Characterization and role of fucose mutarotase in mammal

Abstract

L-fucose for mammalian glycosylation contains an anomeric carbon atom generating α- and β-L-fucoses. Based on sequence comparison of mouse and human homologs with the procaryotic fucose mutarotases (FucU) characterized, we investigated their function in mammalian cell culture system as well as in the mouse model system for FucU deficiency. By NMR measurement with saturation difference analysis, the purified mammalian mutarotases were demonstrated to be involved in an interconversion between the two anomeric forms with comparable efficiency as that of the E. coli FucU. The mouse gene was widely expressed in various tissues and cell lines, including kidney, liver, and pancreas, although expression was marginal in testis. By generating stably expressed cell lines for mutarotase genes in HepG2, it was shown that fucose incorporations into cellular proteins were increased as demonstrated by an incorporation of radiolabeled fucose into the cells. Furthermore, intracellular levels of GDP-L-fucose, measured with HPLC, were enhanced by an overproduction of cellular mutarotase, which was reversed by gene silencing of mutarotase based on RNA interference. In addition, we observed intracellular accumulation of lipid due to over-fucosylated lipases as well as a change in cell morphology. The results suggest that the mammalian mutarotase is functional in facilitated incorporation of fucose through the salvage pathway. We observed a change in sexual behavior in the mutant mouse, in which the gene for fucose mutarotase (FucU) was homozygously knocked out. The mutant female mouse was characterized to be defeminized by avoiding the sexual mounting by male mouse and greatly lowering the lordosis behavior, an indicator of sexual receptivity in female. Rare intromission occurs through forced approach by male without involving lordosis. The mutant female even exhibited a masculine behavior such as mounting female partner. This abnormality in recognizing sexual partner is lik...