cDNA cloning and expression of the mouse Na/H antiporter (NHE-1) and a potential splice variant

A cDNA for the Mus musculus Na/H exchanger-isoform 1 (NHE-1) was identified in a BALB/c myoblast library by its hybridization to rat NHE-1 sequences. Analysis of the clone showed it to display extensive homology with NHE-1 clones from other mammalian species; however, the region of interspecific homology was abruptly interrupted in the midst of the open reading frame by 166 bp of unrelated sequence. This extra sequence is likely to be an unspliced intron 9. Aside from the retained intron 9, the NHE-1 cDNA clone is otherwise fully processed, with all of the other ten introns removed and containing a poly(A) tract. From PCR results this variant represents a significant but minor population of NHE-1 RNAs. The variant message does associate with polysomes thereby suggesting it to be translated into protein. The location of the retained intron in the carboxy terminus of the protein is such that its translation would produce a protein predicted to be still capable of effecting Na and H translocation but whose regulatory features would be markedly altered. Amino acid sequence comparison of the mouse NHE-1 (derived from the fully processed message) with that of other mammalian species demonstrated two exceptionally divergent regions; the C-terminal cytoplasmic tail (residues 750-790), containing a region of 6-8 contiguous acidic amino acids variably composed of aspartate and glutamate residues, and the N-terminal extracellular domain that includes an N-linked glycosylation site (residues 60-80).