Dystroglycans The widely expressed α and β dystroglycans make up the core of the DAPC, establishing the transmembrane link between laminin-2 and dystrophin. Both proteins are produced from a single post-translationally modified polypeptide, and are heavily glycosylated prior to being sorted to their respective extracellular and transmembrane locations. These glycosylation patterns are developmentally regulated and largely correlate with the diversity of binding partners in different tissues (Winder, 2001). Total deletion of dystroglycan in mouse is embryonic lethal owing to disruption of formation of the extraembryonic basement membrane, and the void of naturally occurring mutations or other dystroglycan-associated diseases suggests that its function is indispensable for survival (Williamson et al., 1997). Disrupting the dystroglycan-dystrophin link causes a Duchenne-like phenotype, whereas disruption of the laminindystroglycan link causes congenital muscular dystrophy. Defects in posttranslational modification of dystroglyan may also be pathogenic, as a nonsense mutation in the glycosyltransferase, Large, is the primary defect in the myodystrophy mouse (Grewal et al., 2001).

Sarcoglycans Five transmembrane proteins, all expressed primarily in skeletal muscle, constitute the sarcoglycan family: α (50 kDa, also called adhalin), β (43 kDa), γ (35 kDa), δ (35 kDa) and ε (50 kDa). The β, γ and δ sarcoglycans co-purify, with β and δ forming an especially tight link, whereas α sarcoglycan may be spatially separated. Dystrophin and γ sarcoglycan can interact directly, and δ sarcoglycan appears to be coordinated to the dystroglycan complex (Chan et al., 1998). Mutations abolishing the expression of any one of the sarcoglycans cause loss of the others from the sarcolemma; the four recessive