Archibald J M, Blouin C, Doolittle W F
Program in Evolutionary Biology, Canadian Institute for Advanced Research, Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada.
J Struct Biol. 2001 Aug;135(2):157-69. doi: 10.1006/jsbi.2001.4353.
Chaperonins are multisubunit protein-folding assemblies. They are composed of two distinct structural classes, which also have a characteristic phylogenetic distribution. Group I chaperonins (called GroEL/cpn60/hsp60) are present in Bacteria and eukaryotic organelles while group II chaperonins are found in Archaea (called the thermosome or TF55) and the cytoplasm of eukaryotes (called CCT or TriC). Gene duplication has been an important force in the evolution of group II chaperonins: Archaea possess one, two, or three homologous chaperonin subunit-encoding genes, and eight distinct CCT gene families (paralogs) have been described in eukaryotes. Phylogenetic analyses indicate that while the duplications in archaeal chaperonin genes have occurred numerous times independently in a lineage-specific fashion, the eight different CCT subunits found in eukaryotes are the products of duplications that occurred early and very likely only once in the evolution of the eukaryotic nuclear genome. Analyses of CCT sequences from diverse eukaryotic species reveal that each of the CCT subunits possesses a suite of invariant subunit-specific amino acid residues ("signatures"). When mapped onto the crystal structure of the archaeal chaperonin from Thermoplasma acidophilum, these signatures are located in the apical, intermediate, and equatorial domains. Regions that were found to be variable in length and/or amino acid sequence were localized primarily to the exterior of the molecule and, significantly, to the extreme tip of the apical domain (the "helical protrusion"). In light of recent biochemical and electron microscopic data describing specific CCT-substrate interactions, our results have implications for the evolution of subunit-specific functions in CCT.
伴侣蛋白是多亚基蛋白质折叠组装体。它们由两种不同的结构类别组成，这两种结构类别也具有特征性的系统发育分布。第一类伴侣蛋白（称为GroEL/cpn60/hsp60）存在于细菌和真核细胞器中，而第二类伴侣蛋白则存在于古菌（称为热体或TF55）和真核生物的细胞质中（称为CCT或TriC）。基因复制一直是第二类伴侣蛋白进化中的重要力量：古菌拥有一个、两个或三个同源伴侣蛋白亚基编码基因，真核生物中已描述了八个不同的CCT基因家族（旁系同源物）。系统发育分析表明，虽然古菌伴侣蛋白基因的复制在一个谱系特异性的方式下独立发生了很多次，但真核生物中发现的八个不同的CCT亚基是在真核细胞核基因组进化早期发生的复制产物，而且很可能只发生过一次。对来自不同真核物种的CCT序列分析表明，每个CCT亚基都拥有一组不变的亚基特异性氨基酸残基（“特征序列”）。当将这些特征序列映射到嗜热栖热菌的古菌伴侣蛋白的晶体结构上时，它们位于顶端、中间和赤道结构域。发现长度和/或氨基酸序列可变的区域主要位于分子的外部，并且显著地位于顶端结构域的极端末端（“螺旋突出部”）。鉴于最近描述特定CCT-底物相互作用的生化和电子显微镜数据，我们的结果对CCT中亚基特异性功能的进化具有启示意义。
