Glycobiology doi:10.1093/glycob/cwv078

Glycosylphosphatidylinositols (GPIs) are used in eukaryotic species to tether proteins to the plasma membrane, and in fungi, a truncated version is used to attach proteins to the cell wall. These processes require a minimal GPI construct of four or five glycans and a lipid. However, GPIs can be further functionalized with additional glycans, with the final structure varying across organisms and proteins, and the biological role of these 'decorations' is unclear. Although the enzymes responsible for the biosynthesis and transfer of the conserved GPI core are known, very few enzymes involved in the latter stages have been identified. Krüger et al. now search for an enzyme capable of adding a mannose group to the end of the GPI core through an α-1,3 glycosidic linkage, a known modification in Aspergillus fumigatus. The authors first identified three enzymes in the A. fumigatus genome annotated as belonging to CAZy family 69, known as α-1,3-mannosyltransferases. Deletion of one of these genes, clpA, followed by isolation of the glycan sequences from the GPI anchored proteins and their analysis by chromatography, mass spectrometry and enzymatic digestion, revealed the influence of clpA: loss of the gene led to shorter saccharide sequences sensitive to degradation by an enzyme specific for bonds internal to the minimal motif, whereas the wild-type strain and a variant of the deletion strain with clpA re-introduced had longer sequences that were protected from digestion. Introduction of clpA into Saccharomyces cerevisiae, which does not normally contain an α-1,3 linked sugar, led to the production of glycans protected from digestion, consistent with the introduction of the linkage. Finally, the authors confirmed that the enzyme is present in the Golgi apparatus, as expected, and requires more than one mannose group in the substrate. This discovery provides new opportunities to examine the role of these modifications in GPI functions.