Cornejo Elias, Subramanian Poorna, Li Zhuo, Jensen Grant J, Komeili Arash
Department of Plant and Microbial Biology, University of California, Berkeley, California, USA.
Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California, USA.
mBio. 2016 Feb 16;7(1):e01898-15. doi: 10.1128/mBio.01898-15.
Magnetotactic bacteria produce chains of membrane-bound organelles that direct the biomineralization of magnetic nanoparticles. These magnetosome compartments are a model for studying the biogenesis and subcellular organization of bacterial organelles. Previous studies have suggested that discrete gene products build and assemble magnetosomes in a stepwise fashion. Here, using an inducible system, we show that the stages of magnetosome formation are highly dynamic and interconnected. During de novo formation, magnetosomes first organize into discontinuous chain fragments that are subsequently connected by the bacterial actin-like protein MamK. We also find that magnetosome membranes are not uniform in size and can grow in a biomineralization-dependent manner. In the absence of biomineralization, magnetosome membranes stall at a diameter of ~50 nm. Those that have initiated biomineralization then expand to significantly larger sizes and accommodate mature magnetic particles. We speculate that such a biomineralization-dependent checkpoint for membrane growth establishes the appropriate conditions within the magnetosome to ensure successful nucleation and growth of magnetic particles.
Magnetotactic bacteria make magnetic nanoparticles inside membrane-bound organelles called magnetosomes; however, it is unclear how the magnetosome membrane controls the biomineralization that occurs within this bacterial organelle. We placed magnetosome formation under inducible control in Magnetospirillum magneticum AMB-1 and used electron cryo-tomography to capture magnetosomes in their near-native state as they form de novo. An inducible system provided the key evidence that magnetosome membranes grow continuously unless they have not properly initiated biomineralization. Our finding that the size of a bacterial organelle impacts its biochemical function is a fundamental advance that impacts our perception of organelle formation and can inform future attempts aimed at creating designer magnetic particles.
趋磁细菌产生膜结合细胞器链，这些细胞器链指导磁性纳米颗粒的生物矿化。这些磁小体隔室是研究细菌细胞器生物发生和亚细胞组织的模型。先前的研究表明，离散的基因产物以逐步的方式构建和组装磁小体。在这里，我们使用诱导系统表明，磁小体形成的各个阶段是高度动态且相互关联的。在从头形成过程中，磁小体首先组织成不连续的链片段，随后由细菌肌动蛋白样蛋白MamK连接起来。我们还发现磁小体膜的大小并不均匀，并且可以以生物矿化依赖的方式生长。在没有生物矿化的情况下，磁小体膜停滞在直径约50纳米。那些已经开始生物矿化的磁小体膜随后会扩展到明显更大的尺寸，并容纳成熟的磁性颗粒。我们推测，这种膜生长的生物矿化依赖检查点在磁小体内建立了适当的条件，以确保磁性颗粒的成功成核和生长。
趋磁细菌在称为磁小体的膜结合细胞器内制造磁性纳米颗粒；然而，尚不清楚磁小体膜如何控制这种细菌细胞器内发生的生物矿化。我们将磁小体的形成置于嗜磁螺菌AMB-1的诱导控制之下，并使用电子冷冻断层扫描技术在磁小体从头形成时以接近天然的状态捕获它们。一个诱导系统提供了关键证据，表明磁小体膜会持续生长，除非它们没有正确启动生物矿化。我们发现细菌细胞器的大小会影响其生化功能，这是一项根本性的进展，影响了我们对细胞器形成的认识，并可为未来旨在制造定制磁性颗粒的尝试提供信息。
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