Ras assemblies and signaling at the membrane

•Ras and its effectors form large complexes. Ras–PI3K is at the membrane; Ras–Raf extends via a long linker to the cytoplasm.•The membrane and its surface are highly crowded; thus, only the PI3K lipid kinase assembly is at the membrane.•The complexes involve a large number of molecules, including additional kinases and scaffolding proteins.•The cytoplasm, but not the crowded membrane surface, can accommodate the large Raf activation and MAPK signaling assemblies Here, we review mechanisms of Ras spatiotemporal clustering with PI3Kα and Raf at the membrane. The large RTK‒Ras‒PI3Kα lipid kinase assembly is at the membrane to generate signaling lipid PIP3. Raf, but not PI3Kα, 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 PI3Kα’s, requires kinase domain dimerization; Ras nanoclusters accomplishing this necessitate Raf’s long linkers. Thus, biophysical and functional constraints shape Ras spatiotemporal assemblies.