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 acid s 1-45) of annexin I and the core of annexin V (amino acids 19-320). T his protein, annexin I(N)-V(C), has a similar Ca2+ requirement for bin ding to phospholipid bilayers of 20% phosphatidylserine (PS)/80% phosp hatidylcholine (PC) as annexin V. In contrast to annexin V, this prote in has a strong potency to aggregate phospholipid vesicles as is shown by turbidimetric measurements and cryo-electron microscopy. Ellipsome try was employed to study quantitatively the phenomenon of phospholipi d 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 c an be inhibited by coadsorption of annexin V on the planar bilayer or by shielding the phospholipid surface of the vesicles with blood coagu lation 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 phosp holipids another phospholipid binding site, that differs from the anne xin V phospholipid binding site. Collectively, the data suggest that t wo-dimensional cluster formation of annexin I(N)-V(C) on a bilayer wit h anionic phospholipids is involved in vesicle adherence.