Functional characterization of the Amanita muscaria monosaccharide transporter, AmMst1

Fungal carbohydrate nutrition is an important aspect of ectomycorrhizal sym biosis. At the plant/fungus interface, fungal and Foot cortical cells compe te for monosaccharides, generated From plant-derived sucrose. Therefore, th e kinetic properties of the monosaccharide uptake systems are decisive for the monosaccharide yield of each partner. For the functional characterizati on of a hexose transporter (AmMst1) of the ectomycorrhizal fungus Amanita m uscaria, the entire cDNA was expressed in a Sacchoromyces cerevisiae strain unable to take up hexoses. Uptake experiments with C-14-labelled monosacch arides resulted in K-M values of 0.46 mM for glucose and 4.20 mM far fructo se, revealing a strong preference of AmMst1 for glucose as substrate. Gluco se uptake by AmMst1 was strongly favoured even in the presence of a large e xcess of fructose. Comparable affinities of AmMst1 for glucose, 3-O-methyl glucose and mannose were obtained. In contrast, AmMst1 imported galactose w ith a much lower efficiency, revealing that this transporter distinguishes pyranoses by steric hindrance at the C-4 position. While yeast contains num erous hexose transporter genes, the AmMst1 gene seems to be the main, if no t the only, hexose transporter that is expressed in A. muscaria, as conclud ed from the comparison of hexose import properties of A. muscaria protoplas ts and AmMst1 expressed in yeast.