Membrane curvature initiates Cdc42-FBP17-N-WASP clustering and actin nucleation.

The architecture of actin networks at the cell surface is regulated by local membrane topology. However, how actin nucleation can respond sensitively to the degree of membrane curvature remains incompletely understood. Using nanolithography to precisely control local membrane curvature, we reconstituted the dynamic interplay of the tri-component Cdc42/FBP17/N-WASP system on a series of deformed membrane sites, resulting in differential actin nucleation. We found that high-curvature sensing is primarily mediated by FBP17 through its intrinsic BAR-domain activity, which then induces the hierarchical assembly of FBP17/N-WASP clusters to activate N-WASP in synergy with Cdc42. This nucleation boost is fine-tuned by modulating the FBP17-to-N-WASP stoichiometry within multivalent macromolecular assemblies according to local curvature radii. At lower-curvature regions, Cdc42 enhances basal FBP17 recruitment to the membrane, enabling detection of shallow curvatures and initiating actin polymerization before high-curvature effects dominate. This establishes a dynamic, curvature radius-dependent cooperativity that links geometric cues to the regulation of actin polymerization, highlighting their interplay in coordinating membrane and actin morphodynamics during complex cellular processes.