma cells, revealing a unique therapeutic opportunity targeting RAS protein stability. PAK1 and PAK2 shielded RAS from LZTR1-dependent degradation by phosphorylating T148, and inhibiting PAK1/2 activity improved RAS-directed therapy. Collectively, these findings reveal a regulatory circuit governing RAS stability that is preferentially active in blood cancers and potentially druggable.