Complete primary structure and tissue expression of chicken pectoralis M- protein

M-Protein (165 kDa) is a structural constituent of myofibrillar M-band in striated muscle. We generated a monoclonal antibody which recognized a 165- kDa protein from chicken pectoralis muscle in immunoblot analysis and stained the M-band under immunofluorescence microscopy. By screening a λgt11 cDNA library from chicken embryonic pectoralis muscle with this antibody, we isolated a cDNA clone encoding the M-protein. Northern blot analysis showed that M-protein mRNA is expressed in pectoralis and cardiac muscle but not in gizzard smooth muscle or non-muscle tissues. Moreover, the anterior latissimus dorsi muscle, which consists almost exclusively of slow fiber types, contains no detectable levels of the mRNA. The full-length cDNA sequence predicted a 1,450-amino acid polypeptide with a calculated molecular weight of 163 x 10³. The encoded protein contains several copies of two different repetitive motifs: five copies of fibronectin type III repeats are in the middle part of the predicted molecule, and two and four copies of the immunoglobulin C2-type repeats are located toward the NH₂-terminal and COOH- terminal regions, respectively. This indicates that M-protein, along with other thick filament-associated proteins such as C-protein, twichin, and titin, belongs to the superfamily of cytoskeletal proteins with immunoglobulin/fibronectin repeats.