Taniguchi M, Sawaki H, Sasakawa H, Hase T, Sugiyama T
Department of Agricultural Chemistry, School of Agriculture, Nagoya University, Chikusa, Japan.
Eur J Biochem. 1992 Mar 1;204(2):611-20. doi: 10.1111/j.1432-1033.1992.tb16674.x.
The cytosolic and mitochondrial isozymes of aspartate aminotransferase (AspAT) function in the C4 dicarboxylate cycle of photosynthesis. We constructed a cDNA library from leaf tissues of Panicum miliaceum, an NAD-malic-enzyme-type C4 plant and screened the library for AspAT isozymes. A full-length cDNA clone for cytosolic AspAT was isolated. This clone contains an open reading frame that encodes 409 amino acids. We also isolated two cDNA clones for different precursors of mitochondrial AspAT. Comparing these two sequences in the coding regions, we found 12 amino acid substitutions out of 28 base substitutions. The encoded amino acid sequences predict that mitochondrial AspAT are synthesized as precursor proteins of 428 amino acid residues, which each consist of a mature enzyme of 400 amino acid residues and a 28-amino-acid presequence. This prediction coincides with the observation that the in vitro translation product of the mRNA for mitochondrial AspAT was substantially larger than the mature form. A comparison of the amino acid sequences of the AspAT isozymes from P. miliaceum with the published sequences for the enzymes from various animals and microorganisms reveals that functionally and/or structurally important residues are almost entirely conserved in all AspAT species.
天冬氨酸氨基转移酶（AspAT）的胞质和线粒体同工酶在光合作用的C4二羧酸循环中发挥作用。我们从NAD - 苹果酸酶型C4植物黍的叶片组织构建了一个cDNA文库，并筛选该文库以寻找AspAT同工酶。分离出了胞质AspAT的一个全长cDNA克隆。该克隆包含一个编码409个氨基酸的开放阅读框。我们还分离出了线粒体AspAT不同前体的两个cDNA克隆。比较这两个编码区的序列，我们发现在28个碱基替换中有12个氨基酸替换。编码的氨基酸序列预测线粒体AspAT作为428个氨基酸残基的前体蛋白被合成，每个前体蛋白由一个400个氨基酸残基的成熟酶和一个28个氨基酸的前序列组成。这一预测与线粒体AspAT mRNA的体外翻译产物比成熟形式大得多的观察结果一致。将黍的AspAT同工酶的氨基酸序列与已发表的来自各种动物和微生物的该酶序列进行比较，结果表明在所有AspAT物种中，功能和/或结构上重要的残基几乎完全保守。
