cDNA Cloning of Tropomyosin from the Anterior Byssus Retractor Muscle of Mussel and Its Structural Integrity from the Deduced Amino Acid Sequence

A cDNA clone encoding tropomyosin was isolated from the anterior byssus retractor muscle (ABRM) of the mussel Mytilus edulis galloprovincialis. It contained in total 1, 123 nucleotides (nt) with 5' non-coding 89 nt, 3' non-coding 182 nt and coding 852 nt for 284 amino acid residues, a size typical of muscle tropomyosin. Genomic DNA digests with EcoRI, EcoRV and HindIII all exhibited three bands when hybridized with a HaeIII DNA fragment of tropomyosin cDNA, suggesting that the mussel has multiple genes encoding muscle tropomyosin and related proteins. The mRNAs encoding ABRM tropomyosin were most abundant in muscle tissues from byssus retractor and adductor muscles. Considerably large amounts were also observed with gill and inner mantle, whereas traces were found in outer mantle and foot. In the deduced amino acid sequence of ABRM tropomyosin, the N-terminal MDAIKKKMV was well conserved, which has been reported to be involved in head-to-tail polymerization of the tropomyosin molecules. Our tropomyosin was also suggested to have a coiled-coil structure composed of two α-helices that showed the heptad repeats (a-b-c-d-e-f-g)n where a and d tended to be occupied by nonpolar residues with further 28-amino acid repeat zones.