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

Abstract

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.

Cloning and Expression of the Histo-blood Group PkUDP-galactose:Galβ1–4Glcβ1-Cer α1,4-Galactosyltransferase

“…The third possibility is unlikely, as the coding region of ␣4Gal-T1 was found in a single exon. However, there are precedents for multiple protein forms originating from such genes by differential usage of splice donor sites of introns placed in 3Ј-UTR (43). Although the Northern analysis with mRNA from human organs only revealed one transcript of 2.3 kilobases (Fig.…”

The sodium/glucose cotransport family SLC5

“…Despite the homology among the proteins (21-70% amino acid identity to SGLT1) there is diversity in gene structure: in eight of the ten genes mapped the coding sequences are found in 14-15 exons (SGLT1-6, NIS, and the Na + -dependent multivitamin transporter SMVT [12,62,66,69,76]). On the other hand the coding sequences for the choline transporter (CHT) and SMIT are contained in eight and one exons respectively [39,47,56]. In SGLT4-6 and SMIT there is evidence for alternative splicing ( [56,62] and E. Turk, unpublished).…”

“…On the other hand the coding sequences for the choline transporter (CHT) and SMIT are contained in eight and one exons respectively [39,47,56]. In SGLT4-6 and SMIT there is evidence for alternative splicing ( [56,62] and E. Turk, unpublished).…”

“…[16]. There is some evidence for alternative splicing of the SMIT gene [56], but the physiological significance is not yet clear. Thyroid-stimulating hormone (TSH) increases NIS expression in the thyroid gland through a cGMP pathway [9].…”

“…Martin, E.M.Wright, unpublished). Also includes data from the original cloning papers and GeneCard (EST, and/or DNA array) G Gene defect S Potential alternative splice sites ([56,62] and E.Turk, unpublished) Original references may be obtained through the Accession numbers and/or the text Note: single nucleotide polymorphisms (SNPs) and variants in the NCBI SNP database are not included.…”