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

Na^+/H^+交换器（NHE，SLC 9家族）是一种电中性转运蛋白，能催化Na^+和H^+在多种动物细胞质膜或其他细胞器膜上的交换。我们发现，钙调磷酸酶B同源蛋白（CHP，其普遍存在的异构体CHP 1）的功能作为一个重要的辅因子，以支持通过与质膜型交换器异构体NHE 1 -5的质膜结构域的生理活性。此外，另一种亚型CHP 2在细胞恶性转化时表达，并在维持癌细胞的高细胞内pH中起作用。我们还分析了CHP的Ca^<2+>结合位点的作用，发现在与NHE 1相互作用时，有两个Ca^<2+>离子仍然结合在CHP的EF-手结合位点上，对维持NHE 1的生理功能起着重要作用。除了H^+转运位点外，H^+调节位点也被认为是NHE 1调节pH的重要位点。我们通过测量NHE 1的反向反应模式（^ Na^+从细胞中流出），发现了NHE 1中额外的H^+结合位点<22>。我们还鉴定了NHE 1中对pH传感器调节重要的关键氨基酸残基（Arg 440，Gly 455，Gly 456）。

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.