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

Na^+/H^+交换剂（NHEs， SLC9家族）是电中性转运体，在各种动物的质膜或其他胞内细胞器膜中催化Na^+和H^+的交换。我们发现钙调磷酸酶B同源蛋白（CHP，其普遍存在的异构体CHP1）通过与质膜型交换器异构体NHE1-5的近膜结构域相互作用，作为支持生理活性的重要辅助因子。此外，另一种异构体CHP2在细胞恶性转化时表达，并在癌细胞中维持高细胞内pH值。我们还分析了CHP的Ca^<2+>-结合位点的作用，得出两个Ca^<2+>离子在细胞膜上与NHE1相互作用时，仍然结合在CHP的EF-hand结合位点上，对维持NHE1的生理功能起重要作用。除了H^+转运位点外，H^+调控位点也被认为是NHE1调控pH的重要位点。我们通过测量NHE1的反向反应（^<22>Na^+从细胞中流出），在NHE1中发现了这些额外的H^+结合位点。我们还鉴定了NHE1中对ph传感器调节重要的关键氨基酸残基（Arg440, Gly455, Gly456）。

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.