Annexin I interactions with human neutrophil specific granules: fusogenicity and coaggregation with plasma membrane vesicles

Abstract

The interactions of annexin I with specific granules isolated from human neutrophils were investigated. Unfractionated cytosol induced Ca 2+-dependent granule self-aggregation and fusion of granules with model phospholipid vesicles. High Ca 2+ concentrations were required for these processes (500–600 μM for the half-maximal rate of granule self-aggregation; 100–200 μM for the half-maximal rate of fusion with phospholipid vesicles). These activities were inhibited by a monoclonal antibody specific for annexin I and immunodepletion of cytosol by this antibody greatly reduced activity, implicating annexin I as the major mediator of these processes in neutrophil cytosol. The fact that the Ca 2+ concentration dependences differed for different membranes suggests that specificity may be contolled by the type of intracellular membrane involved and the local Ca 2+ concentration. Trypsin treatment of granules enhanced the rate of fusion of phospholipid vesicles with granules, suggesting that access to phospholipids in the granule membrane may be modulated by granule proteins or that a fusogenic protein factor in the granule membrane is activated by trypsin treatment. Coaggregation of specific granules with plasma membrane vesicles mediated by Ca 2+ and annexin I was suggested by the fact that granules preincubated with Ca 2+, cytosol and plasma membrane vesicles blocked the fusion of subsequently added phospholipid vesicles with the plasma membrane vesicles. These data suggest a role for annexin I as part of a multiprotein system involved in membrane—membrane contact necessary for exocytosis of specific granules in human neutrophils.