Topological analysis of the Na+/H+ exchanger.

The mammalian Na+/H+ exchanger isoform 1 (NHE1) is a ubiquitously expressed integral membrane protein present in mammalian cells. It is made up of a hydrophobic 500 amino acid membrane domain that transports and removes protons from within cells, and a regulatory intracellular cytosolic domain made of approximately 315 amino acids. Determining the structure of NHE1 is critical for both an understanding of the Na+/H+ exchange mechanism of transport, and in the design of new improved inhibitors for use in treatment of several diseases in which it is involved. Differing models of the NHE1 protein have been proposed. The first model suggested by two groups proposes that amino acids 1-500 form a 12 transmembrane segment spanning region in which amino acids 1-127 form two transmembrane segments, and amino acids 315-411 form a single transmembrane segment with membrane associated segments. A second model based on the structure of the Escherichia coli Na+/H+ exchanger protein proposes an overall similar topology, but suggests amino acids 1-127 are removed as a signal sequence and are not present in the mature protein. It also suggests a different topology of amino acids 315-411 to form three transmembrane segments. We used cysteine scanning accessibility and examination of glycosylation of the mature protein to characterize the NHE1 protein. Our results demonstrate that the model of NHE1 is correct which suggests that amino acids 1-127 form two transmembrane segments that remain connected to the mature protein, and the segment between amino acids 315-411 is one transmembrane segment.