Splicing
A vast array of tropomyosin isoforms are generated by using a combination of different genes and alternative splicing. In mammals, regardless of the gene, transcription is initiated at the start of either exon 1a or 1b. Depending on which promoter and initial exon used, the tropomyosin isoforms can be categorized as either high molecular weight (HMW, 284 amino acids) or low molecular weight (LMW, 248). HMW isoforms express exon 1a and either 2a or 2b, while LMW isoforms express exon 1b. To date, all known tropomyosins contain exons 3-9. Alternative splicing can occur at exon 6, with the mutually exclusive choice of exon 6a or 6b. At the c-terminus the transcript is spliced again at exon 9, with the choice of exon 9a, 9b, 9c or 9d.
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