Synemin may function to directly link muscle cell intermediate filaments to both myofibrillar Z-lines and costameres

Synemin is a large intermediate filament (IF) protein that has been identified in all types of muscle cells in association with desmin- and/or vimentin-containing IFs. Our previous studies (Bellin, R. M., Sernett, S. W., Becker, B., Ip, W., Huiatt, T. W., and Robson, R. M. (1999) J. Biol. Chem. 274, 29493-29499) demonstrated that synemin forms heteropolymeric IFs with major IF proteins and contains a binding site for the myofibrillar Z-line protein alpha -actinin. By utilizing blot overlay assays, we show herein that synemin also interacts with the costameric protein vinculin. Furthermore, extensive assays utilizing the Gal4 yeast two-hybrid system demonstrate interactions of synemin with desmin and vimentin and additionally define more precisely the protein subdomains involved in the synemin/alpha -actinin and synemin/vinculin interact-ions. The C-terminal similar to 300-amino acid region of synemin binds to the N-terminal head and central rod domains of alpha -actinin and the similar to 150-amino acid C-terminal tail of vinculin. Overall, these interactions indicate that synemin may anchor IFs to myofibrillar Z-lines via interactions with alpha -actinin and to costameres at the sarcolemma via interactions with vinculin and/or alpha -actinin. These linkages would enable the IFs to directly link all cellular myofibrils and to anchor the peripheral layer of myofibrils to the costameres.