Phosphorylation of myogenin in chick myotubes: regulation by electrical activity and by protein kinase C. Implications for acetylcholine receptor gene expression.

We have analyzed the potential role of myogenin in the regulation by electrical activity of the expression of the acetylcholine receptor (AChR) alpha-subunit gene in cultured chick embryonic myotubes. The state of phosphorylation of myogenin was followed by 32P-labeling and immunoprecipitation with an anti-myogenin antibody. In electrically active myotubes myogenin is phosphorylated, while it is dephosphorylated in electrically silent myotubes following tetrodotoxin (TTX) treatment. Accordingly, nuclear protein kinase C (PKC) activity decreases in TTX-treated myotubes. Myogenin dephosphorylation is also observed upon incubation of myotubes with GF109203X, a pharmacological agent which specifically inhibits PKC activity. Both treatments cause similar increases in the expression of the AChR protein. The effects are not additive. Thus TTX and GF109203X most probably affect a common process. Recombinant chick myogenin binds to myogenic sites (E boxes) present in the AChR alpha-subunit promoter but loses this binding capacity after phosphorylation. As a working hypothesis we propose that repression of AChR biosynthesis by electrical activity results, at least partly, from phosphorylation of myogenin via the PKC pathway.