Cloning and sequencing of a rabbit cDNA encoding an intestinal and kidney-specific Na+/H+ exchanger isoform (NHE-3)

We previously cloned, sequenced, and expressed two distinct mammalian Na+/H+ exchanger isoforms (NHE-1 and NHE-2). We report here the cloning of a composite cDNA which encodes a third mammalian isoform (NHE-3), which is expressed specifically in intestine and kidney. The protein deduced from the longest open reading frame of this composite sequence has 832 amino acids with a calculated Mr of 92,747. The hydrophobicity plot of NHE-3 is very similar to that of NHE-1 and NHE-2. NHE-3 is also predicted to have 10-12 membrane-spanning domains and a long cytoplasmic domain which contains putative protein kinase phosphorylation motifs. NHE-3 exhibits overall 41% amino acid identity with NHE-1. NHE-3 is likely a glycoprotein as it has one potential N-linked glycosylation site, which is conserved in all NHEs identified. Northern blot analysis of poly(A+) RNA isolated from rabbit ileum using NHE-3 cDNA as a probe hybridized to a single 5.4-kilobase transcript. More detailed tissue distribution of message was performed by ribonuclease protection assay. It was found that NHE-3 message is only expressed in intestine and kidney, with the kidney cortex having the most abundant message, followed by intestine and kidney medulla. In intestine, ileum and ascending colon have the same amount of message, with much lesser amounts in jejunum. The message is absent from duodenum and descending colon, which lack the neutral NaCl absorptive process. Thus, NHE-3 might be involved in Na+ absorption in intestinal and renal epithelial cells.