Department of Microbiology and Molecular Genetics, McGovern Medical School, The University of Texas Health Science Center at Houston (UTHealth), Houston, Texas, USA.
Mol Microbiol. 2023 Sep;120(3):397-407. doi: 10.1111/mmi.15127. Epub 2023 Jul 17.
The β-barrel assembly machinery (Bam) complex facilitates the assembly of outer membrane proteins (OMPs) in gram-negative bacteria. The Bam complex is conserved and essential for bacterial viability and consists of five subunits, BamA-E. BamA is the transmembrane component, and its β-barrel domain opens laterally to allow folding and insertion of incoming OMPs. The remaining components are regulatory, among which only BamD is essential. Previous studies suggested that BamB regulates BamA directly, while BamE and BamC serve as BamD regulators. However, specific molecular details of their functions remain unknown. Our previous research demonstrated that BamE plays a specialized role in assembling the complex between the lipoprotein RcsF and its OMP partners, required for the Regulator of Capsule Synthesis (Rcs) stress response. Here, we used RcsF/OmpA as a model substrate to investigate BamE function. Our results challenge the current view that BamE only serves as a BamD regulator. We show that BamE also directly interacts with BamA. BamE interaction with both BamA and BamD is important for function. Our genetic and biochemical analysis shows that BamE stabilizes the Bam complex and promotes bidirectional signaling interaction between BamA and BamD. This BamE function becomes essential when direct BamA/BamD communication is impeded.
β-桶膜装配机器（Bam）复合物促进革兰氏阴性菌的外膜蛋白（OMPs）的装配。Bam 复合物是保守的，对于细菌的生存力是必需的，由五个亚基组成，BamA-E。BamA 是跨膜成分，其β-桶域侧向开放，允许传入的 OMPs 折叠和插入。其余的成分是调节的，其中只有 BamD 是必需的。先前的研究表明，BamB 直接调节 BamA，而 BamE 和 BamC 作为 BamD 的调节剂。然而，它们功能的具体分子细节仍然未知。我们之前的研究表明，BamE 在组装脂蛋白 RcsF 与其 OMP 伙伴之间的复合物中发挥专门作用，这是 Capsule Synthesis（Rcs）应激反应所必需的。在这里，我们使用 RcsF/OmpA 作为模型底物来研究 BamE 的功能。我们的结果挑战了 BamE 仅作为 BamD 调节剂的当前观点。我们表明 BamE 还直接与 BamA 相互作用。BamE 与 BamA 和 BamD 的相互作用对于功能是重要的。我们的遗传和生化分析表明，BamE 稳定了 Bam 复合物并促进了 BamA 和 BamD 之间的双向信号相互作用。当直接的 BamA/BamD 通信受到阻碍时，这种 BamE 功能变得至关重要。
