CAPS (mammalian UNC-31) protein localizes to membranes involved in dense-core vesicle exocytosis

CAPS is a neural/endocrine-specific protein discovered as a cytosolic factor required for Ca2+-activated dense-core vesicle (DCV) exocytosis in permeable neuroendocrine cells. We report that CAPS is also a membrane-associated, peripherally bound protein in brain homogenates that localizes selectively to plasma membranes and to DCVs but not to small clear synaptic vesicles (SVs). CAPS exhibits high affinity and saturable binding to DCVs by interaction with bilayer phospholipids. Specific CAPS antibodies inhibit Ca2+-activated norepinephrine release from lysed synaptosomes that contain membrane-associated CAPS, indicating that membrane-bound CAPS is essential for neural DCV exocytosis. CAPS is a functional component of the exocytotic machinery that localizes selectively to DCVs, and it may confer distinct regulatory features on neuropeptide and biogenic amine transmitter secretion.