Katzmann Emanuel, Scheffel André, Gruska Manuela, Plitzko Jürgen M, Schüler Dirk
Ludwig-Maximillians-Universität München, Department Biologie I, Bereich Mikrobiologie, Biozentrum der LMU, D-82152 Martinsried, Germany.
Mol Microbiol. 2010 Jul 1;77(1):208-24. doi: 10.1111/j.1365-2958.2010.07202.x. Epub 2010 May 12.
Magnetotactic bacteria synthesize magnetosomes, which are unique organelles consisting of membrane-enclosed magnetite crystals. For magnetic orientation individual magnetosome particles are assembled into well-organized chains. The actin-like MamK and the acidic MamJ proteins were previously implicated in chain assembly. While MamK was suggested to form magnetosome-associated cytoskeletal filaments, MamJ is assumed to attach the magnetosome vesicles to these structures. Although the deletion of either mamK in Magnetospirillum magneticum, or mamJ in Magnetospirillum gryphiswaldense affected chain formation, the previously observed phenotypes were not fully consistent, suggesting different mechanisms of magnetosome chain assembly in both organisms. Here we show that in M. gryphiswaldense MamK is not absolutely required for chain formation. Straight chains, albeit shorter, fragmented and ectopic, were still formed in a mamK deletion mutant, although magnetosome filaments were absent as shown by cryo-electron tomography. Loss of MamK also resulted in reduced numbers of magnetite crystals and magnetosome vesicles and led to the mislocalization of MamJ. In addition, extensive analysis of wild type and mutant cells revealed previously unidentified ultrastructural characteristics in M. gryphiswaldense. Our results suggest that, despite of their functional equivalence, loss of MamK proteins in different bacteria may result in distinct phenotypes, which might be due to a species-specific genetic context.
趋磁细菌合成磁小体，磁小体是由膜包裹的磁铁矿晶体组成的独特细胞器。为了进行磁定向，单个磁小体颗粒组装成排列有序的链。肌动蛋白样蛋白MamK和酸性蛋白MamJ先前被认为与链组装有关。虽然有人提出MamK形成与磁小体相关的细胞骨架丝，但MamJ被认为将磁小体囊泡附着到这些结构上。尽管在嗜磁螺菌中删除mamK或在格氏嗜盐碱螺旋菌中删除mamJ都会影响链的形成，但先前观察到的表型并不完全一致，这表明两种生物体中磁小体链组装的机制不同。在这里，我们表明在格氏嗜盐碱螺旋菌中，链的形成并非绝对需要MamK。在mamK缺失突变体中仍然形成了直链，尽管较短、有片段且位置异常，尽管冷冻电子断层扫描显示不存在磁小体丝。MamK的缺失还导致磁铁矿晶体和磁小体囊泡数量减少，并导致MamJ的定位错误。此外，对野生型和突变体细胞的广泛分析揭示了格氏嗜盐碱螺旋菌中以前未发现的超微结构特征。我们的结果表明，尽管它们功能等效，但不同细菌中MamK蛋白的缺失可能导致不同的表型，这可能是由于物种特异性的遗传背景所致。
