The Na+/H+ exchanger as a proton regulatory system : its molecular structure and transport mechanism

Na~+/H~+交换器(NHEs，SLC9家族)是多种动物体内催化质膜或细胞内细胞膜Na~(++)和H~(++)交换的电子转运体。我们发现，钙调神经磷酸酶B同源蛋白(CHP，其普遍存在的异构体CHP1)通过与质膜型交换异构体NHE1-5的膜旁结构域相互作用，发挥着支持生理活性的重要辅助因子的作用。此外，另一种异构体CHP2在细胞恶性转化时表达，并在维持癌细胞内高pH的过程中发挥作用。我们还分析了CHP的Ca~(2+)与Gt~(2+)结合部位的作用，得出结论：当CHP与NHE1在细胞膜上相互作用时，仍有两个Ca~(2+)~(2+)&Gt~(2+)离子与CHP的EF-手性结合部位结合，并在维持NHE1的生理功能中发挥重要作用。我们通过测量NHE1(^&lt；22&gt；Na+从细胞外流)的反向模式反应，在NHE1中发现了这样的额外的H~+结合位点。我们还鉴定了NHE1的关键氨基酸残基(Arg440、Gly455、Gly456)，这些氨基酸对调节pH传感器很重要。

The Na^+/H^+ exchanger (NHEs, SLC9 family) are electroneutral transporters that catalyze the exchange of Na^+ and H^+ in plasma membranes or other intracellular organellar membranes in various animal species. We discovered that calcineurin B homologous protein (CHP, its ubiquitous isoform CHP1) functions as an essential cofactor to support the physiological activity by interacting with the juxtamembrane domain of the plasma membrane-type exchanger isoforms NHE1-5. Furthermore, another isoform CHP2 becomes expressed when cells are malignantly transformed and plays a role in maintaining high intracellular pH in cancer cells. We also analyzed the role of Ca^<2+>-binding sites of CHP and concluded that two Ca^<2+> ions remain to be bound to EF-hand binding sites of CHP when interacts with NHE1 in cell membranes and play an important role in maintaining the physiological function of NHE1.In addition to H^+-transport site, H^+-regulatory site have been recognized to be important for pH regulation by NHE1. We discovered such additional H^+-binding sites in NHE1 by measuring the reverse mode reaction of NHE1 (^<22>Na^+ efflux from the cells). We also identified critical amino acid residues (Arg440, Gly455, Gly456) in NHE1 important for regulation of pH-sensor.