Department of Plant & Microbial Biology, University of California, Berkeley, CA 94720.
Department of Bacteriology, University of Wisconsin-Madison, Madison, WI 53706.
Proc Natl Acad Sci U S A. 2022 Feb 8;119(6). doi: 10.1073/pnas.2111745119.
Magnetosomes are lipid-bound organelles that direct the biomineralization of magnetic nanoparticles in magnetotactic bacteria. Magnetosome membranes are not uniform in size and can grow in a biomineralization-dependent manner. However, the underlying mechanisms of magnetosome membrane growth regulation remain unclear. Using cryoelectron tomography, we systematically examined mutants with defects at various stages of magnetosome formation to identify factors involved in controlling membrane growth. We found that a conserved serine protease, MamE, plays a key role in magnetosome membrane growth regulation. When the protease activity of MamE is disrupted, magnetosome membrane growth is restricted, which, in turn, limits the size of the magnetite particles. Consistent with this finding, the upstream regulators of MamE protease activity, MamO and MamM, are also required for magnetosome membrane growth. We then used a combination of candidate and comparative proteomics approaches to identify Mms6 and MamD as two MamE substrates. Mms6 does not appear to participate in magnetosome membrane growth. However, in the absence of MamD, magnetosome membranes grow to a larger size than the wild type. Furthermore, when the cleavage of MamD by MamE protease is blocked, magnetosome membrane growth and biomineralization are severely inhibited, phenocopying the MamE protease-inactive mutant. We therefore propose that the growth of magnetosome membranes is controlled by a protease-mediated switch through processing of MamD. Overall, our work shows that, like many eukaryotic systems, bacteria control the growth and size of biominerals by manipulating the physical properties of intracellular organelles.
磁小体是一种脂质结合的细胞器，它指导趋磁细菌中磁性纳米颗粒的生物矿化。磁小体膜的大小不均匀，可以以生物矿化依赖的方式生长。然而，磁小体膜生长调控的潜在机制仍不清楚。使用冷冻电子断层扫描技术，我们系统地检查了在磁小体形成的各个阶段有缺陷的突变体，以鉴定参与控制膜生长的因素。我们发现，一种保守的丝氨酸蛋白酶 MamE 在磁小体膜生长调控中发挥关键作用。当 MamE 的蛋白酶活性被破坏时，磁小体膜的生长受到限制，进而限制了磁铁矿颗粒的大小。这一发现与 MamE 蛋白酶活性的上游调节因子 MamO 和 MamM 对磁小体膜生长的要求一致。然后，我们使用候选蛋白组学和比较蛋白组学方法的组合来鉴定 Mms6 和 MamD 是 MamE 的两个底物。Mms6 似乎不参与磁小体膜的生长。然而，在缺乏 MamD 的情况下，磁小体膜的生长比野生型更大。此外，当 MamE 蛋白酶对 MamD 的切割被阻断时，磁小体膜的生长和生物矿化受到严重抑制，这与 MamE 蛋白酶失活突变体的表型相同。因此，我们提出磁小体膜的生长是通过 MamD 的蛋白酶介导的切割来控制的。总的来说，我们的工作表明，与许多真核系统一样，细菌通过操纵细胞内细胞器的物理性质来控制生物矿物质的生长和大小。
