Live-cell target engagement of allosteric MEKi on MEK-RAF/KSR-14-3-3 complexes

The RAS-mitogen-activated protein kinase (MAPK) pathway includes KSR, RAF, MEK and the phospho-regulatory sensor 14-3-3. Specific assemblies among these components drive various diseases and likely dictate efficacy for numerous targeted therapies, including allosteric MEK inhibitors (MEKi). However, directly measuring drug interactions on physiological RAS-MAPK complexes in live cells has been inherently challenging to query and therefore remains poorly understood. Here we present a series of NanoBRET-based assays to quantify direct target engagement of MEKi on MEK1 and higher-order MEK1-bound complexes with ARAF, BRAF, CRAF, KSR1 and KSR2 in the presence and absence of 14-3-3 in living cells. We find distinct MEKi preferences among these complexes that can be compiled to generate inhibitor binding profiles. Further, these assays can report on the influence of the pathogenic mutants such as BRAF-V600E mutant on MEKi binding. Taken together, these approaches can be used as a platform to screen for compounds intended to target specific complexes in the RAS-MAPK cascade. Development of live-cell target engagement approaches to query MEK-bound binary and ternary complexes reveals the distinct pharmacology of clinical MEK inhibitors at specific assemblies composed of MEK, RAF, KSR and 14-3-3.

Automated summary

This study presents a NanoBRET-based assay to quantify the direct target engagement of MEK inhibitors on MEK1 and its complexes with ARAF, BRAF, CRAF, KSR1 and KSR2 in living cells. The study reveals the preferences of MEK inhibitors among these complexes and their binding profiles. Furthermore, the assay can also report on the effect of pathogenic mutations on MEK inhibitor binding. These methods are important for screening compounds targeting specific complexes in the RAS-MAPK cascade.