Characterization of muscle filamin isoforms suggests a possible role of ?-filamin/ABP-L in sarcomeric Z-disc formation

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Abstract
Filamin, also called actin binding protein‐280, is a dimeric protein that cross‐links actin filaments in the cortical cytoplasm. In addition to this ubiquitously expressed isoform (FLN1), a second isoform (ABP‐L/γ‐filamin) was recently identified that is highly expressed in mammalian striated muscles. A monoclonal antibody was developed, that enabled us to identify filamin as a Z‐disc protein in mammalian striated muscles by immunocytochemistry and immunoelectron microscopy. In addition, filamin was identified as a component of intercalated discs in mammalian cardiac muscle and of myotendinous junctions in skeletal muscle. Northern and Western blots showed that both, ABP‐L/γ‐filamin mRNA and protein, are absent from proliferating cultured human skeletal muscle cells. This muscle specific filamin isoform is, however, up‐regulated immediately after the induction of differentiation. In cultured myotubes, ABP‐L/γ‐filamin localises in Z‐discs already at the first stages of Z‐disc formation, suggesting that ABP‐L/γ‐filamin might play a role in Z‐disc assembly. Cell Motil. Cytoskeleton 45:149–162, 2000.