Department of Biology, University of Leicester, University Road, Leicester LE17RH, UK.
Plant J. 2011 Jun;66(5):877-89. doi: 10.1111/j.1365-313X.2011.04551.x. Epub 2011 Apr 4.
The Tic20 protein was identified in pea (Pisum sativum) as a component of the chloroplast protein import apparatus. In Arabidopsis, there are four Tic20 homologues, termed atTic20-I, atTic20-IV, atTic20-II and atTic20-V, all with predicted topological similarity to the pea protein (psTic20). Analysis of Tic20 sequences from many species indicated that they are phylogenetically unrelated to mitochondrial Tim17-22-23 proteins, and that they form two evolutionarily conserved subgroups [characterized by psTic20/atTic20-I/IV (Group 1) and atTic20-II/V (Group 2)]. Like psTic20, all four Arabidopsis proteins have a predicted transit peptide consistent with targeting to the inner envelope. Envelope localization of each one was confirmed by analysis of YFP fusions. RT-PCR and microarray data revealed that the four genes are expressed throughout development. To assess the functional significance of the genes, T-DNA mutants were identified. Homozygous tic20-I plants had an albino phenotype that correlated with abnormal chloroplast development and reduced levels of chloroplast proteins. However, knockouts for the other three genes were indistinguishable from the wild type. To test for redundancy, double and triple mutants were studied; apart from those involving tic20-I, none was distinguishable from the wild type. The tic20-I tic20-II and tic20-I tic20-V double mutants were albino, like the corresponding tic20-I parent. In contrast, tic20-I tic20-IV double homozygotes could not be identified, due to gametophytic and embryonic lethality. Redundancy between atTic20-I and atTic20-IV was confirmed by complementation analysis. Thus, atTic20-I and atTic20-IV are the major functional Tic20 isoforms in Arabidopsis, with partially overlapping roles. While the Group 2 proteins have been conserved over approximately 1.2 billion (1.2 × 10(9) ) years, they are not essential for normal development.
Tic20 蛋白在豌豆（Pisum sativum）中被鉴定为叶绿体蛋白输入装置的组成部分。在拟南芥中，有四个 Tic20 同源物，称为 atTic20-I、atTic20-IV、atTic20-II 和 atTic20-V，它们都具有与豌豆蛋白（psTic20）预测的拓扑相似性。对来自许多物种的 Tic20 序列的分析表明，它们在系统发育上与线粒体 Tim17-22-23 蛋白无关，并且它们形成两个进化上保守的亚组[以 psTic20/atTic20-I/IV（第 1 组）和 atTic20-II/V（第 2 组）为特征]。与 psTic20 一样，拟南芥的所有四种蛋白质都具有预测的转运肽，与靶向内被膜一致。通过对 YFP 融合物的分析证实了每种蛋白质的被膜定位。RT-PCR 和微阵列数据显示，这四个基因在整个发育过程中都有表达。为了评估这些基因的功能意义，鉴定了 T-DNA 突变体。tic20-I 纯合子植物表现出白化表型，与叶绿体发育异常和叶绿体蛋白水平降低有关。然而，其他三个基因的敲除突变体与野生型没有区别。为了测试冗余性，研究了双突变体和三突变体；除了那些涉及 tic20-I 的突变体之外，其他突变体与野生型没有区别。tic20-I tic20-II 和 tic20-I tic20-V 双突变体是白化的，与相应的 tic20-I 亲本相同。相比之下，tic20-I tic20-IV 双纯合子由于配子体和胚胎致死性而无法鉴定。通过互补分析证实了 atTic20-I 和 atTic20-IV 之间的冗余性。因此，atTic20-I 和 atTic20-IV 是拟南芥中主要的 Tic20 同工型，具有部分重叠的作用。虽然第 2 组蛋白在大约 12 亿年（1.2×10^9 ）的时间里得到了保守，但它们对于正常发育并不是必需的。
