Actin dynamics in cell motility and chemotaxis

Actin networks are continuously reorganized in cells that rapidly change their shape. Applying total internal reflection fluorescence (TIRF) microscopy at acquisition rates of 10 to 20 Hz, we measured an average growth rate of 3 m x sec-1 for filamentous actin structures throughout the entire substrate-attached cortex of Dictyostelium cells. New filaments often proceed along pre-existing ones, resulting in bundle formation concurrent with filament growth. In cells that orientate in a gradient of chemoattractant, prominent actin assemblies enriched in the Arp2/3 complex are inserted into the network, primarily at the base of filopods that point into the direction of the gradient. The Arp2/3 complex promotes actin nucleation and branching of the filaments. This complex is inhibited by coronin, a WD40-repeat protein. Accordingly, coronin is recruited to a zone behind the leading edge or to other sites where actin assembly ceases. We propose that high turnover rates of actin filaments confer the plasticity to the cell cortex that is required for rapid accommodation to external stimuli.Actin networks are continuously reorganized in cells that rapidly change their shape. Applying total internal reflection fluorescence (TIRF) microscopy at acquisition rates of 10 to 20 Hz, we measured an average growth rate of 3 m x sec-1 for filamentous actin structures throughout the entire substrate-attached cortex of Dictyostelium cells. New filaments often proceed along pre-existing ones, resulting in bundle formation concurrent with filament growth. In cells that orientate in a gradient of chemoattractant, prominent actin assemblies enriched in the Arp2/3 complex are inserted into the network, primarily at the base of filopods that point into the direction of the gradient. The Arp2/3 complex promotes actin nucleation and branching of the filaments. This complex is inhibited by coronin, a WD40-repeat protein. Accordingly, coronin is recruited to a zone behind the leading edge or to other sites where actin assembly ceases. We propose that high turnover rates of actin filaments confer the plasticity to the cell cortex that is required for rapid accommodation to external stimuli.