A novel hybrid alpha-tropomyosin in fibroblasts is produced by alternative splicing of transcripts from the skeletal muscle alpha-tropomyosin gene.

A single alpha-tropomyosin gene in the Japanese quail exhibits tightly controlled expression of multiple isoforms by an alternative splicing mechanism. We have isolated two full length cDNA clones, cC401 and cC402, from quail embryonic myofibers which differentiated in culture. cC402 is the major skeletal muscle alpha-tropomyosin, expressed in adult skeletal muscle and in embryonic muscle culture. cC401 exhibits a novel hybrid primary structure; smooth muscle structure at the carboxyl terminus is combined with skeletal muscle primary structure for the amino-terminal 90% of the protein. The cC401-type tropomyosin, unlike other smooth muscle or cytoskeletal tropomyosins, thus retains two muscle-type troponin-binding regions, for troponin I at amino acid residues 41-80 and for troponin T near amino acid residue 190, while switching to a different troponin binding site at residues 258-284. cC401-type mRNA is expressed at high levels in cultured skin fibroblasts and at low levels in cultured embryonic myoblasts and myofibers, but not in adult muscle, liver, or embryonic skin. A single quail genomic clone encodes both sets of alternative exons found in cC401 and cC402; the different transcripts from this gene share the same 5' ends. Two additional alpha-tropomyosin isoforms have been detected in quail tissues by S1 mapping experiments. Comparison of the cC401 protein with 11 other tropomyosin protein sequences shows that the quail alpha-tropomyosin gene has three different pairs of mutually exclusive exons which are alternatively spliced in nonrandom combinations in specific tissues. The implications of these findings are discussed in relation to mechanisms that regulate alternative splicing.