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Abstract：
The presence of a cell membrane is one of the major structural components defining life. Recent phylogenomic analyses have supported the hypothesis that the last universal common ancestor (LUCA) was likely a diderm. Yet, the mechanisms that guided outer membrane (OM) biogenesis remain unknown. Thermotogae is an early-branching phylum with a unique OM, the toga. Here, we use cryo-electron tomography to characterize the in situ cell envelope architecture of Thermotoga maritima and show that the toga is made of extended sheaths of β-barrel trimers supporting small (~200 nm) membrane patches. Lipidomic analyses identified the same major lipid species in the inner membrane (IM) and toga, including the rare to bacteria membrane-spanning ether-bound diabolic acids (DAs). Proteomic analyses revealed that the toga was composed of multiple SLH-domain containing Ompα and novel β-barrel proteins, and homology searches detected variable conservations of these proteins across the phylum. These results highlight that, in contrast to the SlpA/OmpM superfamily of proteins, Thermotoga possess a highly diverse bipartite OM-tethering system. We discuss the implications of our findings with respect to other early-branching phyla and propose that a toga-like intermediate may have facilitated monoderm-to-diderm cell envelope transitions.
摘要：
细胞膜的存在是定义生命的主要结构成分之一。最近的系统基因组学分析支持了最后一个普遍共同祖先（LUCA）可能是表皮的假设。然而，引导外膜(OM)生物发生的机制尚不清楚.Thermotogae是一个具有独特OM的早期分支门，Toga.这里,我们使用低温电子层析成像技术来表征Thermotogamaritima的原位细胞包膜结构，并显示toga由支持小（〜200nm）膜斑块的β桶三聚体的延伸鞘制成。脂质组学分析确定了内膜（IM）和toga中相同的主要脂质种类，包括罕见的细菌跨膜醚结合的双功能酸（DAs）。蛋白质组学分析显示，该toga由多个含有Ompα的SLH结构域和新型β桶蛋白组成，同源性搜索在整个门中检测到这些蛋白质的可变保守性。这些结果突出表明，与SlpA/OmpM超家族的蛋白质相反，Thermotoga拥有高度多样化的双向OM网络共享系统。我们讨论了我们的发现对其他早期分支门的影响，并提出了类似于toga的中间体可能促进了单胚层到双胚层的细胞包膜过渡。
