Spatial Relationship of the C-Terminal Domains of Dystrophin and β-Dystroglycan in Cardiac Muscle Support a Direct Molecular Interaction at the Plasma Membrane Interface

Dystrophin and beta-dystroglycan are components of a complex of at least nine proteins (the dystrophin-glycoprotein complex) that physically link the membrane cytoskeleton in skeletal and cardiac muscle, through the plasma membrane, to the extracellular matrix. Mutations in the dystrophin gene, which result in an absence or a quantitative or qualitative alteration of dystrophin, cause a subset of familial dilated cardiomyopathies as well as Duchenne and Becker muscular dystrophy. Biochemical studies on isolated skeletal muscle molecules indicate that dystrophin is bound to the glycoprotein complex via beta-dystroglycan, with the C-terminus of beta-dystroglycan binding to the cysteine-rich domain and first half of the C-terminal domain of dystrophin. Ultrastructural labeling has demonstrated a close spatial relationship between dystrophin and beta-dystroglycan in intact skeletal muscle, but no previous ultrastructural labeling studies have examined the dystrophin/beta-dystroglycan interaction in cardiac muscle. In the present study, we have applied complementary immunoconfocal microscopy and double immunogold fracture-label, a freeze-fracture cytochemical technique that allows high-resolution visualization of labeled membrane components in thin section and in platinum-carbon replicas, to investigate the spatial relationship between dystrophin and beta-dystroglycan in rat cardiac muscle. When immunogold probes of two different sizes for the two proteins were used, "doublets" representing side-by-side antibody labeling were demonstrated in en face views at the level of the plasma membrane. The results support the conclusions that dystrophin and beta-dystroglycan directly interact at the cytoplasmic face of the rat cardiac muscle plasma membrane.