Patel Ramesh, Hsu Shih-Chi, Bédard Jocelyn, Inoue Kentaro, Jarvis Paul
Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom.
Plant Physiol. 2008 Sep;148(1):235-45. doi: 10.1104/pp.108.122754. Epub 2008 Jul 11.
beta-Barrel proteins of the Omp85 (Outer membrane protein, 85 kD) superfamily exist in the outer membranes of Gram-negative bacteria, mitochondria, and chloroplasts. Prominent Omp85 proteins in bacteria and mitochondria mediate biogenesis of other beta-barrel proteins and are indispensable for viability. In Arabidopsis (Arabidopsis thaliana) chloroplasts, there are two distinct types of Omp85-related protein: Toc75 (Translocon at the outer envelope membrane of chloroplasts, 75 kD) and OEP80 (Outer Envelope Protein, 80 kD). Toc75 functions as a preprotein translocation channel during chloroplast import, but the role of OEP80 remains elusive. We characterized three T-DNA mutants of the Arabidopsis OEP80 (AtOEP80) gene. Selectable markers associated with the oep80-1 and oep80-2 insertions segregated abnormally, suggesting embryo lethality of the homozygous genotypes. Indeed, no homozygotes were identified among >100 individuals, and heterozygotes of both mutants produced approximately 25% aborted seeds upon self-pollination. Embryo arrest occurred at a relatively late stage (globular embryo proper) as revealed by analysis using Nomarski optics microscopy. This is substantially later than arrest caused by loss of the principal Toc75 isoform, atToc75-III (two-cell stage), suggesting a more specialized role for AtOEP80. Surprisingly, the oep80-3 T-DNA (located in exon 1 between the first and second ATG codons of the open reading frame) did not cause any detectable developmental defects or affect the size of the AtOEP80 protein in chloroplasts. This indicates that the N-terminal region of AtOEP80 is not essential for the targeting, biogenesis, or functionality of the protein, in contrast with atToc75-III, which requires a bipartite targeting sequence.
Omp85（外膜蛋白，85kD）超家族的β-桶状蛋白存在于革兰氏阴性菌、线粒体和叶绿体的外膜中。细菌和线粒体中显著的Omp85蛋白介导其他β-桶状蛋白的生物合成，对生存能力不可或缺。在拟南芥叶绿体中，有两种不同类型的Omp85相关蛋白：Toc75（叶绿体外被膜易位子，75kD）和OEP80（外被膜蛋白，80kD）。Toc75在叶绿体导入过程中作为前体蛋白易位通道发挥作用，但OEP80的作用仍不清楚。我们对拟南芥OEP80（AtOEP80）基因的三个T-DNA突变体进行了表征。与oep80-1和oep80-2插入相关的选择标记分离异常，表明纯合基因型的胚胎致死性。事实上，在超过100个个体中未鉴定出纯合子，并且两个突变体的杂合子在自花授粉后产生约25%的败育种子。使用Nomarski光学显微镜分析表明，胚胎停滞发生在相对较晚的阶段（球形胚期）。这比主要Toc75异构体atToc75-III缺失导致的停滞（二细胞期）要晚得多，表明AtOEP80具有更特殊的作用。令人惊讶的是，oep80-3 T-DNA（位于开放阅读框第一个和第二个ATG密码子之间的外显子1中）没有引起任何可检测到的发育缺陷，也不影响叶绿体中AtOEP80蛋白的大小。这表明AtOEP80的N端区域对于该蛋白的靶向、生物合成或功能不是必需的，这与需要双部分靶向序列的atToc75-III相反。
