Sarcoglycan isoforms in skeletal muscle

The heterotetrameric sarcoglycan complex, composed of alpha-, beta-, gamma-, and delta-sarcoglycans, is an important component of the dystrophin-associated glycoprotein assembly in striated muscle. Mutations in any of the four genes encoding sarcoglycans cause a deficiency in all sarcoglycans in the sarcolemma and produce one of four types of limb-girdle muscular dystrophy. A fifth widely expressed sarcoglycan, epsilon-sarcoglycan, has been recently described. epsilon-Sarcoglycan is homologous to alpha-sarcoglycan, but whether it associates with the other sarcoglycans in muscle is not known. In this study, we use wild type and alpha-sarcoglycan-deficient mice to analyze the localization and association of sarcoglycans in skeletal muscle in vivo. The amounts of beta-, gamma-, and delta-sarcoglycans are reduced in alpha-sarcoglycan mutants, whereas the amount of epsilon-sarcoglycan is unchanged. We show here that epsilon-sarcoglycan is complexed with beta-, gamma-, and delta-sarcoglycans in both wild type and alpha-sarcoglycan mutant mice. We also use C2C12 myocytes to study the temporal expression and organization of sarcoglycan complexes during muscle cell differentiation in vitro. In C2C12 cells, alpha- and epsilon-sarcoglycans form separate complexes with beta-, gamma-, and delta-sarcoglycans. Both types of complexes are expressed at the cell surface and presumed to be functional. These results suggest that epsilon-sarcoglycan serves a function similar to that of alpha-sarcoglycan and that residual beta-, gamma-, and delta-sarcoglycan seen in mutant mice and alpha-sarcoglycan-deficient patients is due to its association with epsilon-sarcoglycan.