Eukaryotic cells advance in phases of protrusion, pause and withdrawal¹. Protrusion occurs in lamellipodia, which are composed of diagonal networks of actin filaments, and withdrawal terminates with the formation of actin bundles parallel to the cell edge. Using correlated live-cell imaging and electron microscopy, we have shown that actin filaments in protruding lamellipodia subtend angles from 15–90° to the front, and that transitions from protrusion to pause are associated with a proportional increase in filaments oriented more parallel to the cell edge. Microspike bundles of actin filaments also showed a wide angular distribution and correspondingly variable bilateral polymerization rates along the cell front. We propose that the angular shift of filaments in lamellipodia serves in adapting to slower protrusion rates while maintaining the filament densities required for structural support; further, we suggest that single filaments and microspike bundles contribute to the construction of the lamella behind and to the formation of the cell edge when protrusion ceases. Our findings provide an explanation for the variable turnover dynamics of actin filaments in lamellipodia observed by fluorescence speckle microscopy² and are inconsistent with a current model of lamellipodia structure that features actin filaments branching at 70° in a dendritic array³.

1. Institute of Molecular Biotechnology, Austrian Academy of Sciences, Dr. Bohr-Gasse 3, 1030, Vienna, Austria.
2. Cytoskeleton Dynamics Group, Helmholtz Centre for Infection Research (HZI), Inhoffen Strasse 7, D-38124 Braunschweig, Germany.

Correspondence to: J. Victor Small¹ e-mail: vic.small@imba.oeaw.ac.at

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