The KSR1 scaffold translocates from the cytosol to the plasma membrane upon Ras activation and coordinates the assembly of a large multiprotein complex that functions to colocalize MEK with its upstream activator Raf and downstream target ERK, thereby promoting signal transmission between the core kinase components of the ERK cascade. To isolate novel regulators that may impact KSR1 function and/or ERK cascade signaling, we used mass spectrometry to further characterize the proteins associating with KSR1 in these high molecular weight complexes. Using this approach, we have identified the heterotetrameric protein kinase, casein kinase 2 (CK2) as a new KSR1-binding partner. Moreover, we find that the KSR1/CK2 interaction is required for KSR1 to maximally facilitate ERK cascade signaling and contributes to the regulation of Raf kinase activity. During this fiscal year, we also discovered that KSR1 undergoes caspase-dependent cleavage in apoptotic cells and that cleavage destroys the scaffolding function of the KSR1 and generates a stable C-terminal fragment (CTF) that can inhibit ERK activation. These findings indicate that cleavage of the KSR1 scaffold is another mechanism whereby caspases down-regulate ERK survival signaling to promote cellular apoptosis.
