In a previous paper, a monoclonal antibody (designated M1.4) that recognized a 240 kDa polypeptide was characterized. This antibody stained the intermediate filaments in several cell lines, and biochemical characteristics of the 240 kDa polypeptide led us to conclude that it was a novel intermediate filament-associated protein, which we termed gyronemin. Here we report that gyronemin is expressed in adult rat organs that contain a substantial smooth muscle component. Taking advantage of this observation, this protein was purified from bovine uterine tissue and, by biochemical, immunological and amino acid sequence analysis, found to be homologous to the actin-associated protein filamin. Three novel monoclonal antibodies raised using purified bovine gyronemin as the immunogen show this protein to be associated with actin-containing stress fibers, although our original M1.4 antibody continued to be localized along vimentin filaments. Since two-dimensional electrophoretic analysis did not demonstrate a difference in either relative molecular mass or isoelectric point of this polypeptide when associated with either filamentous system, we conclude that filamin is a bifunctional protein capable of associating with both the intermediate filament and actin cytoskeletal systems.

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