Hastings K E, Bucher E A, Emerson C P
J Biol Chem. 1985 Nov 5;260(25):13699-703.
We describe the isolation and sequence analysis of quail muscle cDNA clones encoding two closely related isoforms of the striated muscle contractile protein, troponin T. The cDNAs represent two troponin T mRNAs that exhibit an unusual sequence relationship. The two mRNAs have identical sequences over hundreds of nucleotides including 3' untranslated regions, but they differ dramatically in a discrete, internally located block of 38 nucleotides. The two alternative sequences of this 38-nucleotide block encode two different but related versions of amino acid residues 230-242, near the C terminus of the protein. These results are consistent with a novel mechanism of troponin T isoform generation by alternative mRNA splicing pathways from a single gene containing two different exons corresponding to amino acids 229-242, as recently proposed by Medford et al. (Medford, R. M., Nguyen, H. T., Destree, A. T., Summers, E., and Nadal-Ginard, B. (1984) Cell 38, 409-421). This proposal was based on analysis of a rat troponin T genomic DNA clone and a cDNA clone corresponding to one of the two alternatively spliced mRNAs. Our analysis of quail troponin T cDNA clones, apparently corresponding to two alternatively spliced mRNA species, provides important new evidence for this novel mechanism of troponin T isoform generation and reveals the differential splicing mechanism to be of great antiquity, antedating the bird-mammal divergence. One of the quail alternative isoform sequences clearly corresponds to one of the rat sequences, but the other quail alternative sequence does not correspond to either of the rat sequences. This result suggests a greater complexity of troponin T gene structure or a greater diversity of troponin T isoform genes than is currently known, and also has implications for the functional significance of the troponin T protein isoform heterogeneity. Comparison of quail and mammal alternative isoform sequences also reveals strongly conserved features which suggest that all the isoform alternative amino acid sequences are variations on a common structural theme.
我们描述了鹌鹑肌肉cDNA克隆的分离和序列分析，这些克隆编码横纹肌收缩蛋白肌钙蛋白T的两种密切相关的同工型。这些cDNA代表两种肌钙蛋白T mRNA，它们呈现出不同寻常的序列关系。这两种mRNA在包括3'非翻译区在内的数百个核苷酸上具有相同的序列，但在一个位于内部的38个核苷酸的离散区域存在显著差异。这个38个核苷酸区域的两种不同序列编码了蛋白质C末端附近氨基酸残基230 - 242的两种不同但相关的版本。这些结果与Medford等人最近提出的一种新机制一致，即通过来自单个基因的可变mRNA剪接途径产生肌钙蛋白T同工型，该基因包含两个对应于氨基酸229 - 242的不同外显子（Medford, R. M., Nguyen, H. T., Destree, A. T., Summers, E., and Nadal-Ginard, B. (1984) Cell 38, 409 - 421）。该提议基于对大鼠肌钙蛋白T基因组DNA克隆和对应于两种可变剪接mRNA之一的cDNA克隆的分析。我们对鹌鹑肌钙蛋白T cDNA克隆的分析，显然对应于两种可变剪接的mRNA种类，为这种肌钙蛋白T同工型产生的新机制提供了重要的新证据，并揭示了差异剪接机制具有悠久的历史，早于鸟类和哺乳动物的分化。鹌鹑的一种可变同工型序列明显对应于大鼠的一种序列，但另一种鹌鹑可变序列与大鼠的任何一种序列都不对应。这一结果表明肌钙蛋白T基因结构比目前已知的更为复杂，或者肌钙蛋白T同工型基因具有更大的多样性，并且也对肌钙蛋白T蛋白质同工型异质性的功能意义有影响。鹌鹑和哺乳动物可变同工型序列的比较还揭示了强烈保守的特征，这表明所有同工型的可变氨基酸序列都是基于一个共同结构主题的变体。
