Tissue-specific isoforms of chicken myomesin are generated by alternative splicing

Myomesin is a high molecular weight protein that is present in the M-band of all fiber types of cross-striated skeletal muscle and heart. We have isolated two cDNAs encoding tissue-specific isoforms of chicken myomesin with calculated molecular masses of 174 kDa in skeletal muscle and 152 kDa in heart, Distinct sequences are found at the 3'-end of the two cDNAs, giving rise to different C-terminal domains. Partial analysis of the gene structure has shown that in chicken, both isoforms are generated by alternative splicing of a composite exon. Amino acid sequences show that the main body of myomesin consists of five fibronectin type III (class I motifs) and seven immunoglobulin-like domains (class Il motifs), An identical structure was found in RI-protein and human 190K protein (the human counterpart of chicken myomesin), and a comparable domain arrangement occurs in the M-band associated protein skelemin. We postulate that myomesin, M-protein, and skelemin belong to the same subfamily of high molecular weight RI-band-associated proteins of the immunoglobulin superfamily and that they probably have the same ancestor in evolution.