Role of plectin in cytoskeleton organization and dynamics.

Plectin and its isoforms are versatile cytoskeletal linker proteins of very large size (>500 kDa) that are abundantly expressed in a wide variety of mammalian tissues and cell types. Earlier studies indicated that plectin molecules were associated with and/or directly bound to subcomponents of all three major cytoskeletal filament networks, the subplasma membrane protein skeleton, and a variety of plasma membrane-cytoskeleton junctional complexes, including those found in epithelia, various types of muscle, and fibroblasts. In conjunction with biochemical data, this led to the concept that plectin plays an important role in cytoskeleton network organization, with consequences for viscoelastic properties of the cytoplasm and the mechanical integrity and resistance of cells and tissues. Several recent findings lent strong support to this concept. One was that a hereditary disease, epidermolysis bullosa simplex (EBS)-MD, characterized by severe skin blistering combined with muscular dystrophy, is caused by defects in the plectin gene. Another was the generation of plectin-deficient mice by targeted inactivation of the gene. Dying shortly after birth, these animals exhibited severe defects in skin, skeletal muscle and heart. Moreover, in vitro studies with cells derived from such animals unmasked an essential new role of plectin as regulator of cellular processes involving actin stress fibers dynamics. Comprehensive analyses of the gene locus in man, mouse, and rat point towards a complex gene expression machinery, comprising an unprecedented diversity of differentially spliced transcripts with distinct 5' starting exons, probably regulated by different promoters. This could provide a basis for cell type-dependent and/or developmentally-controlled expression of plectin isoforms, exerting different functions through binding to distinct partners. Based on its versatile functions and structural diversification plectin emerges as a prototype cytolinker protein among a family of proteins sharing partial structural homology and functions.