Data from: Neofunctionalization within the Omp85 protein superfamily during chloroplast evolution

The Toc75 and OEP80 proteins reside in the chloroplast outer envelope membrane. Both are members of the Omp85 superfamily of β-barrel proteins, and both are essential in Arabidopsis plants with important roles throughout development. Toc75 forms the translocation channel of the TOC complex, which is responsible for importing nucleus-encoded proteins into chloroplasts, while the function of OEP80 remains uncertain. Deficiency of Toc75 in plants that have artificially reduced OEP80 levels suggests that the latter may be involved in the biogenesis of β-barrel proteins, in similar fashion to Omp85-related proteins in other systems. To elucidate the evolutionary relationship between the two proteins, we conducted a phylogenetic analysis using 48 sequences from diverse species. This indicated that Toc75 and OEP80 belong to sister groups in the Omp85 superfamily, and originate from a gene duplication in an ancient eukaryotic organism > 1.2 billion years ago. Our analysis also supports the notion that the Toc75 family has undergone a phase of neofunctionalization to accommodate the organelle’s newly acquired need to import proteins.

Toc75与OEP80蛋白定位于叶绿体外被膜（chloroplast outer envelope membrane）。二者均属于β桶状蛋白（β-barrel proteins）的Omp85超家族（Omp85 superfamily），且在拟南芥（Arabidopsis）中为必需基因，在植物整个生长发育过程中发挥关键作用。Toc75是TOC复合物（TOC complex）的转运通道，负责将细胞核编码的蛋白转运至叶绿体中；而OEP80的功能目前仍未明确。在人工下调OEP80表达水平的植株中，Toc75的缺失实验提示，OEP80可能参与β桶状蛋白的生物发生过程，其功能模式与其他系统中的Omp85相关蛋白类似。为阐明这两种蛋白之间的进化关系，我们利用来自不同物种的48条序列开展了系统发育分析（phylogenetic analysis）。分析结果显示，Toc75与OEP80隶属于Omp85超家族中的姐妹分支，其起源可追溯至12亿多年前古代真核生物的一次基因复制事件。我们的分析还支持以下观点：Toc75家族经历了新功能化（neofunctionalization）阶段，以适应该细胞器新获得的蛋白转运需求。