The rare sugar N-acetylated viosamine is a major component of Mimivirus fibers

The giant virus Mimivirus encodes an autonomous glycosylation system that is thought to be responsible for the formation of complex and unusual glycans composing the fibers surrounding its icosahedral capsid, including the dideoxyhexose viosamine. Previous studies have identified a gene cluster in the virus genome, encoding enzymes involved in nucleotide-sugar production and glycan formation, but the functional characterization of these enzymes and the full identification of the glycans found in viral fibers remain incomplete. Because viosamine is typically found in acylated forms, we suspected that one of the genes might encode an acyltransferase, providing directions to our functional annotations. Bioinformatic analyses indicated that the L142 protein contains an N-terminal acyltransferase domain and a predicted C-terminal glycosyltransferase. Sequence analysis of the structural model of the L142 N-terminal domain indicated significant homology with some characterized sugar acetyltransferases that modify the C-4 amino group in the bacillosamine or perosamine biosynthetic pathways. Using mass spectrometry and NMR analyses, we confirmed that the L142 N-terminal domain is a sugar acetyltransferase, catalyzing the transfer of an acetyl moiety from acetyl-CoA to the C-4 amino group of UDP-d-viosamine. The presence of acetylated viosamine in vivo has also been confirmed on the glycosylated viral fibers, using GC-MS and NMR. This study represents the first report of a virally encoded sugar acetyltransferase.