Phosphorylation of the carboxyl-terminal region of dystrophin.

Dystrophin is a protein product of the gene responsible for Duchenne and Becker muscular dystrophy. The protein is localized to the inner surface of sarcolemma and is associated with a group of membrane (glyco)proteins. Dystrophin links cytoskeletal actins via the dystrophin-associated protein complex to extracellular matrix protein, laminin. This structural organization implicates the role of dystrophin in stabilizing the sarcolemma of muscle fibers. Precisely how dystrophin functions is far from clear. The presence of an array of isoforms of the C-terminal region of dystrophin suggests that dystrophin may have functions other than structural. In agreement, many potential phosphorylation sites are found in the C-terminal region of dystrophin, and the C-terminal region of dystrophin is phosphorylated both in vitro and in vivo by many protein kinases, including MAP kinase, p34cdc2 kinase, CaM kinase, and casein kinase, and is dephosphorylated by calcineurin. The C-terminal domain of dystrophin is also a substrate for hierarchical phosphorylation by casein kinase-2 and GSK-3. These observations, in accordance with the finding that the cysteine-rich region binds to Ca2+, Zn2+, and calmodulin, suggest an active involvement of dystrophin in transducing signals across muscle sarcolemma. Phosphorylation-dephosphorylation of the C-terminal region of dystrophin may play a role in regulating dystrophin-protein interactions and (or) transducing signal from the extracellular matrix via the dystrophin molecule to the cytoskeleton.