Kinase Suppressor of RAS (KSR)

The RAS-RAF-MEK-ERK signalling pathway relays extracellular stimuli to changes in cellular function and gene expression. Aberrant activation of this pathway through oncogenic mutations is responsible for a large proportion of human cancer.

KSR (kinase suppressor of RAS) functions as an essential scaffolding protein to coordinate the assembly of RAF-MEK-ERK complexes.

We integrated structural and biochemical studies to understand how KSR promotes stimulatory RAF phosphorylation of MEK. From the crystal structure of the kinase domain of KSR2 (KSR2KD) in complex with MEK1 we found that KSR2KD and MEK1 interactions are mediated by their respective activation segments and C-lobe alpha-G helices.

Analogous to BRAF, KSR2 self-associates through a side-to-side interface involving Arg718, a residue identified in a genetic screen as a suppressor of RAS signalling. ATP is bound to the KSR2KD catalytic site, and KSR2 kinase activity towards MEK1 was demonstrated by in vitro assays and chemical genetics. In the KSR2KD:MEK1 complex the activation segments of both kinases are mutually constrained, and KSR2 adopts an inactive conformation. BRAF allosterically stimulates the kinase activity of KSR2, dependent on formation of a side-to-side KSR2:BRAF heterodimer.

Furthermore, KSR2:BRAF heterodimerisation results in an increase of MEK phosphorylation by BRAF through KSR2 relaying a signal from BRAF to release the activation segment of MEK for phosphorylation. These data suggested that KSR interacts with a regulatory RAF molecule in cis to induce a conformational switch of MEK, facilitating MEK’s phosphorylation by a separate catalytic RAF molecule in trans.

Above: Structure of the KSR2(KD)–MEK1 heterodimer. a, Overall view of thecomplex showing the face-to-face configuration of KSR2 andMEK1. KSR loss-of-function mutations1–3 are shown with C atoms in cyan. Sites of MEK1 phosphorylation (Ser 72, 218 and 222) are indicated. b, Details of the beta-sheet formed between the activation segments of MEK1 and KSR2. c, Details of alpha-G helix contacts. From Brennan et al., (2011).