The carboxyl-terminal isoforms of troponin T (TnT), α and β, differing in the sequence of a region near the COOH terminus, arise from alternative splicing of a primary transcript of the TnT gene and are expressed in a tissue-specific and developmentally regulated manner (Medford, R. M., Nguyen, H. T., Destree, A. T., Summers, E., and Nadal-Ginard, B. (1984) Cell 38, 409-421). To date, the β isoform has not been studied directly at the protein level. To explore the potential functional differences between the α and β sequences, we isolated two rabbit skeletal TnT cDNA clones: a full-length cDNA for a β isoform and a partial-length cDNA for an α isoform. Two restriction fragments derived from the cDNA clones were used to direct overexpression, in Escherichia coli, of two TnT fragments, T2p-α and T2p-β, each containing the last 108 amino acid residues of the α or β isoform of TnT. Using purified T2p-α and T2p-β along with fluorescent derivatives of troponin C (TnC) and αα-tropomyosin (Tm), we showed that T2p-α bound more strongly to TnC than did T2p-β both in the presence and absence of Ca2+, and exhibited a higher affinity for Tm than did T2p-β. More interestingly, the Ca2+ affinities of the Ca2+-specific regulatory sites of TnC in the T2p-α TnC complex were found to be 3-fold higher than in T2p-β·TnC complex. These results support the hypothesis that the sequence divergence between the α and β isoforms of TnT may have functional significance in possibly contributing to the determination of the Ca2+ sensitivity of muscle fibers.
|Number of pages||5|
|Journal||Journal of Biological Chemistry|
|State||Published - Nov 15 1992|
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