Smets Bent, Boschker Henricus T S, Wetherington Maxwell T, Lelong Gérald, Hidalgo-Martinez Silvia, Polerecky Lubos, Nuyts Gert, De Wael Karolien, Meysman Filip J R
Department of Biology, University of Antwerp, Antwerp, Belgium.
Department of Biotechnology, Delft University of Technology, Delft, Netherlands.
Front Microbiol. 2024 Mar 8;15:1208033. doi: 10.3389/fmicb.2024.1208033. eCollection 2024.
Cable bacteria embed a network of conductive protein fibers in their cell envelope that efficiently guides electron transport over distances spanning up to several centimeters. This form of long-distance electron transport is unique in biology and is mediated by a metalloprotein with a sulfur-coordinated nickel (Ni) cofactor. However, the molecular structure of this cofactor remains presently unknown. Here, we applied multi-wavelength Raman microscopy to identify cell compounds linked to the unique cable bacterium physiology, combined with stable isotope labeling, and orientation-dependent and ultralow-frequency Raman microscopy to gain insight into the structure and organization of this novel Ni-cofactor. Raman spectra of native cable bacterium filaments reveal vibrational modes originating from cytochromes, polyphosphate granules, proteins, as well as the Ni-cofactor. After selective extraction of the conductive fiber network from the cell envelope, the Raman spectrum becomes simpler, and primarily retains vibrational modes associated with the Ni-cofactor. These Ni-cofactor modes exhibit intense Raman scattering as well as a strong orientation-dependent response. The signal intensity is particularly elevated when the polarization of incident laser light is parallel to the direction of the conductive fibers. This orientation dependence allows to selectively identify the modes that are associated with the Ni-cofactor. We identified 13 such modes, some of which display strong Raman signals across the entire range of applied wavelengths (405-1,064 nm). Assignment of vibrational modes, supported by stable isotope labeling, suggest that the structure of the Ni-cofactor shares a resemblance with that of nickel bis(1,2-dithiolene) complexes. Overall, our results indicate that cable bacteria have evolved a unique cofactor structure that does not resemble any of the known Ni-cofactors in biology.
电缆细菌在其细胞膜中嵌入了一个导电蛋白纤维网络，该网络能有效地引导电子在长达数厘米的距离上进行传输。这种长距离电子传输形式在生物学中是独一无二的，它由一种含有硫配位镍（Ni）辅因子的金属蛋白介导。然而，目前这种辅因子的分子结构尚不清楚。在这里，我们应用多波长拉曼显微镜来识别与电缆细菌独特生理相关的细胞化合物，并结合稳定同位素标记以及取向依赖和超低频拉曼显微镜，以深入了解这种新型镍辅因子的结构和组织。天然电缆细菌丝的拉曼光谱揭示了来自细胞色素、多聚磷酸盐颗粒、蛋白质以及镍辅因子的振动模式。从细胞膜中选择性提取导电纤维网络后，拉曼光谱变得更简单，并且主要保留了与镍辅因子相关的振动模式。这些镍辅因子模式表现出强烈的拉曼散射以及强烈的取向依赖响应。当入射激光的偏振方向与导电纤维方向平行时，信号强度会特别升高。这种取向依赖性使得能够选择性地识别与镍辅因子相关的模式。我们识别出了13种这样的模式，其中一些在整个应用波长范围（405 - 1064纳米）内都显示出强烈的拉曼信号。在稳定同位素标记的支持下，对振动模式的归属表明，镍辅因子的结构与双（1,2 - 二硫烯）镍配合物的结构有相似之处。总体而言，我们的结果表明，电缆细菌进化出了一种独特的辅因子结构，与生物学中任何已知的镍辅因子都不同。
