Journal Article

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

Li-Yuan Chen, Chi-Jiunn Pan, Jeng-Jer Shieh and Janice Yang Chou

in Human Molecular Genetics

Volume 11, issue 25, pages 3199-3207
Published in print December 2002 | ISSN: 0964-6906
Published online December 2002 | e-ISSN: 1460-2083 | DOI: http://dx.doi.org/10.1093/hmg/11.25.3199
Structure–function analysis of the glucose-6-phosphate transporter deficient in glycogen storage disease type Ib

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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.

Journal Article.  5506 words.  Illustrated.

Subjects: Genetics and Genomics

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