Department of Life Sciences, Ben Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel.
Proc Natl Acad Sci U S A. 2011 Aug 16;108(33):E480-7. doi: 10.1073/pnas.1103367108. Epub 2011 Jul 22.
The magnetosome, a biomineralizing organelle within magnetotactic bacteria, allows their navigation along geomagnetic fields. Magnetosomes are membrane-bound compartments containing magnetic nanoparticles and organized into a chain within the cell, the assembly and biomineralization of magnetosomes are controlled by magnetosome-associated proteins. Here, we describe the crystal structures of the magnetosome-associated protein, MamA, from Magnetospirillum magneticum AMB-1 and Magnetospirillum gryphiswaldense MSR-1. MamA folds as a sequential tetra-trico-peptide repeat (TPR) protein with a unique hook-like shape. Analysis of the MamA structures indicates two distinct domains that can undergo conformational changes. Furthermore, structural analysis of seven crystal forms verified that the core of MamA is not affected by crystallization conditions and identified three protein-protein interaction sites, namely a concave site, a convex site, and a putative TPR repeat. Additionally, relying on transmission electron microscopy and size exclusion chromatography, we show that highly stable complexes form upon MamA homooligomerization. Disruption of the MamA putative TPR motif or N-terminal domain led to protein mislocalization in vivo and prevented MamA oligomerization in vitro. We, therefore, propose that MamA self-assembles through its putative TPR motif and its concave site to create a large homooligomeric scaffold which can interact with other magnetosome-associated proteins via the MamA convex site. We discuss the structural basis for TPR homooligomerization that allows the proper function of a prokaryotic organelle.
磁小体是趋磁细菌内的一种生物矿化细胞器，使它们能够沿着地磁场导航。磁小体是一种膜结合的隔室，包含磁性纳米颗粒，并在细胞内排列成链状，磁小体的组装和生物矿化受磁小体相关蛋白的控制。在这里，我们描述了趋磁螺菌 AMB-1 和格氏嗜甲基菌 MSR-1 中的磁小体相关蛋白 MamA 的晶体结构。MamA 折叠为具有独特钩状形状的连续四三肽重复（TPR）蛋白。MamA 结构分析表明存在两个可以发生构象变化的不同结构域。此外，对七种晶体形式的结构分析证实，MamA 的核心不受结晶条件的影响，并确定了三个蛋白质-蛋白质相互作用位点，即凹面位点、凸面位点和假定的 TPR 重复。此外，基于透射电子显微镜和尺寸排阻色谱，我们表明 MamA 同源寡聚体形成高度稳定的复合物。破坏 MamA 假定的 TPR 模体或 N 端结构域导致体内蛋白质定位错误，并阻止 MamA 在体外寡聚化。因此，我们提出 MamA 通过其假定的 TPR 模体及其凹面位点自组装，形成一个可以通过 MamA 凸面位点与其他磁小体相关蛋白相互作用的大型同源寡聚体支架。我们讨论了允许原核细胞器正常功能的 TPR 同源寡聚化的结构基础。
