Department of Structural Biology, Max Planck Institute of Biophysics, Frankfurt, Germany.
Molecular Microbiology and Bioenergetics, Institute of Molecular Biosciences, Goethe University Frankfurt, Frankfurt, Germany.
Elife. 2017 Dec 27;6:e30483. doi: 10.7554/eLife.30483.
Secretins form multimeric channels across the outer membrane of Gram-negative bacteria that mediate the import or export of substrates and/or extrusion of type IV pili. The secretin complex of is an oligomer of the 757-residue PilQ protein, essential for DNA uptake and pilus extrusion. Here, we present the cryo-EM structure of this bifunctional complex at a resolution of ~7 Å using a new reconstruction protocol. Thirteen protomers form a large periplasmic domain of six stacked rings and a secretin domain in the outer membrane. A homology model of the PilQ protein was fitted into the cryo-EM map. A crown-like structure outside the outer membrane capping the secretin was found not to be part of PilQ. Mutations in the secretin domain disrupted the crown and abolished DNA uptake, suggesting a central role of the crown in natural transformation.
分泌孔蛋白在革兰氏阴性细菌的外膜上形成多聚体通道，介导底物的输入或输出和/或 IV 型菌毛的外排。的分泌孔复合物是由 757 个残基的 PilQ 蛋白组成的寡聚体，对于 DNA 的摄取和菌毛的外排是必不可少的。在这里，我们使用新的重建方案，以约 7 Å 的分辨率呈现了这种双功能复合物的冷冻电镜结构。十三个原聚体形成了一个大的周质域，由六个堆叠的环和一个外膜中的分泌孔域组成。将 PilQ 蛋白的同源模型拟合到冷冻电镜图谱中。在外膜外发现一个冠状结构，覆盖在分泌孔上方，但它不是 PilQ 的一部分。分泌孔域的突变破坏了冠状结构并阻止了 DNA 的摄取，这表明冠状结构在自然转化中起着核心作用。
