Association of plectin with Z-discs is a prerequisite for the formation ofthe intermyofibrillar desmin cytoskeleton

Plectin is a high-molecular mass protein (approximate to 500 kd) that binds actin, intermediate filaments, and microtubules. Mutations of the plectin gene cause a generalized blistering skin disorder and muscular dystrophy. I n adult muscle, plectin is colocalized with desmin at structures forming th e intermyofibrillar scaffold and beneath the plasma membrane. To study the involvement of plectin in myofibrillogenesis, we analyzed the spatial and t emporal expression patterns of plectin in cultured differentiating human sk eletal muscle cells and its relationship to desmin intermediate filaments d uring this process. Northern and Western blot analyses demonstrated that at least two different plectin isoforms are expressed at all developmental st ages from proliferating myoblasts to mature myotubes. Using immunocytochemi stry, we show that the localization of plectin dramatically changes from a network-like distribution into a cross-striated distribution during maturat ion of myocytes. Double immunofluorescence experiments revealed that desmin and plectin are colocalized in premyofibrillar stages and in mature myotub es. Interestingly, plectin was often found to localize to the periphery of Z-discs during the actual alignment of neighboring myofibrils, and an obvio us cross-striated plectin staining pattern was observed before desmin was l ocalized in the Z-disc region. We conclude that the association of plectin with Z-discs is an early event in the lateral alignment of myofibrils that precedes the formation of the intermyofibrillar desmin cytoskeleton.