Abstract
Two known tissue-specific tropomyosin (TM) isoforms are produced from the rodent beta-TM gene. Skeletal muscle beta-TM uses the alternative exons 6b and 9a and the exon 9a-associated poly(A) site. Fibroblast and smooth muscle TM-1 use exons 6a and 9b and the exon-9b associated poly(A) site. We have identified a new skeletal muscle beta-TM isoform, beta-TM2. beta-TM2 contains exon 6b (muscle) and exon 9b (nonmuscle). Full-length beta-TM2 cDNA clones were isolated from a cDNA library of mouse muscle BC3H1 cells. Its mRNA was also found in mouse skeletal muscle tissue but not in other tissues. beta-TM2 mRNA level and protein synthesis are differentiation-dependent, with a transient high level in the early stages of myogenesis both in BC3H1 cells and in mouse embryo limbs. Trace amounts of beta-TM3 mRNA, the other hybrid form (exons 6a + 9a), were found in less differentiated BC3H1 cells, mouse uterus, heart, and 3T3 fibroblasts but not skeletal muscle tissue. Thus, the selection of the two alternative exons appears to be controlled independently. Furthermore, during myogenesis, there is a sequential switch in the internal alternative exon, the terminal exon, and the poly(A) site from the nonmuscle to the muscle type.
Highlights
Wehavebeenusing mouse muscle-like BC3H1 cells as a system for studying T M expression during myogenesis
Because of the elevated levels of P-TM2 in the early stagoefs differentiation of BC:%HcIells, we speculated that this TM isoform might be developmentally controlled in animals. To explore this possibility we examined the presence of P-TM2
In agreement with the results shown in HC,HIcells, this experiment shows a decrease of' TM-1 and Ij-TM2
Summary
Wehavebeenusing mouse muscle-like BC3H1 cells as a system for studying T M expression during myogenesis. Our previous work (9) hasshown that inproducing P-TM mRNA, early stages of myogenesis both in BC3HIcells and in the downstream exon 9b-associated poly(A) site is used. Differentiated BCSHlcells and inmouse skeletal muscle tissue, the selection of the two alternative exoanpspears theexon Sa-associated poly(A)site becomes preferentially to be controlled independently. Myogenesis, there is a sequential switch in thienternal thechickenP-TM gene (10, alsocalled TM-1 alternative exon, the terminal exon, and the poly(A) gene in Ref. 11)has a structure similar to the rodent P-TM site from the nonmuscle to the muscle type. Gene, additionalmRNAs havebeenshown to be produced from it, including those using an internal promoter and different combinations of alternative exons (12). Tropomyosin (TM)' is one of a number of contractile ously unknown, whether any transcript other than proteins that exist as families of isoforms whose expression is tissue-specific (reviewed in Ref. 1).It stabilizes actin fila-
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