Glycobiology, doi:10.1093/glycob/cwt089

Credit: IGS-UMR 7256 CNRS-AMU

The exterior of Mimivirus, a member of the nucleocytoplasmic large DNA virus group, is modified with rhamnose, viosamine, glucose and N-acetylglucosamine (GlcNAc) sugars. Recent results have shown that Mimivirus contains the biosynthetic pathways for UDP-L-rhamnose and UDP-D-viosamine. However, as these sugars are produced in small amounts—if at all—by host cells, it was unclear whether Mimivirus would be able to synthesize GlcNAc, which is prevalent in eukaryotes. Piacente et al. now identify and characterize three Mimivirus enzymes that together mimic the eukaryotic biosynthetic pathway to UDP-GlcNAc but incorporate prokaryotic elements. The first enzyme, encoded by gene L619, is more homologous to the eukaryotic glutamine:fructose-6-phosphate aminotransferase (GFAT) than the bacterial GlmS. However, like its bacterial counterparts, the enzyme is missing an insertion between two domains and is not feedback regulated by UDP-GlcNAc, which may provide an advantage to the quickly replicating virus. L316 encodes the second enzyme, which, by converting L619's product glucosamine 6-phosphate to the eukaryotic intermediate acetylglucosamine 6-phosphate rather than the bacterial intermediate glucosamine 1-phosphate, defines the pathway as eukaryotic-like. The final enzyme, encoded by R689, performs a reaction in the eukaryotic pathway but is most homologous to a bacterial enzyme. These functions, along with additional phylogenetic analysis, suggest that the pathway was not acquired by horizontal gene transfer but may represent an ancient pathway for a now-universal process.