A structural model of a Ras-Raf signalosome

A structural model of a K-Ras nanocluster that promotes the stability and accessibility of active K-Ras and creates composite interfaces that facilitate Raf binding provides a framework to unravel MAPK signaling. The protein K-Ras functions as a molecular switch in signaling pathways regulating cell growth. In the human mitogen-activated protein kinase (MAPK) pathway, which is implicated in many cancers, multiple K-Ras proteins are thought to assemble at the cell membrane with Ras effector proteins from the Raf family. Here we propose an atomistic structural model for such an assembly. Our starting point was an asymmetric guanosine triphosphate-mediated K-Ras dimer model, which we generated using unbiased molecular dynamics simulations and verified with mutagenesis experiments. Adding further K-Ras monomers in a head-to-tail fashion led to a compact helical assembly, a model we validated using electron microscopy and cell-based experiments. This assembly stabilizes K-Ras in its active state and presents composite interfaces to facilitate Raf binding. Guided by existing experimental data, we then positioned C-Raf, the downstream kinase MEK1 and accessory proteins (Galectin-3 and 14-3-3 sigma) on and around the helical assembly. The resulting Ras-Raf signalosome model offers an explanation for a large body of data on MAPK signaling.

Automated summary

A structural model of a K-Ras nanocluster is proposed, which promotes the stability and accessibility of active K-Ras and facilitates Raf binding through composite interfaces. This assembly plays a crucial role in regulating MAPK signaling.