Kowata Hikaru, Tochigi Saeko, Takahashi Hideyuki, Kojima Seiji
Graduate School of Life Sciences, Tohoku University, Sendai, Japan.
Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, Japan.
J Bacteriol. 2017 Sep 5;199(19). doi: 10.1128/JB.00371-17. Print 2017 Oct 1.
The outer membrane of heterotrophic Gram-negative bacteria plays the role of a selective permeability barrier that prevents the influx of toxic compounds while allowing the nonspecific passage of small hydrophilic nutrients through porin channels. Compared with heterotrophic Gram-negative bacteria, the outer membrane properties of cyanobacteria, which are Gram-negative photoautotrophs, are not clearly understood. In this study, using small carbohydrates, amino acids, and inorganic ions as permeation probes, we determined the outer membrane permeability of sp. strain PCC 6803 in intact cells and in proteoliposomes reconstituted with outer membrane proteins. The permeability of this cyanobacterium was >20-fold lower than that of The predominant outer membrane proteins Slr1841, Slr1908, and Slr0042 were not permeable to organic nutrients and allowed only the passage of inorganic ions. Only the less abundant outer membrane protein Slr1270, a homolog of the export channel TolC, was permeable to organic solutes. The activity of Slr1270 as a channel was verified in a recombinant Slr1270-producing outer membrane. The lack of putative porins and the low outer membrane permeability appear to suit the cyanobacterial autotrophic lifestyle; the highly impermeable outer membrane would be advantageous to cellular survival by protecting the cell from toxic compounds, especially when the cellular physiology is not dependent on the uptake of organic nutrients. Because the outer membrane of Gram-negative bacteria affects the flux rates for various substances into and out of the cell, its permeability is closely associated with cellular physiology. The outer membrane properties of cyanobacteria, which are photoautotrophic Gram-negative bacteria, are not clearly understood. Here, we examined the outer membrane of sp. strain PCC 6803. We revealed that it is relatively permeable to inorganic ions but is markedly less permeable to organic nutrients, with >20-fold lower permeability than the outer membrane of Such permeability appears to fit the cyanobacterial lifestyle, in which the diffusion pathway for inorganic solutes may suffice to sustain the autotrophic physiology, illustrating a link between outer membrane permeability and the cellular lifestyle.
异养革兰氏阴性菌的外膜起着选择性渗透屏障的作用，可阻止有毒化合物的流入，同时允许小的亲水性营养物质通过孔蛋白通道非特异性通过。与异养革兰氏阴性菌相比，作为革兰氏阴性光合自养生物的蓝细菌的外膜特性尚不清楚。在本研究中，我们使用小碳水化合物、氨基酸和无机离子作为渗透探针，测定了集胞藻属PCC 6803菌株在完整细胞和用外膜蛋白重构的蛋白脂质体中的外膜通透性。这种蓝细菌的通透性比大肠杆菌低20倍以上。主要的外膜蛋白Slr1841、Slr1908和Slr0042对有机营养物不可渗透，仅允许无机离子通过。只有含量较少的外膜蛋白Slr1270（一种与输出通道TolC同源的蛋白）对有机溶质具有通透性。Slr1270作为通道的活性在产生重组Slr1270的外膜中得到了验证。缺乏假定的孔蛋白和较低的外膜通透性似乎适合蓝细菌的自养生活方式；高度不可渗透的外膜通过保护细胞免受有毒化合物的侵害，对细胞存活有利，尤其是当细胞生理不依赖于有机营养物的摄取时。由于革兰氏阴性菌的外膜影响各种物质进出细胞的通量率，其通透性与细胞生理密切相关。作为光合自养革兰氏阴性菌的蓝细菌的外膜特性尚不清楚。在此，我们研究了集胞藻属PCC 6803菌株的外膜。我们发现它对无机离子相对通透，但对有机营养物的通透性明显较低，比大肠杆菌外膜的通透性低2倍以上。这种通透性似乎适合蓝细菌的生活方式，其中无机溶质的扩散途径可能足以维持自养生理，说明了外膜通透性与细胞生活方式之间的联系。
