MOLECULAR-CLONING, SEQUENCING, TISSUE DISTRIBUTION, AND FUNCTIONAL EXPRESSION OF A NA-2)( H+ EXCHANGER (NHE)

The present studies demonstrate cloning, sequencing, tissue distributi on, and functional expression of a Na+/H+ exchanger which was isolated from a rat intestinal cDNA library. The cloned cDNA recognizes two tr anscripts in poly(A)+ RNA from the stomach, jejunum, ileum, liver, lar ge intestine, and uterus. Based on deduced amino acid sequences, this clone shares sequence homology with the other known Na+/H+ exchanger i soforms (NHE-1, NHE-3, and NHE-4) except for its 5' end. Overall, the protein exhibits 47.8%, 41.2%, and 56.2% amino acid sequence identity to NHE-1, NHE-3, and NHE-4, respectively. The hydropathy profile of th e predicted protein shows 10 transmembrane domains, suggesting a prote in with transport characteristics. The tissue distribution differs fro m that of the other Na+/H+ exchanger isoforms. The cDNA hybridizes to two closely related transcripts in the mRNA of these tissues, which su ggests that the predominant transcript of this clone is alternatively spliced. Transfection of this cDNA into Na+/H+ exchanger-deficient mut ant fibroblasts (PS120 cells) results in functional Na+/H+ exchange ac tivity. These data suggest that we have cloned a member of the Na+/Hexchanger family with tissue-specific expression. We suggest the desig nation of NHE-2 for this Na+/H+ exchanger.