In quiescent cells, KSR1 is phosphorylated on Ser297 by an unknown kinase and on Ser392 by C-TAK1, creating docking sites for 14-3-3. The 14-3-3–KSR1 interaction sequesters KSR1 and MEK1, to which it is constitutively bound, in the
CA 074 [9]. Both KSR1 and its target, c-Raf-1, are also constitutively bound to the PP2A core enzyme (subunits A + C) [10]. Impedes mitogenic signal propagation (IMP) also interacts with the KSR1 N-terminus, maintaining KSR1 inactive [11]. During growth factor stimulation (such as via EGF), the KSR1/PP2A and c-Raf-1/PP2A complexes acquire the PP2A regulatory B subunit, and the PP2A holoenzyme dephosphorylates KSR1 Ser392 and c-Raf-1 Ser259, leading to partial release of 14-3-3 from each protein [10]. For KSR1, this step exposes a MAPK binding site, while for c-Raf-1 it confers activated Ras binding at the
plasma membrane. In addition, Ras-GTP binds IMP, relieving KSR1 inhibition [11]. KSR1 dissociation from IMP and displacement of 14-3-3 on Ser392 leads to rapid plasma membrane translocation of KSR1, localizing MEK1 and MAPK to membrane-bound c-Raf-1. This series of events, and the inability of multiple groups to observe KSR1 kinase activity toward c-Raf-1, has led to the predominating position in the field that KSR1 functions as a scaffold protein only [6], [8] and [12]. In this model, KSR1 appears to coordinate signaling through the c-Raf-1/MEK/MAPK module, but has no other direct impact on c-Raf-1 activation.
