Actin filament uncapping localizes to ruffling lamellae and rocketing vesicles

Abstract

Regulated actin filament assembly is critical for eukaryotic cell physiology. Actin filaments are polar structures¹, and those with free high affinity or barbed ends are crucial for actin dynamics and cell motility². Actin filament barbed-end-capping proteins inhibit filament elongation after binding³, and their regulated disassociation is proposed to provide a source of free filament ends to drive processes dependent on actin polymerization⁴. To examine whether dissociation of actin filament capping proteins occurs with the correct spatio-temporal constraints to contribute to regulated actin assembly in live cells, I measured the dissociation of an actin capping protein, gelsolin, from actin in cells using a variation of fluorescence resonance energy transfer (FRET). Uncapping was found to occur in cells at sites of active actin assembly, including protruding lamellae and rocketing vesicles, with the correct spatio-temporal properties to provide sites of actin filament polymerization during protrusion. These observations are consistent with models where uncapping of existing filaments provides sites of actin filament elongation.