Attachment and entry
A number of viruses, such as HCV, Dengue virus, Ebola virus, SARS coronavirus, West Nile virus, Rift Valley fever virus and Japanese encephalitis virus use N-linked glycans as attachment and entry receptors by interacting with cellular lectins DC-SIGN, L-SIGN, LSECtin, ASGP-R or mannose receptor (Becker et al. 1995; Lin et al. 2003; Lozach et al. 2003; Gramberg et al. 2005; Davis et al. 2006; Miller et al. 2008; Chen et al. 2014; Phoenix et al. 2016; Wang et al. 2016). Mature N-glycans have been suggested to influence Lassa virus binding to cell entry receptor α-dystroglycan (Shrivastava-Ranjan et al. 2016). In addition, specific N-glycans on HIV-1 have been shown to affect CD4 receptor binding on T cells or regulate coreceptor usage (Nakayama et al. 1998; Pollakis et al. 2001; Francois and Balzarini 2011; Lombardi et al. 2015). Systemic functional analysis of HIV-1 gp160 potential N-glycosylation sites has identified a number of additional glycosites affecting infectivity (Wang et al. 2013). In addition, a certain N-glycan on HIV-1 gp41 was shown to have a consistent effect in several viral strains (Mathys and Balzarini 2014). Specific sialylated N-glycans on VZV gB have been identified that are important for interaction with myelin-associated glycoprotein and crucial for cell–cell fusion (Suenaga et al. 2015), and sialylation of virions in general was shown to affect entry of HSV-1 (Teuton and Brandt 2007). Similarly, specific N-linked glycans on HSV-2 gB influenced viral entry (Luo et al. 2015). An N-glycosylation site on IAV HA has also been shown to affect viral entry indirectly by modulating the avidity and specificity for sialosides (Wu et al. 2017). Finally, distinct putative N-glycosylation site mutations on HCV E1 and E2 resulted in diminished entry and altered CD81 binding (Goffard et al. 2005; Falkowska et al. 2007; Helle et al. 2010).
In contrast to the rather well documented roles of N-linked glycans, there are very few known examples, where specific O-glycans participate in virus–host interaction. Among these, two sialylated O-glycans on HSV-1 gB have been identified that determine binding to cellular receptor paired immunoglobulin-like type 2 receptor α residing on immune cells (Wang et al. 2009; Arii et al. 2010; Bagdonaite et al. 2015). In addition, HSV-1 O-glycans are involved in a few other aspects of viral binding to the host cell, as deletion of the densely O-glycosylated region of attachment factor gC affects both the binding affinity to the cell surface, and the release of progeny virus via modulation of interactions with cell surface glycosaminoglycans (Altgarde et al. 2015). Another example includes the carbohydrate-dependent binding of filoviruses to the macrophage galactose lectin that is known to recognize GalNAc-O-glycans (Takada et al. 2004). In hepatitis C virus, the mutation of several putative O-glycosites has also been shown to decrease HCV E2 affinity for CD81, suggesting that O-glycans might be of a more general importance in mediating interaction with host cells (Falkowska et al. 2007). O-glycoproteomic analysis of Hendra virus glycoprotein G has recently sprouted the first, to the best of our knowledge, systematic functional analysis of known O-glycosylation sites, revealing a multitude of functions including attachment and entry to host cells (Colgrave et al. 2012; Stone et al. 2016). Importantly, most of the functions could also be identified in analogous O-glycosites of a closely related Nipah virus (Stone et al. 2016). Interestingly, only in Nipah virus a single O-glycosite significantly affected Ephrin B2 receptor binding (Stone et al. 2016). This is an important example of conserved O-glycan function between closely related viruses, and presents intriguing possibilities in the light of recently identified consistent O-glycosylation patterns of several herpesviruses. Besides confirming the HSV-1 O-glycosites involved in binding PILRα in vitro, a few other O-glycosites residing in protein regions expected to influence HSV-1 attachment via cell entry receptors nectin-1 and HVEM (Carfi et al. 2001; Krummenacher et al. 2005; Heldwein et al. 2006; Di Giovine et al. 2011; Gallagher et al. 2014), or VZV attachment to receptor insulin degrading enzyme (Berarducci et al. 2010) were identified (Bagdonaite et al. 2015, 2016). This is another example how MS-based approaches combined with structural knowledge can help narrow down a list of O-glycosite candidates for focused studies. In conclusion, glycans on viral entry proteins are widely used for modulation of receptor binding and entry, with both N-linked and O-linked glycans having the capacity to affect the interaction (Figure 2, top left panel). In some cases, though, it seems to be an effect of conformational stability, rather than direct interaction (Stone et al. 2016).