Isoform 1 acts as a RAB27A effector protein and plays a role in cytotoxic granule exocytosis in lymphocytes. It is required for cytotoxic granule docking at the immunologic synapse. Isoform 4 binds phosphatidylserine (PS) and phosphatidylinositol-4,5-bisphosphate (PIP2) and promotes the recruitment of glucagon-containing granules to the cell membrane in pancreatic alpha cells. Binding to PS is inhibited by Ca(2+) while binding to PIP2 is Ca(2+) insensitive. Monomer. Binds NRXN1. Interacts with RAB27B (By similarity). Binds RAB27A that has been activated by GTP-binding. Recruited on vesicular structures in cytotoxic T-lymphocytes (CTL) by RAB27A. In the pancreatic alpha cells distributed in both peripheral and anterior regions. Localizes on the glucagon granules in the cell periphery. Isoform 1 is expressed in hematopoietic lineages with a strong expression in CD4 and CD8 T-lymphocytes. It is also widely expressed in nonhematopoietic tissues. Isoform 5 is expressed only in nonhematopoietic tissues. Isoform 4 is expressed in pancreatic alpha cells. The RabBD domain mediates interaction with RAB27A and recruitment on to vesicular structures in cytotoxic T-lymphocytes (CTL). The C2 1 domain mediates binding to phosphatidylserine (PS) and phosphatidylinositol 4,5-bisphosphate (PIP2) and localization to the cell membrane. Isoform 1 is highly susceptible to proteolytic degradation and is stabilized by the interaction with RAB27A. Extended N-terminus. Probable cloning artifact. Truncated N-terminus. Sequencing errors. Truncated N-terminus.