CcsBA is a cytochrome c synthetase that also functions in heme transport
CcsBA is a cytochrome c synthetase that also functions in heme transport
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DOI：
10.1073/pnas.0903132106
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发表时间：
2009-06-23
影响因子：
11.1
通讯作者：
Kranz, Robert G.
中科院分区：
文献类型：
--
作者：
Frawley, Elaine R.;Kranz, Robert G.
关于血红素从其合成位点运输到血红素蛋白组装位点的情况知之甚少。我们描述了一个完整的膜蛋白，允许捕获内源性血红素阐明贩运机制。我们发现，CcsBA，一个代表的一个超家族的整体膜蛋白参与细胞色素c的生物合成，出口和保护血红素氧化。CcsBA具有10个跨膜结构域（TMD），并在大肠杆菌周质中重建细胞色素c合成;因此，CcsBA是细胞色素c合成酶。纯化的CcsBA在“外部血红素结合结构域”中含有血红素，其中两个外部组氨酸被示出作为轴向配体，其保护血红素铁免受氧化。这可能是合成酶的活性位点。此外，TMDs中的两个保守组氨酸是血红素前往外部血红素结合结构域所必需的。值得注意的是，CcsBA的功能与突变，这些TMD组氨酸是纠正外源性咪唑，结果类似于纠正血红素结合肌红蛋白时，其近端组氨酸突变。这些数据表明，CcsBA的双层内有一个血红素结合位点，CcsBA是一个血红素通道。
Little is known about trafficking of heme from its sites of synthesis to sites of heme-protein assembly. We describe an integral membrane protein that allows trapping of endogenous heme to elucidate trafficking mechanisms. We show that CcsBA, a representative of a superfamily of integral membrane proteins involved in cytochrome c biosynthesis, exports and protects heme from oxidation. CcsBA has 10 transmembrane domains (TMDs) and reconstitutes cytochrome c synthesis in the Escherichia coli periplasm; thus, CcsBA is a cytochrome c synthetase. Purified CcsBA contains heme in an "external heme binding domain'' for which two external histidines are shown to serve as axial ligands that protect the heme iron from oxidation. This is likely the active site of the synthetase. Furthermore, two conserved histidines in TMDs are required for heme to travel to the external heme binding domain. Remarkably, the function of CcsBA with mutations in these TMD histidines is corrected by exogenous imidazole, a result analogous to correction of heme binding by myoglobin when its proximal histidine is mutated. These data suggest that CcsBA has a heme binding site within the bilayer and that CcsBA is a heme channel.