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

## TL;DR

This study shows how membrane curvature triggers the clustering of specific proteins to control actin nucleation, linking cell geometry to dynamic processes.

## Contribution

The study reveals a curvature-dependent mechanism for actin nucleation involving FBP17, N-WASP, and Cdc42.

## Key findings

- FBP17 senses high membrane curvature and clusters N-WASP in a stoichiometry-sensitive manner.
- Cdc42 enhances FBP17 and N-WASP recruitment at low curvature to initiate early nucleation.
- High curvature increases actin nucleation through FBP17 oligomerization and N-WASP activation.

## Abstract

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.

Tightly curved membranes are known to promote actin nucleation. This study shows that this nucleation can be initiated by the cooperative and spontaneous assembly of FBP17, N-WASP, and Cdc42 at sites of low curvature radius.

FBP17 senses curvature and concentrates N-WASP at highly curved sites, inducing nanoclustering in a stoichiometry-sensitive manner.Cdc42 boosts initial membrane recruitment of FBP17 and N-WASP at low curvature to prime early nucleation.High curvature speeds up actin nucleation, with FBP17 oligomerization enhancing N-WASP activation and Arp2/3 activity.FBP17–N-WASP–Cdc42 cooperatively form curvature-dependent nucleation hotspots that couple global Cdc42 signals to local geometry.

FBP17 senses curvature and concentrates N-WASP at highly curved sites, inducing nanoclustering in a stoichiometry-sensitive manner.

Cdc42 boosts initial membrane recruitment of FBP17 and N-WASP at low curvature to prime early nucleation.

High curvature speeds up actin nucleation, with FBP17 oligomerization enhancing N-WASP activation and Arp2/3 activity.

FBP17–N-WASP–Cdc42 cooperatively form curvature-dependent nucleation hotspots that couple global Cdc42 signals to local geometry.

The BAR domain of FBP17 sites regions of high membrane curvature and, along with N-WASP, creates actin nucleation hotspots.