AGGREGATION OF PHOSPHOLIPID-VESICLES BY A CHIMERIC PROTEIN WITH THE N-TERMINUS OF ANNEXIN-I AND THE CORE OF ANNEXIN-V

A chimeric protein was produced with the N-terminal domain (amino acids 1-45) of annexin I and the core of annexin V (amino acids 19-320). This protein, annexin I(N)-V(C), has a similar Ca2+ requirement for binding to phospholipid bilayers of 20% phosphatidylserine (PS)/80% phosphatidylcholine (PC) as annexin V. In contrast to annexin V, this protein has a strong potency to aggregate phospholipid vesicles as is shown by turbidimetric measurements and cryo-electron microscopy. Ellipsometry was employed to study quantitatively the phenomenon of phospholipid vesicle adhesion to annexin I(N)-V(C) bound to a planar phospholipid bilayer. The amount of phospholipid vesicles bound by annexin I(N)-V(C) on the planar bilayer is proportional to its surface coverage and can be inhibited by coadsorption of annexin V on the planar bilayer or by shielding the phospholipid surface of the vesicles with blood coagulation factor Va. Annexin I(N)-V(C), like annexin V, does not bind to pure PC bilayers, but its adsorption on anionic phospholipid bilayers brings about the capacity to bind pure PC vesicles. This suggests that annexin I(N)-V(C) generates or exposes after binding to anionic phospholipids another phospholipid binding site, that differs from the annexin V phospholipid binding site. Collectively, the data suggest that two-dimensional cluster formation of annexin I(N)-V(C) on a bilayer with anionic phospholipids is involved in vesicle adherence.