Elucidating the Implications of Norovirus N- and O-Glycosylation, O-GlcNAcylation, and Phosphorylation

Abstract

Norovirus is the most common cause of foodborne gastroenteritis, affecting millions of people worldwide annually. Among the ten genotypes (GI-GX) of norovirus, only GI, GII, GIV, GVIII, and GIX infect humans. Some genotypes reportedly exhibit post-translational modifications (PTMs), including N- and O-glycosylation, O-GlcNAcylation, and phosphorylation, in their viral antigens. PTMs have been linked to increased viral genome replication, viral particle release, and virulence. Owing to breakthroughs in mass spectrometry (MS) technologies, more PTMs have been discovered in recent years and have contributed significantly to preventing and treating infectious diseases. However, the mechanisms by which PTMs act on noroviruses remain poorly understood. In this section, we outline the current knowledge of the three common types of PTM and investigate their impact on norovirus pathogenesis. Moreover, we summarize the strategies and techniques for the identification of PTMs.

Keywords: N- and O-glycosylation; O-GlcNAcylation; norovirus; phosphorylation.

Figure 1

Replication cycle of noroviruses and protein post-translational modifications. (1) Attachment: Human norovirus (HuNoV) attaches to the HBGAs on the host cell surface, allowing viral entrance. Note that fucosylation of HBGA by fucosyltransferase 2 (FUT2) is required for certain genotypes of norovirus infection. In contrast, deamination on Asn373 of norovirus capsid protein VP1 impairs the recognition of HBGAs. (2) Binding. (3) Uncoating through undefined pathways [80]. (4) Translation: The positive-sense RNA genome may serve as a template for viral protein translation. After translation, viral proteins could undergo N- or O-glycosylation at the endoplasmic reticulum or Golgi apparatus. Mechanisms of noroviral protein glycosylation remain unclear. (5) Assembly: viral proteins assemble to form new viral particles. (6) Release: viral particle release from host cell [81]. Abbreviations: HBGA, histo-blood group antigen.

Figure 2

Replication cycle of noroviruses and post-translational modification crosstalk between phosphorylation and O-GlcNAcylation. Co-occurring PTMs on proteins are common, and PTM crosstalk between phosphorylation and O-GlcNAcylation is the most common since they will compete for the same residues (serine/threonine residues). Phosphorylation is required for viral function. In contrast, O-GlcNAcylation of norovirus capsid protein VP1 could interfere with receptor binding. Studies on the O-GlcNAcylation of noroviruses are few and lack direct evidence from receptor binding assays. Abbreviations: HBGA, histo-blood group antigen; VP1, major capsid protein VP1; ATP, adenosine triphosphate; ADP, adenosine diphosphate; OGT, O-linked N-acetylglucosamine (GlcNAc) transferase; OGA, O-GlcNAcase; O-GlcNAc, O-linked N-acetylglucosamine; UDP-GlcNAc, uridine diphosphate N-acetylglucosamine.

Figure 3

PTM crosstalk between O-GlcNAcylation and phosphorylation. Phosphorylation occurs when protein kinase attaches a phosphate group from ATP to the substrate; in contrast, PPase removes a phosphate group from a phosphoprotein. Similarly, OGT adds O-GlcNAc from UDP-GlcNAc to the substrates for O-GlcNAcylation. Conversely, OGA removes O-GlcNAc. These two processes are both reversible. Because phosphorylation and O-GlcNAcylation can compete for serine and threonine residues, PTM crosstalk occurs [83]. Abbreviations: O-GlcNAc, O-linked β-N-acetylglucosamine; OGT, O-GlcNAc transferase; OGA, O-GlcNAcase; Ser, serine; Thr, threonine; ATP, adenosine triphosphate; ADT, adenosine diphosphate; UPD, uridine diphosphate; PPase, protein phosphatase.