Datamining and functional environmental genomics reassesses the phylogenetics and functional diversity of fungal monosaccharide transporters. fungal monosaccharide transporters

Sugar transporters are essential components of primary carbon metabolism and have been extensively studied to control sugar uptake by both yeasts and filamentous fungi used in fermentation processes. We collected published information on characterized fungal sugar porters and showed that this protein family can be split into phylogenetically distinct clades. While several of them encompass transporters that seemingly specialized on specific “sugar-related” molecules (e.g. myo-inositol, charged sugar analogs), others include mostly either mono- or di/oligosaccharide low-specificity transporters. To address the issue of substrate specificity of fungal sugar transporters, we screened “multi-species” soil eukaryotic cDNA libraries for mannose transporters, a sugar of biotechnological relevance that had never been used to select transporters. Of the 19 environmental sugar transporters obtained (mostly of Basidiomycota and Ascomycota origin), one belonged to the unusual “fucose H+ symporter” family, which is only known in Fungi for a rhamnose transporter in Aspergillus niger. Functional analysis of the 19 transporters by expression in yeast and/or in Xenopus oocytes for electrophysiological measurements indicated that most of them showed a preference for D-mannose over other tested D-C6 (glucose, fructose, galactose) or D-C5 (xylose) sugars as a substrate. For the several glucose and fructose-negative transporters, growth of the corresponding recombinant yeast strains was prevented on mannose in the presence of one of these sugars that may act by blocking the protein in a closed conformation. These results highlight to potential of environmental genomics to explore in a context of biotechnology the functional diversity of key fungal protein families.