Structure of pre-S2 N- and O-linked glycans in surface proteins from different genotypes of hepatitis B virus

The middle-sized (M) surface proteins of hepatitis B virus (HBV) and other orthohepadnaviruses contain a conserved N-glycan in their pre-S2 domain, which is essential for the secretion of viral particles. Recently, we also found O-glycans in the pre-S2 domain of M protein from woodchuck hepatitis virus (WHV) and HBV genotype D. Since the O-glycosylation motif is not conserved in all genotypes of HBV, the glycosylation patterns of HBV genotypes A and C were analysed. Pre-S2 (glyco)peptides were released from HBV-carrier-derived HBV subviral particles by tryptic digestion, purified by reversed-phase HPLC and identified by amino acid and amino-terminal sequence analysis as well as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Pre-S2 N-glycans were characterized by anion-exchange chromatography, methylation analysis and on-target sequential exoglycosidase digestions in combination with MALDI-TOF-MS, demonstrating the presence of partially sialylated diantennary complex-type oligosaccharides in all genotypes examined. Pre-S2 O-glycans were characterized by on-target sequential exoglycosidase digestions in combination with MALDI-TOF-MS. The pre-S2 domain of M protein and, to a minor extent, of L (large) protein from HBV genotype C and D was partially O-glycosylated by Neu5Ac(alpha2-3)Gal(beta1-3)GalNAcalpha- or Gal(beta1-3)GalNAcalpha-units at Thr-37 within a conserved sequence context. Genotype A, containing no Thr at position 37 or 38, was not O-glycosylated. Analytical data further revealed that M protein is mostly amino-terminally acetylated in all examined genotypes and that the terminal methionine is partially oxidized. The findings may be relevant for the secretion and the immunogenicity of HBV.