Viruses are infectious agents that hijack the host cell machinery in order to replicate and generate progeny. Viral infection is initiated by attachment to host cell receptors, and typical viral receptors are cell-surface-borne molecules such as proteins or glycan structures. Sialylated glycans (glycans bearing sialic acids) and glycosaminoglycans (GAGs) represent major classes of carbohydrate receptors and have been implicated in facilitating viral entry for many viruses. As interactions between viruses and sialic acids have been extensively reviewed in the past, this review provides an overview of the current state of structural knowledge about interactions between non-enveloped human viruses and GAGs. We focus here on adeno-associated viruses, human papilloma viruses (HPVs), and polyomaviruses, as at least some structural information about the interactions of these viruses with GAGs is available. We also discuss the multivalent potential for GAG binding, highlighting the importance of charged interactions and positively charged amino acids at the binding sites, and point out challenges that remain in the field.
BK Polyomavirus (BKPyV) is an opportunistic pathogen that causes nephropathy in kidney transplant recipients. The BKPyV major capsid protein, VP1, engages gangliosides, lipid-linked sialylated glycans at the cell surface, to gain entry into cells. Here, we characterise the influence of VP1 mutations observed in patients with persistent post-transplant BKPyV replication on ganglioside binding, VP1 protein structure, and the tropism of the virus in two renal cell lines: 293TT and immortalised renal tubular epithelial cells. Infectious entry of single mutants E73Q, E73A and the triple mutant A72V-E73Q-E82Q (VQQ) remained sialic acid-dependent. These three variants acquired binding to a-series gangliosides, including GD1a, although only E73Q was able to infect GD1a-supplemented LNCaP or GM95 cells. Crystal structures of the three mutants showed a clear shift of the BC2 loop in mutants E73A and VQQ that correlated with the inability of these VP1 variants to infect ganglioside complemented cells. On the other hand, the double mutant K69N-E82Q lost the ability to bind sialic acid, with the K69N mutation leading to a steric clash which precludes sialic acid binding. Nevertheless, this mutant retained significant infectivity in 293TT cells that was not dependent on heparan sulphate proteoglycans, implying that an unknown sialic acid-independent entry receptor for BKPyV exists.
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