The membrane skeleton protein supervillin binds tightly to both F-actin and membranes and can potentiate androgen receptor activity in non-muscle cells. We report that muscle, which constitutes the principal tissue source for supervillin sequences, contains a approximately 250 kDa isoform of supervillin that localizes within nuclei and with dystrophin at costameres, regions of F-actin membrane attachment in skeletal muscle. The gene encoding this protein, 'archvillin' (Latin, archi; Greek, archos; 'principal' or 'chief'), contains an evolutionarily conserved, muscle-specific 5' leader sequence. Archvillin cDNAs also contain four exons that encode approximately 47 kDa of additional muscle-specific protein sequence in the form of two inserts within the function-rich N-terminus of supervillin. The first of these muscle-specific inserts contains two conserved nuclear targeting signals in addition to those found in sequences shared with supervillin. Archvillin, like supervillin, binds directly to radiolabeled F-actin and co-fractionates with plasma membranes. Colocalization of archvillin with membrane-associated actin filaments, non-muscle myosin II, and--to a lesser extent--vinculin was observed in myoblasts. Striking localizations of archvillin protein and mRNA were observed at the tips of differentiating myotubes. Transfected protein chimeras containing archvillin insert sequences inhibited myotube formation, consistent with a dominant-negative effect during early myogenesis. These data suggest that archvillin is among the first costameric proteins to assemble during myogenesis and that it contributes to myogenic membrane structure and differentiation.
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