Novel Ca2+-binding Protein (CAPS) Related to UNC-31 Required for Ca2+-activated Exocytosis

Abstract

Exocytotic secretion in neuroendocrine cells is actiNeurotransmitter and peptide hormone secretion are mediated by the fusion of secretory vesicles with the plasma membrane, an exocytotic process that requires ATP and is strongly dependent upon and activated by Ca 2ϩ . Insights into the molecular basis of regulated membrane fusion have been provided by the identification of several required synaptic proteins such as vesicle-associated membrane protein/synaptobrevin, syntaxin and SNAP-25, which are substrates for clostridial neurotoxin proteases (1) and receptors for N-ethylmaleimide-sensitive factor/SNAP proteins (2), and synaptotagmin, a vesicle Ca 2ϩ

Rapid regulated dense-core vesicle exocytosis requires the CAPS protein

“…As anticipated from previous studies on the selective expression of CAPS in neural and endocrine tissues (7,10), CAPS was found to be expressed in rat melanotrophs (Fig. 1A).…”

“…Consistent with a distinct role in dense-core vesicle exocytosis, CAPS was found to be essential for Ca 2ϩ -dependent norepinephrine but not glutamate secretion in functional studies with permeable brain synaptosomes (9). In addition, the phenotype of loss-offunction mutants in Caenorhabditis elegans CAPS (UNC-31 protein) is distinct from that of mutants in other synaptic proteins such as synaptobrevin (10,11). The unc-31 phenotype has been attributed in part to a deficit in the secretion of transmitters stored in dense-core vesicles such as the biogenic amines (12).…”

“…CAPS initially was characterized as a cytosolic factor required for Ca 2ϩ -activated dense-core vesicle exocytosis in permeabilized neuroendocrine cells (7,10). Two recent studies indicate that CAPS may be an essential constituent that selectively functions in the dense-core vesicle but not synaptic vesicle exocytotic pathway.…”

“…Studies of regulated dense-core vesicle exocytosis in permeable PC12 cells or membrane fractions from these cells (7,10,35,36) revealed that CAPS is essential for a late Ca 2ϩ -triggered step beyond vesicle docking and priming. To determine the stage at which CAPS is required for exocytosis in melanotrophs, we stimulated exocytosis by flash photolysis of caged Ca 2ϩ (15) while monitoring C m increases (14).…”

Mechanisms of Exocytosis in Insulin‐Secreting B‐Cells and Glucagon‐Secreting A‐Cells

“…Thus, the size of readily releasable pool depends on the concentration of cytosolic Ca 2π that is present during the minute or so before the initiation of exocytosis (Påhlman & Barg, unpublished results). The previous finding that exogenous Ca 2π -chelators inhibit exocytosis in endocrine cells (Ämmälä et al 1993;Chow et al 1996) (Chen et al 1999), the Ca 2π -dependent phosphatase calcineurin (Renström et al 1996b), CAPS (Ca 2π -dependent activator protein for secretion; Ann et al 1997), and Ca 2π /calmodulin kinase II (Ämmälä et al 1993;Gromada et al 1999). Blockage of the latter inhibited the slow phase, and suggested a myosin-actin-dependent step in the granule recruitment (Bi et al 1997).…”