Ca(++)-calmodulin-dependent phosphorylation and dephosphorylation of rat parotid secretion granules.

In our studies on the control of fusion of secretion granules with the plasma membrane which occurs during exocytosis, we have recently found that Ca++ and calmodulin stimulated the fusion of isolated parotid secretion granules with isolated inside-out plasma membrane vesicles. We are now examining the possibility that they do so by stimulating protein phosphorylation. Secretion granules were isolated from rat parotid by differential and gradient centrifugation and incubated with [Y32P]ATP. The granules were solubilized with sodium dodecylsulfate and the proteins resolved on a 7-15% polyacrylamide gel. Calmodulin plus Ca++, but neither alone, stimulated phosphorylation of four proteins with molecular masses of 64, 58, 55 and 31 kDa, and decreased the phosphorylation of a 36 kDa protein. Trifluoperazine and calmidazolium inhibited these effects. The results suggest that Ca++ and calmodulin may facilitate fusion of secretion granules with the plasma membrane by changing the phosphorylation state of one or more secretion granule proteins.