Isoform 11 acts as a RAB27A effector protein and plays a role in cytotoxic granule exocytosis in lymphocytes. Required for cytotoxic granule docking at the immunologic synapse. Isoform 1 may play a role in melanosome transport and vesicle trafficking. It controls melanosome distribution in the cell periphery and regulates melanocyte morphology. Isoform 1 acts as a positive mediator of mucus secretion by the surface mucus cells of the stomach. Mediates basal mucus secretion by gastric surface cells by promoting the proper granule biognesis and docking of mucus granules with the apical plasma membrane. Monomer. Binds NRXN1. Binds RAB27A that has been activated by GTP-binding. Interacts with RAB27B. Bound to melanosomes. Isoform 1 is localized mainly on peripheral melanosomes but not on less mature melanosomes around the nucleus. In the pancreatic alpha cells distributed in both peripheral and anterior regions. Localizes on the glucagon granules in the cell periphery. Additional isoforms seem to exist. Highly expressed in brain, lung, kidney, testis and in embryos after day 7. Detected at lower levels in skeletal muscle. Expressed in pancreatic alpha cells. Isoform 6 is highly expressed in brain, but not detectable in the other tissues tested. Isoform 1 is expressed abundantly in the stomach and is predominantly localized at the apical region of gastric-surface mucus cells. Isoform 11 is expressed in cytotoxic T-lymphocytes (CTL). The RabBD domain mediates interaction with RAB27A. The C2 1 domain mediates localization to the cell membrane. Isoform 1 is highly susceptible to proteolytic degradation and is stabilized by the interaction with RAB27A. Mice show a reduced number of mucus granules, a deficiency of granule docking with the apical plasma membrane in the gastric-surface mucus cells and reduction of mucus secretion by gastric primary cells. Due to intron retention. Due to intron retention.