Joint BioEnergy Institute, 5885 Hollis Street, Emeryville, CA, 94608, USA.
Department of Chemical & Biomolecular Engineering, University of California, Berkeley, CA, 94720, USA.
Mol Microbiol. 2019 Nov;112(5):1564-1575. doi: 10.1111/mmi.14380. Epub 2019 Sep 17.
Hopanoids are a class of membrane lipids found in diverse bacterial lineages, but their physiological roles are not well understood. The ethanol fermenter Zymomonas mobilis features the highest measured concentration of hopanoids, leading to the hypothesis that these lipids can protect against the solvent toxicity. However, the lack of genetic tools for manipulating hopanoid composition in this bacterium has limited their further functional analysis. Due to the polyploidy (>50 genome copies per cell) of Z. mobilis, we found that disruptions of essential hopanoid biosynthesis (hpn) genes act as genetic knockdowns, reliably modulating the abundance of different hopanoid species. Using a set of hpn transposon mutants, we demonstrate that both reduced hopanoid content and modified hopanoid polar head group composition mediate growth and survival in ethanol. In contrast, the amount of hopanoids, but not their head group composition, contributes to fitness at low pH. Spectroscopic analysis of bacterial-derived liposomes showed that hopanoids protect against several ethanol-driven phase transitions in membrane structure, including lipid interdigitation and bilayer dissolution. We propose that hopanoids act through a combination of hydrophobic and inter-lipid hydrogen bonding interactions to stabilize bacterial membranes during solvent stress.
藿烷类化合物是一类存在于多种细菌谱系中的膜脂，但它们的生理作用尚不清楚。乙醇发酵菌运动发酵单胞菌具有最高的藿烷类化合物测量浓度，这导致了这样一种假设，即这些脂质可以防止溶剂毒性。然而，由于该细菌缺乏用于操纵藿烷类化合物组成的遗传工具，因此限制了对其进一步的功能分析。由于运动发酵单胞菌的多倍体性（每个细胞>50 个基因组拷贝），我们发现必需的藿烷类化合物生物合成（hpn）基因的缺失可作为遗传敲低，可靠地调节不同藿烷类化合物的丰度。使用一组 hpn 转座子突变体，我们证明了藿烷类化合物含量的降低和极性头部基团组成的改变均可介导乙醇中的生长和存活。相比之下，藿烷类化合物的数量而不是其头部基团组成，对低 pH 值下的适应性有贡献。对细菌衍生的脂质体的光谱分析表明，藿烷类化合物可防止膜结构中的几种由乙醇驱动的相变，包括脂质交错和双层溶解。我们提出，藿烷类化合物通过疏水和脂质间氢键相互作用的组合来稳定溶剂胁迫下的细菌膜。
