A mechanism for substrate quality control in outer membrane protein assembly

• 成果类型: Article
• 署名作者: Combs, Ashton N.; Konovalova, Anna; Silhavy, Thomas J.
• 署名单位: Princeton University; University of Texas System; University of Texas Health Science Center Houston
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.2536912123
• 发表日期: 2026-05-26
• 页码: e2536912123
• 关键词: GRAM-NEGATIVE BACTERIA; outer membrane proteins; BAM complex; protein folding; protein quality control; yaet complex; bama; machine; biogenesis; identification; expression; component; insertion; roles

摘要

The assembly of beta-barrel proteins into the outer membrane (OM) of Gram-negative bacteria is catalyzed by the beta-barrel assembly machine (Bam) complex, which consists of two essential proteins, the BamA beta-barrel and the lipoprotein BamD, and three nonessential lipoproteins BamBCE. While it is well established that BamD serves an essential role in regulating the activity of BamA, the physiological reasons underpinning the need for BamD-mediated regulation of beta-barrel assembly are unclear. Here, we demonstrate that BamD-mediated regulation of BamA functions as a mechanism of substrate quality control that ensures the efficient assembly of beta-barrel proteins into the OM. Through the use of substrate C-terminal fragments and multiple alleles of bamA and bamD that prevent effective regulation of BamA by BamD, we show that BamD activity is necessary to prevent the accumulation of defective beta-barrel substrates on BamA. Notably, these bamAD alleles all confer resistance to the Bam complex inhibitor MRL-494 in a manner that correlates with the degree to which BamD activity is bypassed, suggesting that MRL-494 inhibits beta-barrel assembly by disrupting BamD-mediated conformational changes in BamA. We further show that BamD activity functions to prevent the uptake of toxic small molecules across the OM through a mechanism that functionally overlaps with that of the substrate quality control protein Skp. Collectively, these results not only establish that BamD, like Skp, functions to ensure proper quality control of beta-barrel substrates but also demonstrate the importance of substrate quality control functions in maintaining the integrity of the OM permeability barrier.