Department of Biochemistry and Molecular Biology, Monash University, Melbourne 3800, Australia.
J Mol Biol. 2012 Sep 28;422(4):545-55. doi: 10.1016/j.jmb.2012.05.035. Epub 2012 Jun 6.
The β-barrel assembly machinery (BAM) complex drives the assembly of β-barrel proteins into the outer membrane of gram-negative bacteria. It is composed of five subunits: BamA, BamB, BamC, BamD, and BamE. We find that the BAM complex isolated from the outer membrane of Escherichia coli consists of a core complex of BamA:B:C:D:E and, in addition, a BamA:B module and a BamC:D module. In the absence of BamC, these modules are destabilized, resulting in increased protease susceptibility of BamD and BamB. While the N-terminus of BamC carries a highly conserved region crucial for stable interaction with BamD, immunofluorescence, immunoprecipitation, and protease-sensitivity assays show that the C-terminal domain of BamC, composed of two helix-grip motifs, is exposed on the surface of E. coli. This unexpected topology of a bacterial lipoprotein is reminiscent of the analogous protein subunits from the mitochondrial β-barrel insertion machinery, the SAM complex. The modular arrangement and topological features provide new insight into the architecture of the BAM complex, towards a better understanding of the mechanism driving β-barrel membrane protein assembly.
β-桶状膜蛋白装配机器（BAM）复合体驱动β-桶状蛋白组装进入革兰氏阴性菌的外膜。它由五个亚基组成：BamA、BamB、BamC、BamD 和 BamE。我们发现从大肠杆菌外膜分离的 BAM 复合体由 BamA:B:C:D:E 的核心复合体组成，此外还有 BamA:B 模块和 BamC:D 模块。在没有 BamC 的情况下，这些模块变得不稳定，导致 BamD 和 BamB 的蛋白酶敏感性增加。虽然 BamC 的 N 端带有高度保守的区域，对与 BamD 的稳定相互作用至关重要，但免疫荧光、免疫沉淀和蛋白酶敏感性测定表明，BamC 的 C 端结构域由两个螺旋夹结构域组成，暴露在大肠杆菌表面。这种细菌脂蛋白的出人意料的拓扑结构让人联想到线粒体β-桶插入机器（SAM 复合体）中的类似蛋白亚基。这种模块化的排列和拓扑特征为 BAM 复合体的结构提供了新的见解，有助于更好地理解驱动β-桶状膜蛋白组装的机制。
