Synexin-like proteins from human polymorphonuclear leukocytes. Identification and characterization of granule-aggregating and membrane-fusing activities

We have used Ca2+-dependent binding to a phospholipid vesicle affinity column to isolate a mixture of three synexin-like proteins from the cytosol of human polymorphonuclear leukocytes (PMN), with relative molecular weights of approximately 67,000, 47,000, and 28,000. Rabbit antibodies raised against bovine liver synexin recognized the 47,000 molecular weight PMN protein. These PMN proteins, like bovine liver synexin, promoted aggregation of isolated PMN specific granules in the presence of Ca2+ and increased the overall rate of Ca2+-induced fusion of liposomes composed of phosphatidate (PA)/phosphatidylethanolamine (PE) (1:3) and phosphatidylserine/PE (1:3), but decreased the rate of spermine-induced fusion of PA/PE (1:3) liposomes. Using fluorescent lipid probes, rapid fusion of PA/PE liposomes with PMN specific granules (50% maximum signal within a few minutes) was observed when 1 mM Ca2+ was added in the presence of both synexin and free arachidonic acid. Dilution of the aqueous contents of liposomes was also observed under the same conditions. The rate of fusion increased monotonically with Ca2+ and arachidonic acid concentrations, but synexin exhibited an optimum concentration. Lack of any one of the components precluded rapid fusion. These results suggest that PMN contain a protein similar to, or identical with, synexin that may be involved in calcium-dependent fusion of intracellular membranes.