β‐barrel outer membrane proteins (β‐OMPs) play critical roles in nutrition acquisition, protein import/export, and other fundamental biological processes. The assembly of β‐OMPs in Gram‐negative bacteria is mediated by the β‐barrel assembly machinery (BAM) complex, yet its precise mechanism remains elusive. Here, we report two structures of the BAM complex in detergents and in nanodisks, and two crystal structures of the BAM complex with bound substrates. Structural analysis indicates that the membrane compositions surrounding the BAM complex could modulate its overall conformations, indicating low energy barriers between different conformational states and a highly dynamic nature of the BAM complex. Importantly, structures of the BAM complex with bound substrates and the related functional analysis show that the first β‐strand of the BamA β‐barrel (β1BamA) in the BAM complex is associated with the last but not the first β‐strand of a β‐OMP substrate via antiparallel β‐strand interactions. These observations are consistent with the β‐signal hypothesis during β‐OMP biogenesis, and suggest that the β1BamA strand in the BAM complex may interact with the last β‐strand of an incoming β‐OMP substrate upon their release from the chaperone‐bound state.

中文翻译：

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识别外膜蛋白底物的β-桶组装机的结构

β-桶外膜蛋白（β-OMPs）在营养获取、蛋白质输入/输出和其他基本生物过程中起着关键作用。β-OMPs 在革兰氏阴性菌中的组装是由 β-桶组装机制 (BAM) 复合体介导的，但其精确机制仍然难以捉摸。在这里，我们报告了洗涤剂和纳米盘中 BAM 复合物的两种结构，以及具有结合底物的 BAM 复合物的两种晶体结构。结构分析表明，围绕 BAM 复合物的膜成分可以调节其整体构象，表明不同构象状态之间的能量势垒较低，并且 BAM 复合物具有高度动态的性质。重要的，具有结合底物的 BAM 复合物的结构和相关功能分析表明，BAM 复合物中 BamA β-桶 (β1BamA) 的第一个 β-链与 β-OMP 的最后一个但不是第一个 β-链相关底物通过反平行的 β 链相互作用。这些观察结果与 β-OMP 生物发生过程中的 β-信号假说一致，并表明 BAM 复合物中的 β1BamA 链在从分子伴侣结合状态释放后可能与传入的 β-OMP 底物的最后一个 β-链相互作用.