Stoichiometry controls activity of phase-separated clusters of actin signaling proteins

TLDR

Biomolecular condensates concentrate macromolecules into membrane‑free foci, often forming via multivalent interactions that drive liquid‑liquid phase separation. Such regulation should be a general feature of signaling systems that assemble through multivalent interactions and drive nonequilibrium outputs. LLPS of the Nephrin‑Nck‑N‑WASP pathway on lipid bilayers boosts actin assembly by prolonging membrane dwell time of N‑WASP and Arp2/3, with the resulting activity being stoichiometry‑dependent and enabled by the condensates’ undefined stoichiometry.

Abstract

Biomolecular condensates concentrate macromolecules into foci without a surrounding membrane. Many condensates appear to form through multivalent interactions that drive liquid-liquid phase separation (LLPS). LLPS increases the specific activity of actin regulatory proteins toward actin assembly by the Arp2/3 complex. We show that this increase occurs because LLPS of the Nephrin-Nck-N-WASP signaling pathway on lipid bilayers increases membrane dwell time of N-WASP and Arp2/3 complex, consequently increasing actin assembly. Dwell time varies with relative stoichiometry of the signaling proteins in the phase-separated clusters, rendering N-WASP and Arp2/3 activity stoichiometry dependent. This mechanism of controlling protein activity is enabled by the stoichiometrically undefined nature of biomolecular condensates. Such regulation should be a general feature of signaling systems that assemble through multivalent interactions and drive nonequilibrium outputs.