Ras assemblies and signaling at the membrane

Here, we review mechanisms of Ras spatiotemporal clustering with PI3K alpha and Raf at the membrane. The large RTK-Ras-PI3K alpha lipid kinase assembly is at the membrane to generate signaling lipid PIP3. Raf, but not PI3K alpha, has long linker extending from the membrane to the kinase domain. This disordered linker stretches into the cytoplasm where Raf's kinase domain side-to-side dimerization and activation is allosterically-driven by MEK under KSR dimers control. The cytoplasm, but not the crowded membrane surface, can accommodate the large Raf's activation and MAPK signaling assemblies, and Raf's disordered linker brings them there. Further, Raf's activation, but not PI3K alpha's, requires kinase domain dimerization; Ras nanoclusters accomplishing this necessitate Raf's long linkers. Thus, biophysical and functional constraints shape Ras spatiotemporal assemblies.

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