Synemin contains the rod domain of intermediate filaments

We have screened a lambda gt11 cDNA library from chicken gizzard muscle with polyclonal antiserum to avian synemin. Immunopositive clones were characterized and sequenced. Computer searches identified primarily intermediate filament (IF) proteins as being homologous to the synemin clones. Synemin's sequence contained all structural features characteristic of the rod domain of IF proteins: (a) its length (approximately 310 amino acids), (b) the heptad repeat pattern of hydrophobic residues of coiled-coil proteins, (c) subdomain structure of the IF rod, by which helical subdomains (1A, 1B, 2A, and 2B) are separated by three short non-helical linker regions, and (d) several potential intrahelical ion pairs along the sequence. We also confirmed the presence of the IF rod domain at the protein level by immunoblotting a proteolytic digest of synemin by using a monoclonal antibody specific to the rod domain of IFs.