Identification of the galactosyltransferase of Cryptococcus neoformans involved in the biosynthesis of basidiomycete-type glycosylinositolphosphoceramide.
The pathogenic fungus Cryptococcus neoformans synthesizes a complex family of glycosylinositolphosphoceramide (GIPC) structures. These glycosphingolipids (GSLs) consist of mannosylinositolphosphoceramide (MIPC) extended by β1-6-linked galactose, a unique structure that has to date only been identified in basidiomycetes. Further extension by up to five mannose residues and a branching xylose has been described. In this study, we identified and determined the gene structure of the enzyme Ggt1, which catalyzes the transfer of a galactose residue to MIPC. Deletion of the gene in C. neoformans resulted in complete loss of GIPCs containing galactose, a phenotype that could be restored by the episomal expression of Ggt1 in the deletion mutant. The entire annotated open reading frame, encoding a C-terminal GT31 galactosyltransferase domain and a large N-terminal domain of unknown function, was required for complementation. Notably, this gene does not encode a predicted signal sequence or transmembrane domain. The demonstration that Ggt1 is responsible for the transfer of a galactose residue to a GSL thus raises questions regarding the topology of this biosynthetic pathway and the function of the N-terminal domain. Phylogenetic analysis of the GGT1 gene shows conservation in hetero- and homobasidiomycetes but no homologs in ascomycetes or outside of the fungal kingdom.
Institute of Microbiology, ETH Zürich, Wolfgang-Pauli-Str. 10, HCI F413, CH-8093 Zürich, Switzerland.
Therese
T
Wohlschlager
Reto
R
Buser
Michael L
ML
Skowyra
Brian C
BC
Haynes
Bernard
B
Henrissat
Tamara L
TL
Doering
Markus
M
Künzler
Markus
M
Aebi
D002518:Ceramides / Q000096:biosynthesis*; D003455:Cryptococcus neoformans / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D062610:Fungal Polysaccharides / Q000096:biosynthesis; D005700:Galactosyltransferases / Q000235:genetics*; D017353:Gene Deletion; D005816:Genetic Complementation Test; D006028:Glycosphingolipids / Q000096:biosynthesis*; D006031:Glycosylation; D010802:Phylogeny; D010957:Plasmids / Q000235:genetics; D017386:Sequence Homology, Amino Acid
D002518:Ceramides; D062610:Fungal Polysaccharides; D006028:Glycosphingolipids; D005700:Galactosyltransferases
GIPC; basidiomycete; fungal glycans; galactosyltransferase; glycolipids