Cysteine string protein expression in mammary epithelial cells

Cysteine string protein (Csp) is a secretory vesicle protein previously dem onstrated to be required for Ca2+-regulated exocytosis in neurons and endoc rine cells. It has been suggested to function by regulating voltage-gated C a2+ channels or, alternatively, to have a more direct effect on the regulat ed exocytotic machinery. Here we demonstrate the expression of Csp in mamma ry epithelial cells and in the KIM-2 mammary cell line. In KIM-2 cells, Csp was found to be associated with a population of small vesicles and showed partial co-distribution with the vesicle protein cellubrevin. KIM-2 cells d o not express detectable levels of voltage-gated Ca2+ channels, ruling thes e out as a site of action. Using the release of transfected growth hormone (GH) as an assay of secretion, we found that GH is secreted in an exclusive ly constitutive manner from KIM-2 cells. Overexpression of Csp1 inhibits re gulated exocytosis in other cell types but has no effect on constitutive GH release by KIM-2 cells. These results suggest that Csp does not have a maj or function in constitutive exocytosis.