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. Ca2+-calmodulin incre ases the rate of phosphorylation of dystrophin 12-fold relative to the EGTA control, while other protein kinase activators, cAMP and cGMP, h ave no effect. Phosphorylation of other proteins in the DGC preparatio n was observed, with a 59-kDa protein also being phosphorylated in a c almodulin-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 agains t CaM kinase II cross-react with DGC blotted onto nitrocellulose. Furt her, purified, baculovirus-expressed CaM kinase II phosphorylates dyst rophin and also phosphorylates at least one of the peptides of dystrop hin which is phosphorylated by the DGC protein kinase activity, as sho wn by tryptic peptide maps. CaM kinase II also phosphorylates other pr oteins present in the DGC preparation that are phosphorylated by the e ndogenous protein kinase. Finally, dystrophin sequence 2618-3074, prod uced by recombinant techniques, is phosphorylated by both the DGC prot ein 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 phosp horylation-operate in the DGC.