Novel Ca2+-binding protein (CAPS) related to UNC-31 required for Ca2+-activated exocytosis

Exocytotic secretion in neuroendocrine cells is activated by cytoplasmic Ca2+ increases. Late post-docking events in dense core vesicle exocytosis in permeable PC12 cells require cytosolic factors for sequential ATP-dependent priming and Ca2+-dependent triggering steps. The cytosolic proteins phosphatidylinositol transfer protein and phosphatidylinositol (4)-phosphate 5-kinase, as well as membrane-bound N-ethylmaleimide-sensitive factor, are required for the ATP-dependent priming step. Following priming, the Ca2+-dependent triggering of vesicle fusion requires an additional cytosolic factor, CAPS, which was purified as a 145-kDa protein. To clarify late Ca2+-dependent events in vesicle fusion, the sequence of rat CAPS cDNA was determined and found to encode a novel protein that is the vertebrate homologue of the Caenorhabditis elegans UNC-31 protein shown genetically to be required for neurosecretion. Recombinant CAPS substituted for cytosol in the Ca2+ triggering step in permeable PC12 cells and exhibited moderate affinity (Kd = 270 microM) Ca2+ binding (2 mol Ca2+/mol CAPS dimer), consistent with a role at a Ca2+-regulated step in exocytosis.