Hou Y M, Shiba K, Mottes C, Schimmel P
Department of Biology, Massachusetts Institute of Technology, Cambridge 02139.
Proc Natl Acad Sci U S A. 1991 Feb 1;88(3):976-80. doi: 10.1073/pnas.88.3.976.
Polypeptide chains of 19 previously studied Escherichia coli aminoacyl-tRNA synthetases are as large as 951 amino acids and, depending on the enzyme, have quaternary structures of alpha, alpha 2, alpha 2 beta 2, and alpha 4. These enzymes have been organized into two classes which are defined by sequence motifs that are associated with specific three-dimensional structures. We isolated, cloned, and sequenced the previously uncharacterized gene for E. coli cysteine-tRNA synthetase (EC 6.1.1.16) and showed that it encodes a protein of 461 amino acids. Biochemical analysis established that the protein is a monomer, thus establishing this enzyme as the smallest known monomeric synthetase. The sequence shows that cysteine-tRNA synthetase is a class I enzyme that is most closely related to a subgroup that includes the much larger methionine-, isoleucine-, leucine-, and valine-tRNA synthetases, which range in size from 677 to 951 amino acids. The amino-terminal 293 amino acids of the cysteine enzyme can be modeled as a nucleotide-binding fold that is more compact than that of its closest relatives by virtue of truncations of two insertions that split the fold. This smaller nucleotide-binding fold accounts for much of the reduced size of the cysteine enzyme and establishes the limit to which the structure of this domain is contracted in the five members of this subgroup of class I enzymes.
此前研究的19种大肠杆菌氨酰-tRNA合成酶的多肽链长达951个氨基酸，根据不同的酶，其四级结构分别为α、α2、α2β2和α4。这些酶被分为两类，分类依据是与特定三维结构相关的序列基序。我们分离、克隆并测序了此前未被鉴定的大肠杆菌半胱氨酰-tRNA合成酶（EC 6.1.1.16）基因，结果表明它编码一个由461个氨基酸组成的蛋白质。生化分析证实该蛋白质为单体，从而确定这种酶是已知最小的单体合成酶。序列分析表明，半胱氨酰-tRNA合成酶是I类酶，与一个亚组关系最为密切，该亚组包括大得多的甲硫氨酰-、异亮氨酰-、亮氨酰-和缬氨酰-tRNA合成酶，其大小在677至951个氨基酸之间。半胱氨酸酶的氨基末端293个氨基酸可模拟为一个核苷酸结合折叠结构，由于两个插入片段的截断使该折叠结构分开，它比其最接近的亲属的折叠结构更紧凑。这种较小的核苷酸结合折叠结构是半胱氨酸酶尺寸减小的主要原因，并确定了I类酶这个亚组的五个成员中该结构域收缩的限度。
