Expression of the cytoskeletal-associated protein filamin in adult rat organs.

Filamin is a well-characterized actin-associated protein first isolated from chicken smooth muscle. Subsequently, this polypeptide and its nonmuscle homolog actin-binding protein have been shown to be expressed in avian muscle tissue, mammalian smooth muscle, mammalian macrophages and other blood cell types, as well as several cultured cell lines. In this report, the occurrence of this polypeptide in adult mammalian organs has been investigated. Immunoblot analysis using three anti-filamin monoclonal antibodies showed that this protein was largely detected in adult rat organs that possess a substantial smooth muscle component. Furthermore, the limited expression of filamin in smooth muscle tissue was corroborated by immunohistochemical analysis. In contrast to avian systems, filamin was never found in detectable quantities in either mammalian cardiac or skeletal muscle. Quantitative immunoblot analysis demonstrated that filamin amounts roughly correlated with the abundance of the smooth muscle component of a given organ, comprising as much as 16.5% of the total SDS-extractable protein in bovine aorta. Work in avian systems and cells in culture has suggested that filamin is a rather ubiquitous cytoskeletal element. By contrast, this work demonstrates that filamin is highly restricted in its expression in mammalian organ systems, in situ.