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Specific assembly pathway of sarcoglycans is dependent on beta‐ and delta‐sarcoglycan

Mutations in sarcoglycans (SG) have been reported to cause autosomal‐recessive limb‐girdle muscular dystrophy (LGMD) and dilated cardiomyopathy. In skeletal and cardiac muscle, sarcoglycans exist as a complex of four transmembrane proteins (α‐, β‐, γ‐, and δ‐SG). In this study, the assembly of the sarcoglycan complex was examined in a heterologous expression system. Our results demonstrated that the assembly process occurs as a discrete stepwise process. We found that β‐SG appears to play an initiating role and its association with δ‐SG is essential for the proper localization of the sarcoglycan complex to the cell membrane. The incorporation of α‐SG into the sarcoglycan complex occurs at the final stage by interaction with γ‐SG. These findings were supported by chemical cross‐linking of endogenous sarcoglycans in cultured myotubes. We have also provided evidence that glycosylation‐defective mutations in β‐SG and a common mutation in γ‐SG (C283Y) disrupt sarcoglycan‐complex formation. Our proposed model for the assembly and structure of sarcoglycans should generate important insight into their function in muscle as well as their role in muscular dystrophies and cardiomyopathies. Muscle Nerve 29: 409–419, 2004