NOVEL CA2-BINDING PROTEIN (CAPS) RELATED TO UNC-31 REQUIRED FOR CA2+-ACTIVATED EXOCYTOSIS()

Exocytotic secretion in neuroendocrine cells is activated by cytoplasm ic Ca2+ increases. Late post-docking events in dense core vesicle exoc ytosis in permeable PC12 cells require cytosolic factors for sequentia l ATP-dependent priming and Ca2+-dependent triggering steps. The cytos olic proteins phosphatidylinositol transfer protein and phosphatidylin ositol (4)-phosphate 5-kinase, as well as membrane-bound N-ethylmaleim ide-sensitive factor, are required for the ATP dependent priming step. Following priming, the Ca2+-dependent triggering of vesicle fusion re quires an additional cytosolic factor, CAPS, which was purified as a 1 45-kDa protein. To clarify late Ca2+-dependent events in vesicle fusio n, the sequence of rat CAPS cDNA was determined and found to encode a novel protein that is the vertebrate homologue of the Caenorhabditis e legans UNC-31 protein shown genetically to be required for neurosecret ion. Recombinant CAPS substituted for cytosol in the Ca2+ triggering s tep in permeable PC12 cells and exhibited moderate affinity (K-d = 270 mu M) Ca2+ binding (2 mol Ca2+/mol CAPS dimer), consistent with a rol e at a Ca2+-regulated step in exocytosis.