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 (alpha-, beta-, gamma-, and delta-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 beta-SG appears to play an initiating role and its association with delta-SG is essential for the proper localization of the sarcoglycan complex to the cell membrane. The incorporation of alpha-SG into the sarcoglycan complex occurs at the final stage by interaction with gamma-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 beta-SG and a common mutation in gamma-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.
Shi
Weixing
W
Sigfried and Janet Weis Center for Research M.C. 26-11, The Geisinger Clinic, 100 North Academy Avenue, Danville, Pennsylvania 17822, USA.
Chen
Zaili
Z
Schottenfeld
Jodi
J
Stahl
Richard C
RC
Kunkel
Louis M
LM
Chan
Yiu-Mo
YM
D000818:Animals; D019556:COS Cells; D009202:Cardiomyopathies / Q000235:genetics; D002454:Cell Differentiation / Q000235:genetics*; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism; D003598:Cytoskeletal Proteins / Q000235:genetics / Q000378:metabolism*; D049030:Dystroglycans; D005347:Fibroblasts; D006031:Glycosylation; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000378:metabolism; D009136:Muscular Dystrophies / Q000235:genetics; D009154:Mutation / Q000235:genetics; D032446:Myoblasts; D011485:Protein Binding / Q000235:genetics; D017434:Protein Structure, Tertiary / Q000235:genetics; D049031:Sarcoglycans; D014162:Transfection
D003598:Cytoskeletal Proteins; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins; D049031:Sarcoglycans; D049030:Dystroglycans