IDENTIFYING REGULATORS OF KSRI USING A PROTEOMICS APPROACH: A FUNCTIONAL ANALYSIS OF THE KSR1/CK2 INTERACTION

Abstract

KSR1 accelerates growth factor-mediated signaling by acting as a molecular scaffold for the Ras pathway. In cells, KSR1 exists in a high molecular weight protein complex, some components of which have yet to be identified. Using a proteomics approach, CK2 was determined to be a novel KSR1-interacting protein. CK2 constitutively binds to the KSR1 atypical C1 domain and this association is dependent upon the KSR1 basic residues K360 and R363. CK2 phosphorylates KSR1 on S518 in vitro and contributes to S518 phosphorylation in vivo; however, the biological consequence of this phosphorylation event is unknown. In response to Ras activation, the interaction with KSR1 functions to localize CK2 to the plasma membrane, where CK2 can phosphorylate Raf-1 on the activating S338 site in a manner dependent on Raf-1. Y341 phosphorylation. Thus, these findings define a role for KSR1 as an upstream regulator of Raf-1 in the Raf-1 activation process.