Structure-function analysis of the glucose-6-phosphate transporter deficient in glycogen storage disease type Ib

Hum Mol Genet. 2002 Dec 1;11(25):3199-207. doi: 10.1093/hmg/11.25.3199.


Glycogen storage disease type Ib (GSD-Ib) is caused by a deficiency in the glucose-6-phosphate transporter (G6PT), a 10 transmembrane domain endoplasmic reticulum protein. To date, 69 G6PT mutations, including 28 missenses and 2 codon deletions, have been identified in GSD-Ib patients. We previously characterized 15 of the missense and one codon deletion mutations using a pSVL-based expression assay. A lack of sensitivity in this assay limited the discrimination between mutations that lead to loss of function and mutations that leave a low residual activity. We now report an improved G6PT assay, based on an adenoviral vector-mediated expression system and its use in the functional characterization of all 30 codon mutations found in GSD-Ib patients. Twenty of the naturally occurring mutations completely abolish microsomal G6P uptake activity while the other 10 mutations, including 5 previously characterized ones, partially inactivate the transporter. This information should greatly facilitate genotype-phenotype correlation. We also report a structure-function analysis of G6PT. In addition to the 3 destabilizing mutations reported previously, we now show that the G50R, C176R, V235del, G339C and G339D mutations also compromise the G6PT stability. Mutation analysis of the amino-terminal domain of G6PT shows that it is required for optimal G6P uptake activity. Finally, we show that degradation of both wild-type and mutant G6PT is inhibited by a potent proteasome inhibitor, lactacystin, demonstrating that G6PT is a substrate for proteasome-mediated degradation.

MeSH terms

  • Acetylcysteine / analogs & derivatives*
  • Acetylcysteine / pharmacology
  • Adenoviridae / enzymology
  • Adenoviridae / genetics
  • Amino Acid Sequence
  • Animals
  • Antiporters / biosynthesis
  • Antiporters / deficiency*
  • Antiporters / genetics*
  • Antiporters / metabolism
  • COS Cells
  • Cell Line
  • Chlorocebus aethiops
  • Cysteine Endopeptidases / physiology
  • Cysteine Proteinase Inhibitors / pharmacology
  • Endoplasmic Reticulum / chemistry
  • Genetic Vectors
  • Glucose-6-Phosphatase / genetics
  • Glucose-6-Phosphate / metabolism
  • Glycogen Storage Disease Type I / enzymology
  • Glycogen Storage Disease Type I / genetics*
  • Glycogen Storage Disease Type I / metabolism
  • Humans
  • Intracellular Membranes / chemistry
  • Microsomes / metabolism
  • Molecular Sequence Data
  • Monosaccharide Transport Proteins / biosynthesis
  • Monosaccharide Transport Proteins / deficiency*
  • Monosaccharide Transport Proteins / genetics*
  • Monosaccharide Transport Proteins / metabolism
  • Multienzyme Complexes / antagonists & inhibitors
  • Multienzyme Complexes / physiology
  • Mutation, Missense
  • Peptides / physiology
  • Proteasome Endopeptidase Complex
  • Protein Structure, Tertiary / physiology
  • Sequence Deletion
  • Structure-Activity Relationship
  • Viral Proteins / genetics


  • Antiporters
  • Cysteine Proteinase Inhibitors
  • Monosaccharide Transport Proteins
  • Multienzyme Complexes
  • Peptides
  • SLC37A4 protein, human
  • Viral Proteins
  • glucose 6-phosphate(transporter)
  • lactacystin
  • Glucose-6-Phosphate
  • Glucose-6-Phosphatase
  • Cysteine Endopeptidases
  • Proteasome Endopeptidase Complex
  • Acetylcysteine