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Ca(2+)-regulated actin and phospholipid binding protein (68 kD-protein) from bovine liver: identification as a homologue for annexin VI and intracellular localization.

An F-actin binding protein was purified from bovine liver by means of DNase I affinity, hydroxylapatite and DEAE-cellulose column chromatographies. It consisted of a single polypeptide chain having an apparent molecular weight of 68,000 with a Stokes radius of 35 A. Electron microscopy of rotary shadowed specimens showed that the 68 kD protein is a globular protein. This protein showed a higher affinity for F-actin in the presence of Ca2+ than in its absence, which is opposite to the actin-binding property shown by nonmuscle alpha-actinin or fimbrin. The 68 kD protein had no F-actin severing and capping activity. Interestingly, the 68 kD protein was found to aggregate liposomes at micromolar Ca2+ concentrations. Immunoblot analysis and partial protein sequence data identified the 68 kD protein as an annexin VI (p68) homologue. Immunocytochemical studies showed that the 68 kD protein was localized along stress fibers as well as membrane ruffles, microspikes and focal contacts, raising the possibility that annexin VI may contribute to control membrane-microfilament interaction in the cell.