Am J Physiol Cell Physiol Journal of Applied Physiology
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Am J Physiol Cell Physiol 276: C1325-C1337, 1999;
0363-6143/99 $5.00
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Vol. 276, Issue 6, C1325-C1337, June 1999

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

Francesca Porcellati1, Yoshiyuki Hosaka1, Tommy Hlaing2, Masaki Togawa1, Dennis D. Larkin1, Anil Karihaloo1, Martin J. Stevens1, Paul D. Killen2, and Douglas A. Greene1

Departments of 1 Internal Medicine and 2 Pathology, University of Michigan Medical School, Ann Arbor, Michigan 48109-0354

myo-Inositol is a ubiquitous intracellular organic osmolyte and phosphoinositide precursor maintained at millimolar intracellular concentrations through the action of membrane-associated Na+-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 homologous to the Na+-glucose cotransporter with a potential protein kinase A phosphorylation site and multiple protein kinase C phosphorylation sites. A consistent ~1.0- to 13.5-kb array of transcripts hybridizing with this cDNA are osmotically induced in a variety of mammalian cells and species, yet SMIT activity appears to vary among different tissues and species. An 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 gene. Recently, this laboratory ascribed multiply sized, osmotically induced SMIT transcripts in human retinal pigment epithelial cells to the alternate utilization of several 3'-untranslated SMIT exons. This article describes an alternate splice donor site within the coding region that extends the open reading frame into the otherwise untranslated 3' exons, potentially generating novel SMIT isoforms. In these isoforms, the last putative transmembrane domain is replaced with intracellular carboxy termini containing a novel potential protein kinase A phosphorylation site and multiple protein kinase C phosphorylation sites, and this could explain the heterogeneity in the regulation and structure of the SMIT.

myo-inositol; retinal pigment epithelial cell; Xenopus oocyte expression system; osmoregulated genes; sodium-glucose cotransporter superfamily; SCL5A3 open reading frame


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