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Impaired trafficking and subcellular localization of a mutant lactase associated with congenital lactase deficiency

Impaired trafficking and subcellular localization of a mutant lactase associated with congenital lactase deficiency

Gastroenterology 136(7): 2295-2303

Congenital lactase deficiency (CLD) is a cause of disaccharide intolerance and malabsorption characterized by watery diarrhea in infants fed breast milk or lactose-containing formulas. The molecular basis of CLD is unknown. Mutations in the coding region of the brush border enzyme lactase phlorizin hydrolase (LPH) were found to cause CLD in a study of 19 Finnish families. We analyzed the effects of one of these mutations, G1363S, on LPH folding, trafficking, and function. We introduced a mutation into the LPH complementary DNA that resulted in the amino acid substitution G1363S. The mutant gene was transiently expressed in COS-1 cells, and the effects were assessed at the protein, structural, and subcellular levels. The mutant protein LPH-G1363S was misfolded and could not exit the endoplasmic reticulum. Interestingly, the mutation creates an additional N-glycosylation site that is characteristic of a temperature-sensitive protein. The intracellular transport and enzymatic activity, but not correct folding, of LPH-G1363S were partially restored by expression at 20 degrees C. However, a form of LPH that contains the mutations G1363S and N1361A, which eliminates the N-glycosylation site, did not restore the features of wild-type LPH. Thus, the additional glycosyl group is not required for the LPH-G1363S defects. This is the first characterization, at the molecular and subcellular levels, of a mutant form of LPH that is involved in the pathogenesis of CLD. Mutant LPH accumulates predominantly in the endoplasmic reticulum but can partially mature at a permissive temperature; these features are unique for a protein involved in a carbohydrate malabsorption defect implicating LPH.

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Accession: 053730717

Download citation: RISBibTeXText

PMID: 19208354

DOI: 10.1053/j.gastro.2009.01.041

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