Interaction of annexins with membranes: the N-terminus as a governing parameter as revealed with a chimeric annexin.

The modulating effect of the variable N-terminus of annexins on the properties of these Ca(2+)-binding proteins was investigated. To this end, the interaction of annexin V and a mutant annexin, INVC, consisting of the N-terminus of annexin I (amino acids 1-45) and the core of annexin V (19-320), with large unilamellar phosphatidylserine (PS) vesicles was examined. In contrast to annexin V, the mutant annexin mediated Ca(2+)-dependent aggregation of the lipid vesicles at neutral pH. However, annexin V induces Ca(2+)-dependent aggregation at mild acidic pH. Moreover, both proteins can engage in hydrophobic interactions with PS vesicles, which results in release of the vesicle contents. These membrane-perturbing properties are expressed by both annexins in the absence of Ca2+ and occur at neutral and mild acidic pH. Interestingly, addition of Ca2+ inhibits annexin V-induced release, but sustains the release induced by the mutant annexin INVC. The Ca(2+)-dependent effects on the release of vesicle contents are reversed upon EDTA addition. Conformational changes revealed by binding of the hydrophobic probe, 4,4'-bis(1-anilino-8-naphthalenesulfonate), underly the observed Ca(2+)-modulated effects on leakage. However, low-pH-mediated aggregation by annexin V does not seem to be related to macroscopic conformational changes. Annexin INVC also affects Ca(2+)-induced fusion of PS vesicles, displaying synergistic properties in conjunction with Ca2+ at neutral pH. By contrast, annexin V does not display similar properties at mild acidic pH, in spite of its ability to aggregate vesicles under such conditions.(ABSTRACT TRUNCATED AT 250 WORDS)