Department of Biochemistry and Molecular Biology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv 69978, Israel.
Department of Protein Evolution, Max Planck Institute for Developmental Biology, Tübingen 72076, Germany.
Proc Natl Acad Sci U S A. 2021 Aug 3;118(31). doi: 10.1073/pnas.2104059118.
Outer-membrane beta barrels (OMBBs) are found in the outer membrane of gram-negative bacteria and eukaryotic organelles. OMBBs fold as antiparallel β-sheets that close onto themselves, forming pores that traverse the membrane. Currently known structures include only one barrel, of 8 to 36 strands, per chain. The lack of multi-OMBB chains is surprising, as most OMBBs form oligomers, and some function only in this state. Using a combination of sensitive sequence comparison methods and coevolutionary analysis tools, we identify many proteins combining multiple beta barrels within a single chain; combinations that include eight-stranded barrels prevail. These multibarrels seem to be the result of independent, lineage-specific fusion and amplification events. The absence of multibarrels that are universally conserved in bacteria with an outer membrane, coupled with their frequent de novo genesis, suggests that their functions are not essential but rather beneficial in specific environments. Adjacent barrels of complementary function within the same chain may allow for functions beyond those of the individual barrels.
外膜β桶（OMBBs）存在于革兰氏阴性细菌和真核细胞器的外膜中。OMBBs 折叠为反平行 β 片层，自身封闭，形成贯穿膜的孔道。目前已知的结构每条链仅包含一个 8 到 36 股的桶。缺乏多 OMBB 链是令人惊讶的，因为大多数 OMBB 形成寡聚体，而有些仅在这种状态下发挥功能。我们使用敏感的序列比较方法和共进化分析工具的组合，识别出许多在单个链中结合多个 β 桶的蛋白质；包括八股桶的组合占主导地位。这些多桶似乎是独立的、谱系特异性融合和扩增事件的结果。在外膜细菌中普遍保守的多桶不存在，加上它们频繁的从头发生，表明它们的功能不是必需的，而是在特定环境中有益的。同一链中具有互补功能的相邻桶可能允许具有超出单个桶的功能。
