The heteropentomeric β-barrel assembly machine (BAM complex) is responsible for folding and inserting a diverse array of β-barrel outer membrane proteins (OMPs) into the outer membrane (OM) of Gram-negative bacteria. The BAM complex contains two essential proteins, the β-barrel OMP BamA and a lipoprotein BamD, whereas the auxiliary lipoproteins BamBCE are individually nonessential. Here, we identify and characterize three bamA mutations, the E-to-K change at position 470 (bamA_E470K), the A-to-P change at position 496 (bamA_A496P), and the A-to-S change at position 499 (bamA_A499S), that suppress the otherwise lethal ΔbamD, ΔbamB ΔbamC ΔbamE, and ΔbamC ΔbamD ΔbamE mutations. The viability of cells lacking different combinations of BAM complex lipoproteins provides the opportunity to examine the role of the individual proteins in OMP assembly. Results show that, in wild-type cells, BamBCE share a redundant function; at least one of these lipoproteins must be present to allow BamD to coordinate productively with BamA. Besides BamA regulation, BamD shares an additional essential function that is redundant with a second function of BamB. Remarkably, bamA_E470K suppresses both, allowing the construction of a BAM complex composed solely of BamA^E470K that is able to assemble OMPs in the absence of BamBCDE. This work demonstrates that the BAM complex lipoproteins do not participate in the catalytic folding of OMP substrates but rather function to increase the efficiency of the assembly process by coordinating and regulating the assembly of diverse OMP substrates.

中文翻译：

---

Baam中的旁路突变揭示了BamBCDE脂蛋白的功能。

异戊二烯β桶装配机器（BAM复合体）负责折叠各种β桶外膜蛋白（OMP）并将其插入革兰氏阴性细菌的外膜（OM）。BAM复合物包含两个必需蛋白，即β-桶状OMP BamA和脂蛋白BamD，而辅助脂蛋白BamBCE分别是不必要的。在这里，我们识别并表征了三个bamA突变，即位置470的E-to-K变化（bamA _E470K），位置496的A-to-P变化（bamA _A496P）和位置A-S的变化499（巴马_A499S），即抑制其他致命Δ BAMD，Δ bamB Δ北广传媒Δ bamE和Δ北广传媒Δ BAMD Δ bamE突变。缺少BAM复杂脂蛋白不同组合的细胞的生存能力提供了检查单个蛋白在OMP组装中的作用的机会。结果表明，在野生型细胞中，BamBCE具有冗余功能。必须存在这些脂蛋白中的至少一种，以使BamD与BamA有效地协同作用。除了BamA调节外，BamD还具有一个额外的基本功能，该功能与BamB的第二个功能是多余的。值得注意的是，bamA _E470K抑制了两者，从而可以构建仅由BamA ^E470K组成的BAM复合体在没有BamBCDE的情况下能够组装OMP。这项工作证明BAM复合脂蛋白不参与OMP底物的催化折叠，而是通过协调和调节各种OMP底物的组装来提高组装过程的效率。