<b>CALMODULIN STIMULATES DEPHOSPHORYLATION OF A 90 kDa PROTEIN IN THE HUMAN INSULINOMA </b><b>CYTOSOL </b>

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.

Ca2+ and pancreatic B-cell function

“…A number of B-cell enzymes have been shown to be activated by Ca2+-calmodulin [18][19][20]. The calmodulin antagonist calmidazol inhibits influx of Ca2+ through voltage-gated I,-type Ca2 + channels in the B-cell [21], suggesting that phosphorylation by calmodulin-dependent protein kinases is involved in the regulation of these channels ( Figure 1).…”