The annexins are a family of proteins that bind acidic phospholipids in the presence of Ca2+. The association of these proteins with the membranes of secretory granules and endosomes indicates these proteins may play a role in membrane trafficking. One member of the family, annexin II, can exist either as a monomer, heterodimer or heterotetramer in conjunction with the S100 protein p11. The ability of annexin II tetramer to bind both membranes and actin in a Ca2+-dependent manner has led to the hypothesis that annexin II may mediate between vesicle and/or plasma membranes and the cortical cytoskeleton. However, despite intensive biochemical characterisation in vitro, the function of this protein in vivo remains a mystery. In this study annexin II function in living cells was analysed in several different ways using green fluorescent protein (GFP) in full length annexin II-GFP chimeras and chimeras consisting of fragments of annexin II fused to GFP. Transfection of different cell lines with these annexin II-GFP constructs and fluorescence assisted cell sorting (FACS) allowed the generation of multiclonal cell populations expressing annexin II-GFP fusion proteins. These cell populations were analysed for effects on physiological functions - such as secretion (in the RBL cell line) or differentiation (of the PC12 cell line). This line of investigation did not yield evidence to support a role for annexin II in either of these processes. Using novel forms of microscopy the localisation of a full length annexin II- GFP chimera (NAII-GFP) was followed in single cells under physiological conditions. Under conditions of stress NAII-GFP was found to become incorporated into novel actin based structures, reminiscent of Listeria rockets, which propelled pinosomes through the cell interior. This form of vesicle locomotion is dependent on actin polymerisation and may represent a hitherto unrecognised form of vesicle transport
