Regulation of actin dynamics by annexin 2

Annexin 2 is a ubiquitous Ca 2+ ‐binding protein that is essential for actin‐dependent vesicle transport. Here, we show that in spontaneously motile cells annexin 2 is concentrated in dynamic actin‐rich protrusions, and that depletion of annexin 2 using siRNA leads to the accumulation of stress fibres and loss of protrusive and retractile activity. Cells co‐expressing annexin 2‐CFP and actin‐YFP exhibit Ca 2+ ‐dependent fluorescense resonance energy transfer throughout the cytoplasm and in membrane ruffles and protrusions, suggesting that annexin 2 may directly interact with actin. This notion was supported by biochemical studies, in which we show that annexin 2 reduces the polymerisation rate of actin monomers in a dose‐dependent manner. By measuring actin polymerisation rates in the presence of barbed‐end and pointed‐end cappers, we further demonstrate that annexin 2 specifically inhibits filament elongation at the barbed ends. These results show that annexin 2 has an essential role in maintaining the plasticity of the dynamic membrane‐associated actin cytoskeleton, and that its activity in this context may be at least partly explained through direct interactions with polymerised and monomeric actin.