Altered protein phosphorylation in murine muscular dystrophy

Protein phosphorylation has been studied in the dydy murine muscular dystrophy, both in intact muscle cells and in various membrane fractions derived from them. The results obtained showed that several polypeptides were more heavily phosphorylated in dystrophic myotubes in culture as well as in dystrophic muscle fibers isolated from tibialis anterior. In vitro phosphorylation studies revealed that a large polypeptide of apparent molecular weight of 170,000-150,000 was phosphorylated under basal conditions (3 mM EGTA) in dydy microsomal membranes. The phosphorylation of this polypeptide was not stimulated further by cAMP, calmodulin, cGMP or 12-O-tetradecanoylphorbol 13-acetate (TPA). Under no condition was the corresponding polypeptide phosphorylated at an appreciable rate in normal microsomal membranes. An antibody raised against the voltage-dependent calcium channel reacted, in an immunoblot assay, with a polypeptide, present in both normal and dydy microsomes, which had migration characteristics identical to the phosphorylated 170-150 kDa polypeptide after one- or two-dimensional gel electrophoresis. Additional differences were identified in the phosphorylation of smaller polypeptides of microsomal membranes. When sarcolemmal membranes of normal and dydy muscle were phosphorylated in vitro, no major differences were observed. These results show the existence of an alteration of protein phosphorylation in dystrophic muscle cells in vitro and in vivo, leading to abnormal phosphorylation of the voltage-dependent calcium channel. The possible causes and consequences of this alteration are discussed.