Permeabilized adrenal chromaffin cells secrete catecholamines by exocytosis in response to micromolar calcium concentrations. Recently, we have demonstrated that chromaffin cells permeabilized with digitonin progressively lose their capacity to secrete due to the release of certain cytosolic proteins essential for exocytosis (Sarafian T., D. Aunis, and M. F. Bader. 1987. J. Biol. Chem. 34:16671-16676). Here we show that one of the released proteins is calpactin I, a calcium-dependent phospholipid-binding protein known to promote in vitro aggregation of chromaffin granules at physiological micromolar calcium levels. The addition of calpactin I into digitonin- or streptolysin-O-permeabilized chromaffin cells with reduced secretory capacity as a result of the leakage of cytosolic proteins partially restores the calcium-dependent secretory activity. This effect is specific of calpactin I since other annexins (p32, p37, p67) do not stimulate secretion at similar or higher concentrations. Calpactin I requires the presence of Mg-ATP, suggesting that a phosphorylating step may regulate the activity of calpactin. Calpactin is unable to restore the secretory activity in cells which have completely lost their cytosolic protein kinase C or in cells having their protein kinase C inhibited by sphingosine or downregulated by long-term incubation with TPA. In contrast, calpactin I prephosphorylated in vitro by purified protein kinase C is able to reconstitute secretion in cells depleted of their protein kinase C activity. This stimulatory effect is also observed with thiophosphorylated calpactin I which is resistant to cellular phosphatases or with phosphorylated calpactin I introduced into cells in the presence of microcystin, a phosphatase inhibitor. These results suggest that calpactin I is involved in the exocytotic machinery by a mechanism which requires phosphorylation by protein kinase C.

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