Molecular Basis for Differential Anion Binding and Proton Coupling in the CI-/H+ Exchanger CIC-ec1
Molecular Basis for Differential Anion Binding and Proton Coupling in the CI-/H+ Exchanger CIC-ec1
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DOI：
10.1021/jacs.5b12062
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发表时间：
2016-03-09
影响因子：
15
通讯作者：
Tajkhorshid, Emad
中科院分区：
文献类型：
--
作者：
Jiang, Tao;Han, Wei;Tajkhorshid, Emad
ClC超家族的CL-/H+转运蛋白是一类普遍存在的膜蛋白，它催化化学计量比耦合的氯和H+跨生物膜交换。CLC转运体将H+交换为卤化物和某些多原子阴离子，但不包括阳离子、F-和较大的生理阴离子，如PO43-和SO42--尽管不同阴离子的转运率相似，但CLC转运体中的H+偶联程度因转运物的化学性质而有很大差异。尽管交换的分子机制尚不清楚，但对细菌ClC-ECL转运体的研究表明，氯与中心阴离子结合部位(S-CEN)的结合是阴离子偶联H+转运的关键。在这里，我们发现，Cl-、F-、NO3-和SCN-在SCEN位置显示出不同的结合配位，并且以不同的方式水化。与差异结合的观察结果一致，CIC-EC1在不同的阴离子存在下，表现出明显不同的能力来支持瞬时水线的形成，这是支持两个H+转移位点(Glu(In)和Glu(EX))连接所必需的。而在生理输送的氯离子存在的情况下，经常观察到连续的水线。F-或NO3-的结合导致形成与用Cl-形成的导线实质上不同的假水线。然而，SCN-的结合完全消除了水线。这些发现提供了CIC-ECL中阴离子结合的结构细节，并揭示了H+传输与阴离子传输(而不是Cl-)传输解耦的原子水平机制。
Cl-/H+ transporters of the CLC superfamily form a ubiquitous class of membrane proteins that catalyze stoichiometrically coupled exchange of Cl- and H+ across biological membranes. CLC transporters exchange H+ for halides and certain polyatomic anions, but exclude cations, F-, and larger physiological anions, such as PO43- and SO42- Despite comparable transport rates of different anions, the H+ coupling in CLC transporters varies significantly depending on the chemical nature of the transported anion. Although the molecular mechanism of exchange remains unknown, studies on bacterial ClC-ecl transporter revealed that Cl- binding to the central anion-binding site (S-cen) is crucial for the anion-coupled H+ transport. Here, we show that Cl-, F-, NO3-, and SCN- display distinct binding coordinations at the Scen site and are hydrated in different manners. Consistent with the observation of differential bindings, CIC-ec1 exhibits markedly variable ability to support the formation of the transient water wires, which are necessary to support the connection of the two H+ transfer sites (Glu(in) and Glu(ex)), in the presence of different anions. While continuous water wires are frequently observed in the presence of physiologically transported Cl-. binding of F- or NO3- leads to the formation of pseudo-water-wires that are substantially different from the wires formed with Cl-. Binding of SCN-, however, eliminates the water wires altogether. These findings provide structural details of anion binding in CIC-ecl and reveal a putative atomic-level mechanism for the decoupling of H+ transport to the transport of anions other than Cl-.