Activation of annexin 7 by protein kinase C in vitro and in vivo

Annexin 7, a Ca2+/GTP-activated membrane fusion protein, is preferentially phosphorylated in intact chromaffin cells, and the levels of annexin 7 phos phorylation increase quantitatively in proportion to the extent of catechol amine secretion, Consistently, various protein kinase C inhibitors proporti onately reduce both secretion and phosphorylation of annexin 7 in these cel ls, In vitro, annexin 7 is quantitatively phosphorylated by protein kinase C to a mole ratio of 2.0, and phosphorylation is extraordinarily sensitive to variables such as pH, calcium, phospholipid, phorbol eater, and annexin 7 concentration. Phosphorylation of annexin 7 by protein kinase C significa ntly potentiates the ability of the protein to fuse phospholipid vesicles a nd lowers the half-maximal concentration of calcium needed for this fusion process. Furthermore, other protein kinases, including cAMP-dependent prote in kinase, cGMP-dependent protein kinase, and protein-tyrosine kinase pp60( c-src), also label annexin 7 with high efficiency but do not have this effe ct on membrane fusion. In the case of pp60(c-src), we note that this kinase , if anything, modestly suppresses the membrane fusion activity of annexin 7. These results thus lead us to hypothesize that annexin 7 may be a positi ve mediator for protein kinase C action in the exocytotic membrane fusion r eaction in chromaffin cells.