Annexins are soluble proteins capable of binding to phospholipid membranes in a calcium-dependent manner. Annexin 3, a 33 kDa protein mainly expressed in neutrophils, aggregates granules in cell-free assays, and a 36 kDa variant of this protein, specifically expressed in monocytes, has recently been identified. To obtain further information on these proteins, we defined their subcellular localization in resting and activated cells by immunofluorescence microscopy. Both proteins were associated with cytoplasmic granules in resting cells. We obtained evidence to indicate that, in neutrophils which possess a heterogenous granule population, annexin 3 was more likely to be associated with the specific granules. In cells activated with phorbol 12-myristate 13-acetate or opsonized zymosan, the 33 kDa and 36 kDa proteins translocated to the plasma or the phagosome membrane. Upon stimulation with A23187, annexin 3 translocated to the plasma membrane only in neutrophils. We also report that while annexin 3 was associated with restricted membranes in intact cells, it binds indiscriminately to every membrane fraction in cell-free assay. In conclusion, association of both forms of annexin 3 with granules suggests that these proteins could be implicated in processes of granule fusion.

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