[Analysis of the diversity of tropomyosin isoforms]

Tropomyosins are a family of actin filament binding proteins. Like many structural proteins, tropomyosin isoform expression involves the use of multiple genes, but diversity is to a large extent generated by alternative processing of RNA. The tropomyosin family consists of 15 to 20 different protein isoforms which are coded by four genes. Each of these genes code for multiple proteins ranging from two up to as many as nine different isoforms. These genes have been named alpha CTM, alpha FTM, alpha STM and beta TM after to the striated muscle specific subunit of tropomyosin which they code. Their multiple coding potential is based upon the existence of multiple exons associated with initiation of transcription, multiple exons associated with polyadenylation signals and multiple mutually exclusive internal exons which are alternatively spliced. The regulation of this process of alternative splicing have been extensively studied both in the case of exons 2a/2b of the alpha FTM gene and in the case of exons 6a/6b of the beta TM gene. In both cases, one exon is specifically used in one type of muscle tissue, exon 2a is smooth muscle specific and exon 6b is skeletal muscle specific. In both cases, alternative splicing involves a combination of negative regulation, on exon 2b in smooth muscle and on exon 6b in non muscle tissues, and of competition in the alternative situation.