MEK1/2 degraders uncover kinase-independent role of MEK1/2 in CRAF stabilization and maturation

The mitogen-activated protein kinase (MAPK) cascade is an evolutionarily conserved signaling pathway comprised of RAS GTPases, and downstream effector kinases (RAF, MEK, and ERK) that promote cell growth and survival. MEK1 and MEK2 (MEK1/2) are ubiquitous members of the dual-specificity MAP2K kinase family that phosphorylate and activate ERK1/2, controlling cellular proliferation. MEK1/2 paralogs share highly similar kinase domains, including their activation loops, which are phosphorylated by the upstream RAF family of kinases ARAF, BRAF, and CRAF. The activation of MEK1/2 by RAF is thoroughly described; RAF directly interacts with and phosphorylates MEK1/2, resulting in MEK1/2 kinase activation and subsequent phosphorylation of ERK1/2. In contrast, how the upstream RAF kinase, particularly CRAF, is activated is highly complex, involving an orchestration of chaperones, phosphorylation, and RAF dimerization, ultimately resulting in the phosphorylation of MEK1/2. The recognized CRAF signaling cascade has MEK1/2 functioning solely as a substrate of CRAF, binding to CRAF after RAS binding and RAF dimerization. However, there is evidence that MEK1 can directly regulate CRAF activity independent of RAS, suggesting that MEK1 has additional functions in the CRAF cycle. Here, using MEK1/2 proteolysis targeting chimeras (PROTACs), we discovered that MEK1/2 have kinase-independent functions in the regulation of CRAF protein stabilization, maturation, and kinase activation. Our findings reshape the current view of the CRAF activation cycle, establishing MEK1/2 binding to CRAF as an early required stabilizing event preceding CRAF-RAS engagement. Thus, MEK1/2 functions as both an activator and substrate of CRAF.