Structures of Merkel Cell Polyomavirus VP1 Complexes Define a Sialic Acid Binding Site Required for Infection

Abstract

The recently discovered human Merkel cell polyomavirus (MCPyV or MCV) causes the aggressive Merkel cell carcinoma (MCC) in the skin of immunocompromised individuals. Conflicting reports suggest that cellular glycans containing sialic acid (Neu5Ac) may play a role in MCPyV infectious entry. To address this question, we solved X-ray structures of the MCPyV major capsid protein VP1 both alone and in complex with several sialylated oligosaccharides. A shallow binding site on the apical surface of the VP1 capsomer recognizes the disaccharide Neu5Ac-α2,3-Gal through a complex network of interactions. MCPyV engages Neu5Ac in an orientation and with contacts that differ markedly from those observed in other polyomavirus complexes with sialylated receptors. Mutations in the Neu5Ac binding site abolish MCPyV infection, highlighting the relevance of the Neu5Ac interaction for MCPyV entry. Our study thus provides a powerful platform for the development of MCPyV-specific vaccines and antivirals. Interestingly, engagement of sialic acid does not interfere with initial attachment of MCPyV to cells, consistent with a previous proposal that attachment is mediated by a class of non-sialylated carbohydrates called glycosaminoglycans. Our results therefore suggest a model in which sialylated glycans serve as secondary, post-attachment co-receptors during MCPyV infectious entry. Since cell-surface glycans typically serve as primary attachment receptors for many viruses, we identify here a new role for glycans in mediating, and perhaps even modulating, post-attachment entry processes.

Structure Analysis of the Major Capsid Proteins of Human Polyomaviruses 6 and 7 Reveals an Obstructed Sialic Acid Binding Site

“…The I-strand is split into two parts named I and I=. As is typical for polyomavirus VP1 structures, rather poor electron density was observed for the CDloops at the base of the HPyV6 and HPyV7 pentamers (25,26,(28)(29)(30)(31)(32). This loop is flexible and assumes different conformations even in the context of the intact virion (25,26,28).…”

“…The presence and roles of VP2 and VP3 seem to differ among polyomavirus species (27). All known structures of polyomavirus VP1 show extended and structurally variable surface loops that emanate from a conserved ␤-sheet core structure formed by strands B, I, D, and G and strands C, H, E, and F (25,26,(28)(29)(30)(31)(32)(33)(34)(35). These surface loops, named BC-, DE-, HI-, and EF-loops after the ␤-strands connected by them, are chiefly responsible for viral antigenicity.…”

“…For each virus, they form a unique virus-host interaction platform that determines host range, cell tropism, viral spread, and pathogenicity. Engagement of sialylated glycan motifs during cell attachment and entry is a common feature of the better-studied orthopolyomaviruses (28)(29)(30)(32)(33)(34)(35)(36). In contrast, the routes of infection, transmission, cell tropism, receptor specificity, and entry pathways of wukipolyomaviruses remain largely mysterious.…”

Crystallographic and Glycan Microarray Analysis of Human Polyomavirus 9 VP1 Identifies N -Glycolyl Neuraminic Acid as a Receptor Candidate

“…VP1 is the major polyomavirus capsid protein, and a collection of available VP1 structures from different polyomaviruses shows that the protein adopts a jelly roll fold and assembles into a homopentamer around a central cavity (24)(25)(26)(27)(28)(29)(30)(31). While the pentameric core structure is conserved in all VP1 proteins, these structures from different polyomaviruses exhibit substantial differences in the sequence, length, and conformation of their surface-exposed loops.…”

“…While the pentameric core structure is conserved in all VP1 proteins, these structures from different polyomaviruses exhibit substantial differences in the sequence, length, and conformation of their surface-exposed loops. These highly variable regions contain the sites for receptor engagement (25)(26)(27)29) as well as antibody binding (21)(22)(23)32). All human polyomavirus VP1 structures known to date engage glycans terminating in 5-N-acetyl neuraminic acid (Neu5Ac) as receptors (27,(29)(30)(31), with, in some cases, drastically different interactions taking place.…”