Cytoskeleton-associated plectin: in situ localization, in vitro reconstitution, and binding to immobilized intermediate filament proteins

The association and interaction of plectin (Mr 300,000) with intermediate filaments and filament subunit proteins were studied. Immunoelectron microscopy of whole mount cytoskeletons from various cultured cell lines (rat glioma C6, mouse BALB/c 3T3, and Chinese hamster ovary) and quick-frozen, deep-etched replicas of Triton X-100-extracted rat embryo fibroblast cells revealed that plectin was primarily located at junction sites and branching points of intermediate filaments. These results were corroborated by in vitro recombination studies using vimentin and plectin purified from C6 cells. Filaments assembled from mixtures of both proteins were extensively crosslinked by oligomeric plectin structures, as demonstrated by electron microscopy of negatively stained and rotary-shadowed specimens as well as by immunoelectron microscopy; the binding of plectin structures on the surface of filaments and cross-link formation occurred without apparent periodicity. Plectin's cross-linking of reconstituted filaments was also shown by ultracentrifugation experiments. As revealed by the rotary-shadowing technique, filament-bound plectin structures were oligomeric and predominantly consisted of a central globular core region of 30-50 nm with extending filaments or filamentous loops. Solid-phase binding to proteolytically degraded vimentin fragments suggested that plectin interacts with the helical rod domain of vimentin, a highly conserved structural element of all intermediate filament proteins. Accordingly, plectin was found to bind to the glial fibrillar acidic protein, the three neurofilament polypeptides, and skin keratins. These results suggest that plectin is a cross-linker of vimentin filaments and possibly also of other intermediate filament types.