Modulates exocytosis of dense-core granules and secretion of hormones in the pancreas and the pituitary. Interacts with vesicles containing negatively charged phospholipids in a Ca(2+)-independent manner. Part of a ternary complex containing STX1A and RAB27A. Can bind both dominant negative and dominant active mutants of RAB27A. Binds STXBP1, RAB3A, RAB8A and RAB27B. Interacts with MYO5A. Detected close to the plasma membrane and on secretory granules. In pancreas, interacts with insulin-containing vesicles. Detected in the pancreatic islet, in particular in insulin-positive beta cells, and in pituitary. May be due to an intron retention.