Differential regulation of basic protein phosphorylation by calcium phospholipid and cyclic-AMP-dependent protein kinases

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Abstract
Myelin basic protein, an 80‐kilodalton (kDa) protein in rat oligodendrocytes, and an 80‐kDa basic protein in neuroblastoma × neonatal Chinese hamster brain explant hybrids were phosphorylated extensively when the cells were treated with either phorbol esters (TPA) or diacylglycerols (e.g., oleyoyl‐acetyl‐glycerol). TPA‐stimulated phosphorylation was inhibited by pre‐incubation with 50 μM psychosine (galactosyl‐sphingosine), confirming that it is mediated through the phospholipid‐dependent protein kinase C (PK‐C). Surprisingly, phosphorylation of these proteins was inhibited by incubation of cells with agents which result in activation of cyclic‐AMP‐dependent protein kinase (dibutyryl cyclic AMP or forskolin). In contrast, phosphorylation of other nonbasic proteins, for example, the oligodendrocyte‐specific 2′,3′‐cyclic nucleotide phosphohydrolase, was stimulated under these conditions (Vartanian et al.: Proceedings of the National Academy of Sciences of the United States of America 85:939, 1988) . The possible role of cyclic AMP in activating specific phosphatases or restricting the availability of diacylglycerol for PK‐C activation is discussed.