Sulfide:quinone oxidoreductase (SQR) is a monotopic membrane flavoprotein present in all domains of life, with multiple roles including sulfide detoxification, homeostasis and energy generation by providing electrons to respiratory or photosynthetic electron transport chains. A type III SQR from the hyperthermophilic archeon Caldivirga maquilingensis has been previously characterized, and its C-terminal amphipathic helices were demonstrated to be responsible for membrane binding. Here, the oligomeric state of this protein was experimentally evaluated by size exclusion chromatography, native gels and crosslinking, and found to be a monomer-dimer-trimer equilibrium. Remarkably, mutant and truncated variants unable to bind to the membrane are able to maintain their oligomeric association. Thus, unlike other related monotopic membrane proteins, the region involved in membrane binding does not influence oligomerization. Furthermore, by studying heterodimers between the WT and mutants, it was concluded that membrane binding requires an oligomer with at least two copies of the protein with intact C-terminal amphipathic helices. A structural homology model of the C. maquilingensis SQR was used to define the flavin- and quinone-binding sites. CmGly12, CmGly16, CmAla77 and CmPro44 were determined to be important for flavin binding. Unexpectedly, CmGly299 is only important for quinone reduction despite its proximity to bound FAD. CmPhe337 and CmPhe362 are also important for quinone binding apparently by direct interaction with the quinone ring, whereas CmLys359, postulated to hydrogen bond to the quinone, seems to have a more structural role. The results presented differentiate the Type III CmSQR from some of its counterparts classified as Type I, II and V.

中文翻译：

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膜结合需要Caldivirga maquilingensis III型硫化物：奎农酮氧化还原酶的低聚状态。

硫化物：醌氧化还原酶（SQR）是一种存在于生活所有领域的单分子膜黄素蛋白，具有多种作用，包括通过向呼吸或光合作用的电子传输链提供电子来实现硫化物的解毒，体内平衡和能量产生。先前已经对来自超嗜热古细菌Caldivirga maquilingensis的III型SQR进行了表征，并证明了其C末端两亲螺旋负责膜结合。在此，通过尺寸排阻色谱，天然凝胶和交联实验评估了该蛋白质的低聚状态，发现其为单体-二聚体-三聚体平衡。显着地，不能结合到膜的突变体和截短的变体能够维持其寡聚缔合。因此，与其他相关的单分子膜蛋白不同，参与膜结合的区域不影响寡聚。此外，通过研究野生型和突变体之间的异二聚体，可以得出结论，膜结合需要具有至少两个拷贝的具有完整C末端两亲性螺旋的蛋白质的寡聚体。使用马氏梭菌SQR的结构同源性模型来确定黄素和醌的结合位点。确定CmGly12，CmGly16，CmAla77和CmPro44对黄素结合很重要。出乎意料的是，尽管CmGly299与结合的FAD接近，但它仅对减少醌很重要。CmPhe337和CmPhe362对醌的结合也很重要，显然是通过与醌环的直接相互作用而实现的，而CmLys359（假定与醌氢键结合）似乎具有更强的结构性作用。