Annexin-mediated membrane fusion of human neutrophil plasma membranes and phospholipid vesicles.

Membrane fusion was studied using human neutrophil plasma membrane preparations and phospholipid vesicles approximately 0.15 microns in diameter and composed of phosphatidylserine and phosphatidylethanolamine in a ratio of 1 to 3. Liposomes were labeled with N-(7-nitrobenzo-2-oxa-1,3-diazol-4-yl (NBD) and lissamine rhodamine B derivatives of phospholipids. Apparent fusion was detected as an increase in fluorescence of the resonance energy transfer donor, NBD, after dilution of the probes into unlabeled membranes. 0.5 mM Ca2+ alone was sufficient to cause substantial fusion of liposomes with a plasma membrane preparation but not with other liposomes. Both annexin I and des(1-9)annexin I caused a substantial increase in the rate of fusion under these conditions while annexin V inhibited fusion. Fusion mediated by des(1-9)annexin I was observed at Ca2+ concentrations as low as approximately 5 microM, suggesting that the truncated form of this protein may be active at physiologically low Ca2+ concentrations. Trypsin treated plasma membranes were incapable of fusion with liposomes, suggesting that plasma membrane proteins may mediate fusion. Liposomes did not fuse with whole cells at any Ca2+ concentration, indicating that the cytoplasmic side of the membrane is involved. These results suggest that annexin I and unidentified plasma membrane proteins may play a role in Ca(2+)-dependent degranulation of human neutrophils.