Structural modeling of an outer membrane electron conduit from a metal-reducing bacterium suggests electron transfer via periplasmic redox partners.
Structural modeling of an outer membrane electron conduit from a metal-reducing bacterium suggests electron transfer via periplasmic redox partners.
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DOI：
10.1074/jbc.ra118.001850
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发表时间：
2018-05-25
期刊：
影响因子：
--
通讯作者：
Clarke TA
中科院分区：
文献类型：
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作者：
Edwards MJ;White GF;Lockwood CW;Lawes MC;Martel A;Harris G;Scott DJ;Richardson DJ;Butt JN;Clarke TA
许多地下微生物将它们的代谢与细胞外基质的还原或氧化偶联。例如，厌氧矿物呼吸细菌可以在呼吸过程中使用外部金属氧化物作为末端电子受体。孔蛋白-细胞色素复合物促进通过细胞内分解代谢过程产生的电子穿过细菌外膜移动到这些末端电子受体。在矿物质还原模型细菌Shewanella oneidensis MR-1中，该复合物由两个十血红素细胞色素（MtrA和MtrC）和外膜β-桶（MtrB）组成。然而，孔蛋白-细胞色素复合物传递电子的结构和机制尚不清楚。在这里，我们使用小角中子散射（SANS）研究跨膜复合物MtrAB和MtrCAB的分子结构。对散射数据进行从头算模拟，得到MtrAB的分子包络尺寸为105 × 60 × 35 nm，MtrCAB的分子包络尺寸为170 × 60 × 45 nm。这些分子包膜的形状表明，MtrC与MtrAB的表面相互作用，从膜表面延伸约70 nm，并允许末端血红素与MtrAB和细胞外受体相互作用。数据还显示，MtrA完全延伸通过MtrB的长度，其中1030 μ m暴露在周质中。含有膜相关的MtrCAB和内化的小四血红素细胞色素（STC）的蛋白脂质体模型表明，MtrCAB可以减少铁（III）柠檬酸与STC作为电子供体，揭示了MtrCAB和STC之间的直接相互作用。两者合计，结构和蛋白脂质体实验支持孔蛋白细胞色素介导的电子传递通过周质细胞色素，如STC。
Many subsurface microorganisms couple their metabolism to the reduction or oxidation of extracellular substrates. For example, anaerobic mineral-respiring bacteria can use external metal oxides as terminal electron acceptors during respiration. Porin–cytochrome complexes facilitate the movement of electrons generated through intracellular catabolic processes across the bacterial outer membrane to these terminal electron acceptors. In the mineral-reducing model bacterium Shewanella oneidensis MR-1, this complex is composed of two decaheme cytochromes (MtrA and MtrC) and an outer-membrane β-barrel (MtrB). However, the structures and mechanisms by which porin–cytochrome complexes transfer electrons are unknown. Here, we used small-angle neutron scattering (SANS) to study the molecular structure of the transmembrane complexes MtrAB and MtrCAB. Ab initio modeling of the scattering data yielded a molecular envelope with dimensions of ∼105 × 60 × 35 Å for MtrAB and ∼170 × 60 × 45 Å for MtrCAB. The shapes of these molecular envelopes suggested that MtrC interacts with the surface of MtrAB, extending ∼70 Å from the membrane surface and allowing the terminal hemes to interact with both MtrAB and an extracellular acceptor. The data also reveal that MtrA fully extends through the length of MtrB, with ∼30 Å being exposed into the periplasm. Proteoliposome models containing membrane-associated MtrCAB and internalized small tetraheme cytochrome (STC) indicate that MtrCAB could reduce Fe(III) citrate with STC as an electron donor, disclosing a direct interaction between MtrCAB and STC. Taken together, both structural and proteoliposome experiments support porin–cytochrome–mediated electron transfer via periplasmic cytochromes such as STC.