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Chapter 21 : Glucose
Category: Microbial Genetics and Molecular Biology; Fungi and Fungal Pathogenesis
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This chapter reviews the regulation of glucose uptake and metabolism in filamentous fungi and highlights the similarities with and differences from mechanisms in yeast. Fungal glucose transporters are classified as high-affinity transporters if the Km for glucose is in the micromolar range and low-affinity transporters if Km for glucose is in the millimolar range. S. cerevisiae contains a large number of proteins that can transport glucose across the yeast cell membrane, 17 of which (Hxt1 through Hxt11p, Hxt13 through Hxt17p, and Gal2p) belong to the yeast glucose transporter family. The use of glucose analogues has been used to determine whether glucose sensing in fungi requires uptake and/or further metabolism of glucose. The role of hexokinases in glucose sensing was confirmed in studies using strains carrying mutations in the three genes encoding sugar-phosphorylating enzymes: HXK1, HXK2, and GLK2. Given the central role of glucose in carbon metabolism and the diverse nutrient sources used by different fungi, it is not surprising that differences in glucose transport, glucose metabolism, glucose signaling, and carbon catabolite repression have arisen through selection.
Phylogenetic tree of putative sugar transporters in A. nidulans. A. nidulans (AN) putative proteins that show similarity to fungal hexose transporters have been characterized in A. muscaria (Mst1), A. nidulans (HxtA and MstE), A. niger (MstA), N. crassa (HGT1 and RCO3), S. cerevisiae (Hxt1p through Hxt11p, Hxt13p through Hxt17p, and Gal2p). T. harzianum (Gtt1), and U. fabae (Hxt1p). The rooted tree was constructed using CLUSTAL ( Thompson et al., 1994 ), PROTDIST and KITSCH ( Felsenstein, 1996 ) through Biomanager at the Australian National Genome Information Service ((http://www.angis.org.au), and TREE-VIEW ( Page, 1996 ). The amino acid sequences used to construct the tree were obtained from the Saccharomyces Genome Database (http://www.yeastgenome.org/), NCBI (http://www.ncbi.nlm.nih.gov/), and the Aspergillus Comparative Database (http://www.broad.mit.edu/annotation/genome/aspergillus_group/MultiHome.html). The human glucose transporter GTR1 was included as an outgroup.
Enzymes of the glycolytic pathway and TCA cycle. In A. oryzae, genes encoding enzymes indicated in a solid box were increased, and those indicated in a dotted box decreased, in mycelia grown in glucose-rich compared to glucose-poor media ( Maeda et al., 2004 ). Abbreviations: HEX, hexokinase/glucokinase; GPI, phosphoglucose isomerase; PFK, phosphofructokinase; FBA, fructose bisphosphatase; FBPA, fructose bisphosphate aldolase; TPI, triose phosphate isomerase; GPD, glyceraldehyde-3-phosphate dehydrogenase; PGK, phosphoglycerate kinase; PGM, phosphoglycerate mutase; ENO, enolase; PK, phosphoenolpyruvate kinase; PDC, pyruvate decarboxylase; ADH, alcohol dehydrogenase I; ALD, acetaldehyde dehydrogenase; ACS, acetyl-CoA synthase; PDH, pyruvate dehydrogenase; CT, citrate synthase; ACO, aconitase; IDH, isocitrate dehydrogenase; KDH, α-ketoglutarate dehydrogenase; SCoS, succinyl-CoA synthase; SDH, succinate dehydrogenase; FUM, fumarate dehydratase; MDH, malate dehydrogenase; PEPCK, phosphoenolpyruvate carboxykinase.
Phylogenetic tree of hexokinases and hexokinase-like proteins from A. nidulans, A. niger, Batrachochytrium dendrobatidis, C. cinereus, Fusarium graminearum, Magnaporthe oryzae (formerly M. grisea) and N. crassa. The rooted tree was constructed with amino acid sequences from the Fungal Genome Initiative at the Broad Institute (http://www.broad.mit.edu/annotation/fgi/) and sequences from A. niger ( Panneman et al., 1996 , 1998 ) by using CLUSTAL ( Thompson et al., 1994 ), PROTDIST and KITSCH ( Felsenstein, 1996 ) through Biomanager at the Australian National Genome Information Service (http://www.angis.org.au), and TREEVIEW ( Page, 1996 ). Only two of the six sequences encoded by the A. niger genome were included ( Panneman et al., 1996 , 1998 ). Human glucokinase was included for comparison.
Conserved motifs within filamentous fungal carbon catabolite repressor proteins. Shown are sequence comparisons of En (Emericella [Aspergillus] nidulans EMBL ENCREA); Ao (A. oryzae EMBL AOR272151); An (A. niger EMBL ANCREA); Aa(Aspergillus aculeatus EMBL AB024314); Gf (Gibberella fujikuroi EMBL GFY16626); Ss (Sclerotinia sclerotiorum EMBL SSCRES); Bc(Botrytis cinerea EMBL BCY16625); Ac (Acremonium chrysogenum EMBL ACH245727); Hg (Humicola grisea EMBL AB003106); Tr (T. reesei EMBL TR27356); Cc (C. carbonum EMBL AF306571); Ma (Metarhizium anisopliae EMBL MACRR1); Nc ( N. crassa EMBL AF055464); and Th (T. harzianum EMBL THCRE1). Amino acid residues in A. nidulans, S. sclerotiorum, and T. reesei referred to in the text are underlined.
Characterization of glucose transporters in filamentous fungi
Comparison of glucose sensing in S. cerevisiae and filamentous fungi
Number of genes encoding hexokinases in filamentous fungi
Comparison of mechanisms of regulation by major carbon catabolite repression proteins in fungi