Phosphorylation of B‐50 Protein by Calcium‐Activated, Phospholipid‐Dependent Protein Kinase and B‐50 Protein Kinase

: B‐50 is a brain‐specific phosphoprotein, the phosphorylation state of which may play a role in the regulation of (poly)phosphoinositide metabolism. Several kinases were tested for their ability to phosphorylate purified B‐50 protein. Only calcium‐activated, phospholipid‐dependent protein kinase (kinase C) and B‐50 protein kinase were able to use B‐50 protein as a substrate. Furthermore, kinase C specifically phosphorylates B‐50 when added to synaptic plasma membranes. We further characterized the sensitivity of kinase C and B‐50 kinase to ACTH (and various fragments), phospholipids, chlorpromazine, and proteolytic activation. Since the sensitivities of both kinases were similar, we conclude that B‐50 protein kinase is a calcium‐dependent, phospholipidstimulated protein kinase of the same type as kinase C.