Natarajan Janani, Moitra Anasuya, Zabel Sussanne, Singh Nidhi, Wagner Samuel, Rapaport Doron
Interfaculty Institute of Biochemistry, University of Tübingen, Tübingen, Germany.
Current address: Center for Bioinformatics (ZBIT), University of Tübingen, Tübingen, Germany.
Microb Cell. 2019 Nov 19;7(1):15-27. doi: 10.15698/mic2020.01.703.
Secretins form large multimeric pores in the outer membrane (OM) of Gram-negative bacteria. These pores are part of type II and III secretion systems (T2SS and T3SS, respectively) and are crucial for pathogenicity. Recent structural studies indicate that secretins form a structure rich in β-strands. However, little is known about the mechanism by which secretins assemble into the OM. Based on the conservation of the biogenesis of β-barrel proteins in bacteria and mitochondria, we used yeast cells as a model system to study the assembly process of secretins. To that end, we analyzed the biogenesis of PulD (T2SS), SsaC (T3SS) and InvG (T3SS) in wild type cells or in cells mutated for known mitochondrial import and assembly factors. Our results suggest that secretins can be expressed in yeast cells, where they are enriched in the mitochondrial fraction. Interestingly, deletion of mitochondrial import receptors like Tom20 and Tom70 reduces the mitochondrial association of PulD but does not affect that of InvG. SsaC shows another dependency pattern and its membrane assembly is enhanced by the absence of Tom70 and compromised in cells lacking Tom20 or the topogenesis of outer membrane β-barrel proteins (TOB) complex component, Mas37. Collectively, these findings suggest that various secretins can follow different pathways to assemble into the bacterial OM.
分泌素在革兰氏阴性菌的外膜中形成大型多聚体孔道。这些孔道分别是II型和III型分泌系统（分别为T2SS和T3SS）的一部分，对致病性至关重要。最近的结构研究表明，分泌素形成富含β链的结构。然而，关于分泌素组装到外膜中的机制知之甚少。基于细菌和线粒体中β桶蛋白生物合成的保守性，我们以酵母细胞为模型系统来研究分泌素的组装过程。为此，我们分析了野生型细胞或已知线粒体导入和组装因子发生突变的细胞中PulD（T2SS）、SsaC（T3SS）和InvG（T3SS）的生物合成。我们的结果表明，分泌素可以在酵母细胞中表达，并富集在线粒体部分。有趣的是，缺失线粒体导入受体如Tom20和Tom70会降低PulD与线粒体的结合，但不影响InvG。SsaC表现出另一种依赖模式，其膜组装在缺失Tom70时增强，而在缺乏Tom20或外膜β桶蛋白（TOB）复合体组分Mas37的细胞中受损。总的来说，这些发现表明，各种分泌素可以通过不同途径组装到细菌外膜中。
