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.
[Isoform 1]: Cytoplasm . Cell membrane . Recruited on vesicular structures in cytotoxic T-lymphocytes (CTL) by RAB27A. .; [Isoform 4]: Cell membrane . 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.
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