Cysteine String Protein Functions Directly in Regulated Exocytosis

Cysteine string protein (Csp) is essential for neurotransmitter release in Drosophila. It has been suggested that Csp functions by regulating the activity of presynaptic Ca(2+) channels, thus controlling exocytosis. We have examined the effect of overexpressing Csp1 in PC12 cells, a neuroendocrine cell line. PC12 cell clones overexpressing Csp1 did not show any changes in morphology, granule number or distribution, or in the levels of other key exocytotic proteins. This overexpression did not affect intracellular Ca(2+) signals after depolarization, suggesting that Csp1 has no gross effect on Ca(2+) channel activity in PC12 cells. In contrast, we show that Csp1 overexpression enhances the extent of exocytosis from permeabilized cells in response to Ca(2+) or GTPγS in the absence of Ca(2+). Because secretion from permeabilized cells is not influenced by Ca(2+) channel activity, this represents the first demonstration that Csp has a direct role in regulated exocytosis.