No abstract

Regulation of protein phosphorylation by Ca" and calmodulin, and by cyclic AMP (cAMP) was examined in the human insulinoma cytosol. A 65 kDa protein is specifically phosphorylated in the presence of Ca" and calmodulin, and a 52 kDa protein in the presence of cAMP, whereas a 58 kDa protein is phosphorylated in both Ca2"-calmodulin-and cAMP-dependent manners. In addition, phosphorylation of a 90 kDa protein is inhibited in the presence of Ca" and calmodulin. The inhibition is partially restored by EGTA or trifiuoperazine (TFP), suggesting the occurrence of a Caz"-calmodulin-stimulated phosphoprotein phosphatase in the human insulinoma cytosol.