Calcium ion and phospholipid-dependent protein kinase in rod outer segment

The protein kinase in a suspension of bovine rod outer segment was activated by the calcium ion. After the enzyme was extracted from the rod outer segment, the enzymatic activity required not only the calcium ion but also a membrane-associated factor for full activity. This factor resisted proteolysis and was extractable with a 2:1 mixture of chloroform/methanol. The factor could be replaced by the phospholipid fraction prepared from the ghosts of erythrocytes. Calcium ion and phospholipid-dependent protein kinase was partially purified by DEAE-Cellulose and Sephadex G-150 column chromatography. The purified protein kinase showed the molecular weight of 8.7 X 10(4) and a sedimentation coefficient of 5.1 S. Among phospholipids, phosphatidylserine and phosphatidylinositol were the most effective as cofactor. Other phospholipids such as phosphatidic acid, diphosphatidylglycerol and phosphatidylethanolamine were less effective. Phosphatidylcholine and sphingomyelin were inactive. Calcium was the most potent of all divalent cations examined for activation of the protein kinase, and full enzymatic activity was obtained at 4 X 10(-4) M. Strontium was 9% as potent as calcium, but other divalent cations such as barium, zinc, cobalt and magnesium had no effect.