Annexin A2 at the interface between F-actin and membranes enriched in phosphatidylinositol 4,5,-bisphosphate.

Vesicle rocketing has been used as a model system for understanding the dynamics of the membrane-associated F-actin cytoskeleton, but in many experimental systems is induced by persistent, non-physiological stimuli. Localised changes in the concentration of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) in membranes stimulate the recruitment of actin-remodelling proteins to their sites of action, regulate their activity and favour vesicle rocketing. The calcium and anionic phospholipid-binding protein annexin A2 is necessary for macropinocytic rocketing and has been shown to bind both PI(4,5)P2 and the barbed-ends of F-actin filaments. Here we show that annexin A2 localises to the comet tails which form constitutively in fibroblasts from patients with Lowe Syndrome. These fibroblasts are deficient in OCRL1, a phosphatidylinositol polyphosphate 5-phosphatase with specificity for PI(4,5)P2. We show that upon depletion of annexin A2 from these cells vesicle rocketing is reduced, and that this is also dependent upon PI(4,5)P2 formation. Annexin A2 co-localised with comet-tails induced by pervanadate and hyperosmotic shock in a basophilic cell line, and in an epithelial cell line upon activation of PKC. In vitro annexin A2 promoted comet formation in a bead-rocketing assay and was sufficient to link F-actin filaments to PI(4,5)P2 containing vesicles. These observations are consistent with a role for annexin A2 as an actin nucleator on PI(4,5)P2-enriched membranes.