1. ¹Department of Neurology, University Hospital Bonn, Bonn, Germany
2. ²Department of Cell Biology, University of Potsdam, Potsdam, Germany
3. ³Division of Cell Biology, German Cancer Research Center, Heidelberg, Germany

Correspondence: Dr. P. F. M. van der Ven, Department of Cell Biology, University of Potsdam, Lennéstr.7a, D–14471 Potsdam, Germany. Fax: 0049 331 977 4861; E-mail pvdven@rz.uni-potsdam.de

Received 14 September 1999.

Abstract

Plectin is a high-molecular mass protein ( 500 kd) that binds actin, intermediate filaments, and microtubules. Mutations of the plectin gene cause a generalized blistering skin disorder and muscular dystrophy. In adult muscle, plectin is colocalized with desmin at structures forming the intermyofibrillar scaffold and beneath the plasma membrane. To study the involvement of plectin in myofibrillogenesis, we analyzed the spatial and temporal expression patterns of plectin in cultured differentiating human skeletal muscle cells and its relationship to desmin intermediate filaments during this process. Northern and Western blot analyses demonstrated that at least two different plectin isoforms are expressed at all developmental stages from proliferating myoblasts to mature myotubes. Using immunocytochemistry, we show that the localization of plectin dramatically changes from a network-like distribution into a cross-striated distribution during maturation of myocytes. Double immunofluorescence experiments revealed that desmin and plectin are colocalized in premyofibrillar stages and in mature myotubes. Interestingly, plectin was often found to localize to the periphery of Z-discs during the actual alignment of neighboring myofibrils, and an obvious cross-striated plectin staining pattern was observed before desmin was localized 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.