title: Human variant glucose-6-phosphate transporter is active in microsomal transport.
abstract: Glycogen storage disease type lb (GSD-lb) is caused by deficiencies in the glucose-6-phosphate transporter (G6PT), which works together with glucose-6-phosphatase to maintain glucose homeostasis. In humans, there are two alternatively spliced transcripts, G6PT and variant G6PT (vG6PT), differing by the inclusion of a 66-bp exon-7 sequence in vG6PT. We have previously shown that the G6PT protein functions as a microsomal glucose-6-phosphate (G6P) transporter, which is anchored to the endoplasmic reticulum by ten transmembrane helices. Here, we demonstrate that vG6PT is also active in microsomal G6P transport. The additional 22 amino acids in vG6PT is predicted to constitute a part of the luminal loop 4. Our data indicate that this loop plays no vital role in microsomal G6P transport. Further, we show that G6PT mRNA is expressed in all organs and tissues examined, but that the vG6PT transcript is expressed exclusively in the brain, heart, and skeletal muscle. These results raise the possibility that mutations in exon-7 of the G6PT gene, which would not perturb glucose homeostasis, might have other deleterious effects.
authors.affiliation: Heritable Disorders Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA.
authors.forename: B
authors.initials: B
authors.lastname: Lin
authors.forename: C J
authors.initials: CJ
authors.lastname: Pan
authors.forename: J Y
authors.initials: JY
authors.lastname: Chou
mesh_terms: D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D017920:Antiporters / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D019556:COS Cells; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D002522:Chlorocebus aethiops; D003001:Cloning, Molecular; D005091:Exons; D014644:Genetic Variation; D019298:Glucose-6-Phosphate / Q000378:metabolism; D005953:Glycogen Storage Disease Type I / Q000235:genetics; D006801:Humans; D008861:Microsomes / Q000378:metabolism*; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D014158:Transcription, Genetic; D014162:Transfection
chemical_list: D017920:Antiporters; D009004:Monosaccharide Transport Proteins; D011994:Recombinant Proteins; C503531:SLC37A4 protein, human; C111261:glucose 6-phosphate(transporter); D019298:Glucose-6-Phosphate