SPATIAL RELATIONSHIP OF THE C-TERMINAL DOMAINS OF DYSTROPHIN AND BETA-DYSTROGLYCAN IN CARDIAC-MUSCLE SUPPORT A DIRECT MOLECULAR INTERACTIONAT THE PLASMA-MEMBRANE INTERFACE

Dystrophin and beta-dystroglycan are components of a complex of at lea st nine proteins (the dystrophin-glycoprotein complex) that physically link the membrane cytoskeleton in skeletal and cardiac muscle, throug h the plasma membrane, to the extracellular matrix. Mutations in the d ystrophin gene, which result in an absence or a quantitative or qualit ative alteration of dystrophin, cause a subset familial dilated cardio myopathies as well as Duchenne and Becker muscular dystrophy. Biochemi cal studies on isolated skeletal muscle molecules indicate that dystro phin is bound to the glycoprotein complex via beta-dystroglycan, with the C-terminus of beta-dystroglycan binding to the cysteine-rich domai n and first half of the C-terminal domain of dystrophin. Ultrastructur al labeling has demonstrated a close spatial relationship between dyst rophin and beta-dystroglycan in intact skeletal muscle, but no previou s ultrastructural labeling studies have examined the dystrophinl/beta- dystroglycan interaction in cardiac muscle. In the present study, we h ave applied complementary immunoconfocal microscopy and double immunog old fracture-label, a freeze-fracture cytochemical technique that allo ws high-resolution visualization of labeled membrane components in thi n section and in platinum-carbon replicas, to investigate the spatial relationship between dystrophin and beta-dystroglycan in rat cardiac m uscle. When immunogold probes of two different sizes for the two prote ins were used, ''doublets'' representing side-by-side antibody labelin g were demonstrated in en face views at the level of the plasma membra ne. The results support the conclusions that dystrophin and beta-dystr oglycan directly interact at the cytoplasmic face of the rat cardiac m uscle plasma membrane.