Signaling dynamics of the KSR1 scaffold complex.

Scaffold proteins contribute to the spatiotemporal control of MAPK signaling and KSR1 is an ERK cascade scaffold that localizes to the plasma membrane in response to growth factor treatment. To better understand the molecular mechanisms of KSR1 function, we examined the interaction of KSR1 with each of the ERK cascade components, Raf, MEK, and ERK. Here, we identify a hydrophobic motif within the proline-rich sequence (PRS) of MEK1 and MEK2 that is required for constitutive binding to KSR1 and find that MEK binding and residues in the KSR1 CA1 region enable KSR1 to form a ternary complex with B-Raf and MEK following growth factor treatment that enhances MEK activation. We also find that docking of active ERK to the KSR1 scaffold allows ERK to phosphorylate KSR1 and B-Raf on feedback S/TP sites. Strikingly, feedback phosphorylation of KSR1 and B-Raf promote their dissociation and result in the release of KSR1 from the plasma membrane. Together, these findings provide unique insight into the signaling dynamics of the KSR1 scaffold and reveal that through regulated interactions with Raf and ERK, KSR1 acts to both potentiate and attenuate ERK cascade activation, thus regulating the intensity and duration of ERK cascade signaling emanating from the plasma membrane during growth factor signaling.