Overlapping and Distinct Roles of Aspergillus fumigatus UDP-glucose 4-Epimerases in Galactose Metabolism and the Synthesis of Galactose-containing Cell Wall Polysaccharides

Mark J Lee,Fabrice N Gravelat,Robert P Cerone,Stefanie D Baptista,Paolo V Campoli,Se-In Choe,Ilia Kravtsov,Evgeny Vinogradov,Carole Creuzenet,Hong Liu,Albert M Berghuis,Jean-Paul Latgé,Scott G Filler,Thierry Fontaine,Donald C Sheppard

doi:10.1074/jbc.m113.522516

Abstract

The cell wall of Aspergillus fumigatus contains two galactose-containing polysaccharides, galactomannan and galactosaminogalactan, whose biosynthetic pathways are not well understood. The A. fumigatus genome contains three genes encoding putative UDP-glucose 4-epimerases, uge3, uge4, and uge5. We undertook this study to elucidate the function of these epimerases. We found that uge4 is minimally expressed and is not required for the synthesis of galactose-containing exopolysaccharides or galactose metabolism. Uge5 is the dominant UDP-glucose 4-epimerase in A. fumigatus and is essential for normal growth in galactose-based medium. Uge5 is required for synthesis of the galactofuranose (Galf) component of galactomannan and contributes galactose to the synthesis of galactosaminogalactan. Uge3 can mediate production of both UDP-galactose and UDP-N-acetylgalactosamine (GalNAc) and is required for the production of galactosaminogalactan but not galactomannan. In the absence of Uge5, Uge3 activity is sufficient for growth on galactose and the synthesis of galactosaminogalactan containing lower levels of galactose but not the synthesis of Galf. A double deletion of uge5 and uge3 blocked growth on galactose and synthesis of both Galf and galactosaminogalactan. This study is the first survey of glucose epimerases in A. fumigatus and contributes to our understanding of the role of these enzymes in metabolism and cell wall synthesis.

Highlights

Aspergillus fumigatus produces two galactose-containing exopolysaccharides, galactomannan and galactosaminogalactan
Uge5 Is the Most Highly Expressed Epimerase Gene in A. fumigatus and Is Required for Normal Galactose Metabolism— We first performed expression analysis of the three epimerase genes in wild-type A. fumigatus grown in Brian medium (Fig. 1)
We hypothesized that Uge5 likely plays an important role in galactose metabolism and cell wall glycan synthesis

Summary

Introduction

Aspergillus fumigatus produces two galactose-containing exopolysaccharides, galactomannan and galactosaminogalactan. Results: Galactosaminogalactan synthesis requires the UDP-glucose 4-epimerases, Uge and Uge, whereas galactomannan synthesis requires Uge alone. The cell wall of Aspergillus fumigatus contains two galactosecontaining polysaccharides, galactomannan and galactosaminogalactan, whose biosynthetic pathways are not well understood. We found that uge is minimally expressed and is not required for the synthesis of galactose-containing exopolysaccharides or galactose metabolism. Uge is required for synthesis of the galactofuranose (Galf) component of galactomannan and contributes galactose to the synthesis of galactosaminogalactan. In the absence of Uge, Uge activity is sufficient for growth on galactose and the synthesis of galactosaminogalactan containing lower levels of galactose but not the synthesis of Galf. This study is the first survey of glucose epimerases in A. fumigatus and contributes to our understanding of the role of these enzymes in metabolism and cell wall synthesis

Methods

Results

Conclusion