Formation of sarcoglycan complex with differentiation in cultured myocytes.

The sarcoglycan complex consists of four transmembrane protein subunits. Mutation of any one of the genes encoding these four subunits causes complete loss or marked decrease in expression of the whole complex, resulting in the phenotype of Duchenne-like autosomal recessive muscular dystrophy, termed sarcoglycanopathy. As the basis for understanding this process, we examined how the sarcoglycan complex is formed and associates with other proteins during myogenic differentiation, using a myogenic cell line. Accumulation of the sarcoglycan subunits and formation of the sarcoglycan complex were accomplished with myotube formation. In protein transport inhibition experiments with blefeldin A, we found that the sarcoglycan complex is formed in the endoplasmic reticulum and then associates with the dystroglycan complex and sarcospan en route from the Golgi apparatus to the cell surface. In early myotubes, limited kinds of incomplete sarcoglycan complexes were observed. Their analyses would provide information on the possible patterns of formation of the sarcoglycan complex.