Effect of divalent metal ions on annexin-mediated aggregation of asolectin liposomes.

Annexins belong to a family of Ca 2+ - and phospholipid-binding proteins that can me- diate the aggregation of granules and vesicles in the presence of Ca 2+ . We have stud- ied the effects of different divalent metal ions on annexin-mediated aggregation of liposomes using annexins isolated from rabbit liver and large unilamellar vesicles pre- pared from soybean asolectin II-S. In the course of these studies, we have found that annexin-mediated aggregation of liposomes can be driven by various earth and transi- tion metal ions other than Ca 2+ . The ability of metal ions to induce annexin-mediated aggregation decreases in the order: Cd 2+ >Ba 2+ ,S r 2+ >C a 2+ >>Mn 2+ >Ni 2+ >>Co 2+ . Annexin-mediated aggregation of vesicles is more selective to metal ions than the binding of annexins to membranes. We speculate that not every type of divalent metal ion can induce conformational change sufficient to promote the interaction of annexins either with two opposing membranes or with opposing protein molecules. Relative concentration ratios of metal ions in the intimate environment may be crucial for the functioning of annexins within specialized tissues and after treatment with toxic metal ions.