Alternate splicing in human Na+-MI cotransporter gene yields differentially regulated transport isoforms

myo-Inositol is a ubiquitous intracellular organic osmolyte and phosphoinos itide precursor maintained at millimolar intracellular concentrations throu gh the action of membrane-associated Nac-myo-inositol cotransporters (SMIT) . Functional cloning and expression of a canine SMIT cDNA, which conferred SMIT activity in Xenopus oocytes, predicted a 718-amino acid peptide homolo gous to the Na+-glucose cotransporter with a potential protein kinase A pho sphorylation site and multiple protein kinase C phosphorylation sites. A co nsistent similar to 1.0- to 13.5-kb array of transcripts hybridizing with t his cDNA are osmotically induced in a variety of mammalian cells and specie s, yet SMIT activity appears to vary among different tissues and species. A n open reading frame on human chromosome 21 (SLC5A3) homologous to that of the canine cDNA (96.5%) is thought to comprise an intronless human SMIT gen e. Recently, this laboratory ascribed multiply sized, osmotically induced S MIT transcripts in human retinal pigment epithelial cells to the alternate utilization of several 3'-untranslated SMIT exons. This article describes a n alternate splice donor site within the coding region that extends the ope n reading frame into the otherwise untranslated 3' exons, potentially gener ating novel SMIT isoforms. In these isoforms, the last putative transmembra ne domain is replaced with intracellular carboxy termini containing a novel potential protein kinase A phosphorylation site and multiple protein kinas e C phosphorylation sites, and this could explain the heterogeneity in the regulation and structure of the SMIT.