CYSTEINE STRING PROTEIN FUNCTIONS DIRECTLY IN REGULATED EXOCYTOSIS

Cysteine string protein (Csp) is essential for neurotransmitter releas e in Drosophila. It has been suggested that Csp functions by regulatin g the activity of presynaptic Ca2+ channels, thus controlling exocytos is. We have examined the effect of overexpressing Csp1 in PC12 cells, a neuroendocrine cell line. PC12 cell clones overexpressing Csp1 did n ot show any changes in morphology, granule number or distribution, or in the levels of other key exocytotic proteins. This overexpression di d not affect intracellular Ca2+ signals after depolarization, suggesti ng that Csp1 has no gross effect on Ca2+ channel activity in PC12 cell s. In contrast, we show that Csp1 overexpression enhances the extent o f exocytosis from permeabilized cells in response to Ca2+ or GTP gamma S in the absence of Ca2+. Because secretion from permeabilized cells is not influenced by Ca2+ channel activity, this represents the first demonstration that Csp has a direct role in regulated exocytosis.