CYSTEINE STRING PROTEIN (CSP) IS AN INSULIN SECRETORY GRANULE-ASSOCIATED PROTEIN REGULATING BETA-CELL EXOCYTOSIS

Cysteine string proteins (CSPs) are novel synaptic vesicle-associated protein components characterized by an N-terminal J-domain and a centr al palmitoylated string of cysteine residues. The cellular localizatio n and functional role of CSP was studied in pancreatic endocrine cells . In situ hybridization and RT-PCR analysis demonstrated CSP mRNA expr ession in insulin-producing cells. CSP1 mRNA was present in pancreatic islets; both CSP1 and CSP2 mRNAs were seen in insulin-secreting cell lines. Punctate CSP-like immunoreactivity (CSP-LI) was demonstrated in most islets of Langerhans cells, acinar cells and nerve fibers of the rat pancreas. Ultrastructural analysis showed CSP-LI in close associa tion with membranes of secretory granules of cells in the endo- and ex ocrine pancreas. Subcellular fractionation of insulinoma cells showed CSP1 (34/36 kDa) in granular fractions; the membrane and cytosol fract ions contained predominantly CSP2 (27 kDa), The fractions also contain ed proteins of 72 and 70 kDa, presumably CSP dimers, CSP1 overexpressi on in INS-1 cells or intracellular administration of CSP antibodies in to mouse ob/ob beta-cells did not affect voltage-dependent Ca2+-channe l activity. Amperometric measurements showed a significant decrease in insulin exocytosis in individual INS-1 cells after CSP1 overexpressio n. We conclude that CSP is associated with insulin secretory granules and that CSP participates in the molecular regulation of insulin exocy tosis by mechanisms not involving changes in the activity of voltage-g ated Ca2+-channels.