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 (OMBB) 位于革兰氏阴性细菌和真核细胞器的外膜中。OMBBs 折叠成反平行的 β-折叠片，它们自身闭合，形成穿过膜的孔。当前已知的结构仅包括一个桶，每条链有 8 至 36 股。多 OMBB 链的缺乏令人惊讶，因为大多数 OMBB 形成低聚物，有些仅在这种状态下起作用。使用灵敏的序列比较方法和协同进化分析工具的组合，我们鉴定了在单个链中结合多个β桶的许多蛋白质；包括八股桶的组合占主导地位。这些多桶似乎是独立的、特定于谱系的融合和放大事件的结果。没有在具有外膜的细菌中普遍保守的多桶，再加上它们频繁的从头发生，表明它们的功能不是必不可少的，而是在特定环境中有益的。同一链内具有互补功能的相邻桶可以允许超出单个桶的功能。