The dystrophin C-terminal domains and interactions with the dystrophin-associated protein complex.

Abstract

Dystrophin plays an important role in skeletal muscle by linking the cytoskeleton and the extracellular matrix. The amino-terminus of dystrophin binds to actin, while the carboxy-terminus associates with the dystrophin-associated protein (DAP) complex. Mutations resulting in the loss of dystrophin or other members of this complex result in several forms of muscular dystrophy, including Duchenne muscular dystrophy. We have generated transgenic/mdx mice expressing full-length dystrophin constructs with consecutive deletions within the C-terminal domains to analyze the interaction of dystrophin with members of the DAP complex and the effects that perturbing these associations have on the dystrophic process. Deletions within the cysteine rich region disrupt the interaction between dystrophin and the DAP complex, leading to a severe dystrophic pathology. Our results indicate that a direct link between dystrophin and $\beta$-dystroglycan is critical for the in vivo function of dystrophin and that association of the sarcoglycan complex with dystrophin and the other DAPs is dependent on this binding. In contrast, dystrophin proteins deleted for the alternatively-spliced region and the extreme C-terminus supported formation of a completely normal DAP complex and were able to prevent the dystrophic pathology observed in mdx mice. These data indicate that the binding site on dystrophin for $\beta$-dystroglycan is a critical functional region of the protein, and suggest that the alternatively-spliced and the C-terminal domains are not required for a functional link between the extracellular matrix and the subsarcolemmal cytoskeleton. In addition to studying the function of the dystrophin C-terminus, we have also begun to search for additional members of the DAP complex. We investigated the PDZ domain-containing protein Mdlg as a candidate for interaction with either dystrophin or its autosomal homologue, utrophin. Mdlg was not found to be present around the entire sarcolemmal membrane of skeletal muscle fibers, as is dystrophin, but instead is concentrated at neuromuscular and myotendinous junctions, in a similar pattern as utrophin. We have also mapped Dlgh1, the gene encoding Mdlg, to mouse chromosome 16 close to a region implicated in tumor suppression, and are continuing to investigate Mdlg as a member of the utrophin-associated protein complex.