Calmodulin-activated phosphorylation of dystrophin

Purified dystrophin glycoprotein complex (DGC) contains an endogenous protein kinase activity which phosphorylates dystrophin. Mg2+ (or Mn2+) and ATP are required for this phosphorylation. Ca(2+)-calmodulin increases the rate of phosphorylation of dystrophin 12-fold relative to the EGTA control, while other protein kinase activators, cAMP and cGMP, have no effect. Phosphorylation of other proteins in the DGC preparation was observed, with a 59-kDa protein also being phosphorylated in a calmodulin-dependent manner. These phosphorylations were all on serine residues. The DGC protein kinase activity also phosphorylates syntide-2, a peptide substrate for CaM kinase II, and antibodies raised against CaM kinase II cross-react with DGC blotted onto nitrocellulose. Further, purified, baculovirus-expressed CaM kinase II phosphorylates dystrophin and also phosphorylates at least one of the peptides of dystrophin which is phosphorylated by the DGC protein kinase activity, as shown by tryptic peptide maps. CaM kinase II also phosphorylates other proteins present in the DGC preparation that are phosphorylated by the endogenous protein kinase. Finally, dystrophin sequence 2618-3074, produced by recombinant techniques, is phosphorylated by both the DGC protein kinase and purified CaM kinase II. Since dystrophin and two other DGC components have also been shown to bind calmodulin, two important components of signal transduction--calmodulin binding and protein phosphorylation--operate in the DGC.