Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan.
Proc Natl Acad Sci U S A. 2010 May 18;107(20):9382-7. doi: 10.1073/pnas.1001870107. Epub 2010 May 3.
The unique ability of magnetotactic bacteria to navigate along a geomagnetic field is accomplished with the help of prokaryotic organelles, magnetosomes. The magnetosomes have well-ordered chain-like structures, comprising membrane-enveloped, nano-sized magnetic crystals, and various types of specifically associated proteins. In this study, we applied atomic force microscopy (AFM) to investigate the spatial configuration of isolated magnetosomes from Magnetospirillum magneticum AMB-1 in near-native buffer conditions. AFM observation revealed organic material with a approximately 7-nm thickness surrounding a magnetite crystal. Small globular proteins, identified as magnetosome-associated protein MamA, were distributed on the mica surface around the magnetosome. Immuno-labeling with AFM showed that MamA is located on the magnetosome surface. In vitro experiments showed that MamA proteins interact with each other and form a high molecular mass complex. These findings suggest that magnetosomes are covered with MamA oligomers in near-native environments. Furthermore, nanodissection revealed that magnetosomes are built with heterogeneous structures that comprise the organic layer. This study provides important clues to the supramolecular architecture of the bacterial organelle, the magnetosome, and insight into the function of the proteins localized in the organelle.
趋磁细菌沿着地球磁场导航的独特能力是借助于原核细胞器——磁小体来实现的。磁小体具有有序的链状结构，由膜包裹的纳米级磁性晶体和各种类型的特定相关蛋白组成。在这项研究中，我们应用原子力显微镜（AFM）在近天然缓冲条件下研究从趋磁螺菌 AMB-1 中分离出的磁小体的空间结构。AFM 观察显示，在磁铁矿晶体周围有一层约 7nm 厚的有机物质。在云母表面上分布着被鉴定为磁小体相关蛋白 MamA 的小球状蛋白。用 AFM 进行免疫标记显示，MamA 位于磁小体表面。体外实验表明，MamA 蛋白相互作用并形成高分子质量复合物。这些发现表明，在近天然环境中，磁小体被 MamA 低聚物覆盖。此外，纳米解剖显示，磁小体由包含有机层的异构结构组成。这项研究为细菌细胞器——磁小体的超分子结构提供了重要线索，并深入了解定位于细胞器中的蛋白质的功能。
