Murray David S, Schumacher Maria A, Brennan Richard G
Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon 97239-3098, USA.
J Biol Chem. 2004 Apr 2;279(14):14365-71. doi: 10.1074/jbc.M313870200. Epub 2004 Jan 15.
The Staphylococcus aureus multidrug-binding protein QacR represses transcription of the plasmid-encoded membrane protein QacA, a multidrug efflux transporter. QacR is induced by multiple structurally dissimilar monovalent and bivalent cationic lipophilic compounds, many of which are effluxed from the cell by QacA via the proton motive force. The multidrug-binding pocket of QacR has been shown to be quite extensive and features several glutamates and multiple aromatic residues. To date, the structure of only one QacR-bivalent cationic drug complex (that of QacR bound to dequalinium) has been determined, and how other longer or shorter bivalent cationic compounds bind is unknown. Here we report the crystal structures of QacR bound to two cytotoxic bivalent diamidines, pentamidine and hexamidine. These compounds are structurally similar, differing by only one methylene carbon in the alkyl chain linker. However, this small difference results in very dissimilar binding modes. Similar to dequalinium, hexamidine spans the multidrug-binding pocket, and its positively charged benzamidine groups are neutralized by residues Glu-57 and Glu-120. Pentamidine binds QacR in a novel fashion whereby one of its benzamidine groups interacts with residue Glu-63, and the other is neutralized by carbonyl and side chain oxygen atoms. Thus, these structures demonstrate that a formal negative charge is not a prerequisite for binding positively charged drugs and underscore the versatility of the QacR and, likely, all multidrug-binding pockets.
金黄色葡萄球菌多药结合蛋白QacR可抑制质粒编码的膜蛋白QacA（一种多药外排转运蛋白）的转录。QacR可被多种结构不同的单价和二价阳离子亲脂性化合物诱导，其中许多化合物可通过质子动力由QacA从细胞中排出。已证明QacR的多药结合口袋相当广泛，具有多个谷氨酸和多个芳香族残基。迄今为止，仅确定了一种QacR-二价阳离子药物复合物（QacR与地喹氯铵结合的复合物）的结构，而其他更长或更短的二价阳离子化合物的结合方式尚不清楚。在此，我们报道了QacR与两种细胞毒性二价双脒（喷他脒和己脒）结合的晶体结构。这些化合物在结构上相似，仅在烷基链连接体中相差一个亚甲基碳。然而，这种微小差异导致了非常不同的结合模式。与地喹氯铵类似，己脒横跨多药结合口袋，其带正电荷的苯甲脒基团被Glu-57和Glu-120残基中和。喷他脒以一种新的方式与QacR结合，其中一个苯甲脒基团与Glu-63残基相互作用，另一个则被羰基和侧链氧原子中和。因此，这些结构表明，形式上的负电荷并非结合带正电荷药物的先决条件，并突出了QacR以及可能所有多药结合口袋的多功能性。
