Institut Pasteur, Université Paris Cité, Genetics of Biofilms Laboratory, Paris, France.
Institut Pasteur, Université Paris Cité, Evolutionary Biology of the Microbial Cell Laboratory, Paris, France.
Nat Commun. 2023 Nov 23;14(1):7642. doi: 10.1038/s41467-023-43411-y.
E. coli and most other diderm bacteria (those with two membranes) have an inner membrane enriched in glycerophospholipids (GPLs) and an asymmetric outer membrane (OM) containing GPLs in its inner leaflet and primarily lipopolysaccharides in its outer leaflet. In E. coli, this lipid asymmetry is maintained by the Mla system which consists of six proteins: the OM lipoprotein MlaA extracts GPLs from the outer leaflet, and the periplasmic chaperone MlaC transfers them across the periplasm to the inner membrane complex MlaBDEF. However, GPL trafficking still remains poorly understood, and has only been studied in a handful of model species. Here, we investigate GPL trafficking in Veillonella parvula, a diderm Firmicute with an Mla system that lacks MlaA and MlaC, but contains an elongated MlaD. V. parvula mla mutants display phenotypes characteristic of disrupted lipid asymmetry which can be suppressed by mutations in tamB, supporting that these two systems have opposite GPL trafficking functions across diverse bacterial lineages. Structural modelling and subcellular localisation assays suggest that V. parvula MlaD forms a transenvelope bridge, comprising a typical inner membrane-localised MCE domain and, in addition, an outer membrane ß-barrel. Phylogenomic analyses indicate that this elongated MlaD type is widely distributed across diderm bacteria and likely forms part of the ancestral functional core of the Mla system, which would be composed of MlaEFD only.
大肠杆菌和大多数其他二型细菌（具有两层膜的细菌）的内膜富含甘油磷脂（GPLs），不对称的外膜（OM）在内层含有 GPLs，在外层主要含有脂多糖。在大肠杆菌中，这种脂质不对称性由 Mla 系统维持，该系统由六个蛋白组成：OM 脂蛋白 MlaA 从外叶层提取 GPLs，周质伴侣 MlaC 将它们穿过周质转移到内膜复合物 MlaBDEF。然而，GPL 转运仍然知之甚少，并且仅在少数几种模式物种中进行了研究。在这里，我们研究了 Veillonella parvula 中的 GPL 转运，它是一种二型 Firmicute，具有缺乏 MlaA 和 MlaC 的 Mla 系统，但含有拉长的 MlaD。V. parvula mla 突变体表现出脂质不对称性破坏的表型，这些表型可以通过 tamB 突变来抑制，这表明这两个系统在不同的细菌谱系中具有相反的 GPL 转运功能。结构建模和亚细胞定位分析表明，V. parvula MlaD 形成跨膜桥，包含典型的内膜定位的 MCE 结构域，此外，还有一个外膜 β-桶。系统发育基因组学分析表明，这种拉长的 MlaD 类型广泛分布于二型细菌中，可能构成 Mla 系统的祖先功能核心的一部分，该核心仅由 MlaEFD 组成。
