Plays a role in cytotoxic granule exocytosis in lymphocytes. Required for both granule maturation and granule docking and priming at the immunologic synapse. Regulates assembly of recycling and late endosomal structures, leading to the formation of an endosomal exocytic compartment that fuses with perforin-containing granules at the immunologic synapse and licences them for exocytosis. Regulates Ca(2+)-dependent secretory lysosome exocytosis in mast cells (By similarity). Interacts with RAB27A and DOC2A. Colocalizes with cytotoxic granules at the plasma membrane. Localizes to endosomal exocytic vesicles. Expressed in lung bronchial epithelium goblet/mucous cells. Also expressed in spleen and testis. Expressed at very low levels in heart muscle, kidney, liver, brain and skeletal muscle. The MHD1 and MHD2 domains mediate localization on recycling endosomes and lysosome. Belongs to the unc-13 family.