Molecular Biophysics Laboratory, Department of Biological Sciences, Indian Institute of Science Education and Research, Bhopal, India.
Protein Sci. 2021 Mar;30(3):624-637. doi: 10.1002/pro.4022. Epub 2021 Jan 16.
The outer membrane of a Gram-negative bacterium is a crucial barrier between the external environment and its internal physiology. This barrier is bridged selectively by β-barrel outer membrane proteins (OMPs). The in vivo folding and biogenesis of OMPs necessitates the assistance of the outer membrane chaperone BamA. Nevertheless, OMPs retain the ability of independent self-assembly in vitro. Hence, it is unclear whether substrate-chaperone dynamics is influenced by the intrinsic ability of OMPs to fold, the magnitude of BamA-OMP interdependence, and the contribution of BamA to the kinetics of OMP assembly. We addressed this by monitoring the assembly kinetics of multiple 8-stranded β-barrel OMP substrates with(out) BamA. We also examined whether BamA is species-specific, or nonspecifically accelerates folding kinetics of substrates from independent species. Our findings reveal BamA as a substrate-independent promiscuous molecular chaperone, which assists the unfolded OMP to overcome the kinetic barrier imposed by the bilayer membrane. We additionally show that while BamA kinetically accelerates OMP folding, the OMP primary sequence remains a vital deciding element in its assembly rate. Our study provides unexpected insights on OMP assembly and the functional relevance of BamA in vivo.
革兰氏阴性菌的外膜是外部环境与其内部生理之间的关键屏障。这种屏障通过β-桶状外膜蛋白（OMP）有选择性地连接。OMP 的体内折叠和生物发生需要外膜伴侣蛋白 BamA 的协助。然而，OMP 在体外保留了独立自组装的能力。因此，目前尚不清楚底物-伴侣蛋白动力学是否受到 OMP 折叠的固有能力、BamA-OMP 相互依存的程度以及 BamA 对 OMP 组装动力学的贡献的影响。我们通过监测具有（无）BamA 的多种 8 链β-桶状 OMP 底物的组装动力学来解决这个问题。我们还研究了 BamA 是否具有物种特异性，或者是否非特异性地加速来自独立物种的底物的折叠动力学。我们的发现揭示了 BamA 是一种与底物无关的混杂分子伴侣，它可以帮助未折叠的 OMP 克服双层膜施加的动力学障碍。我们还表明，虽然 BamA 在动力学上加速了 OMP 的折叠，但 OMP 的一级序列仍然是其组装速率的重要决定因素。我们的研究为 OMP 组装提供了意想不到的见解，并揭示了 BamA 在体内的功能相关性。
