Crystal structure of the human astrovirus capsid spike

Abstract

Astroviruses are single-stranded, plus-sense RNA viruses that infect both mammals and birds, causing gastroenteritis and other extraintestinal diseases. Clinical studies have established astroviruses as the second leading cause of viral diarrhea in young children. Here we report the crystal structure of the human astrovirus dimeric surface spike determined to 1.8-Å resolution. The overall structure of each spike/projection domain has a unique three-layered β-sandwiches fold, with a core, six-stranded β-barrel structure that is also found in the hepatitis E virus capsid protrusions, suggesting a closer phylogenetic relationship between these two viruses than previously acknowledged. Based on a hepatitis E virus capsid model, we performed homology modeling and produced a complete, T ¼ 3 astrovirus capsid model with features remarkably similar to those observed in a cryoelectron microscopy reconstruction image of a human astrovirus. Mapping conserved residues onto the astrovirus projection domain revealed a putative receptor binding site with amino acid compositions characteristic for polysaccharide recognition. Our results will have an important impact on future characterization of astrovirus structure and function, and will likely have practical applications in the development of vaccines and antivirals.structural protein | naked virus | attachment domain A stroviruses are small, nonenveloped, icosahedral viruses with a positive-sense, single-stranded RNA genome of approximately 7 kb in size. With members infecting mammals and avian species, the family of Astroviridae consists of two genera, Mammastrovirus and Avastrovirus (1, 2). Human astroviruses, eight serotypes in total, are one of the leading causes of gastroenteritis in young children, elderly people, and immunocompromised adults. Sporadic astrovirus infections as well as large-scale outbreaks in susceptible populations have been described (2). In addition to humans, astroviruses can infect a wide range of wild and domestic animals, causing gastroenteritis in most mammals and both intestinal and extraintestinal diseases in birds (1).The genomic RNA of astroviruses is polyadenylated with three ORFs. ORF1a and the downstream overlapping ORF1b encode two nonstructural polyproteins, nsp1a and nsp1ab. The nsp1ab protein is produced from both ORF1a and ORF1b through a translational frame-shift mechanism (3). Polyproteins nsp1a and nsp1ab are processed by a viral 3C-like serine protease and other cellular proteases into several proteins (e.g., the RNAdependent RNA polymerase and the serine protease) that are likely to function in the replication of the viral genome (4). ORF2 at the 3′ end of the viral genome encodes the viral capsid protein (CP) (5) and is found in both genomic and subgenomic RNAs. It is unclear whether the astrovirus RNA contains a 5′ cap. Viral capping enzymes such as guanylyltranferase and methyltransferases have not been identified in the astrovirus genome.When overexpressed in eukaryotic hosts, the 796-aa CP of the serotype 2 astrovirus was ...

Lineage diversification and recombination in type-4 human astroviruses

“…The HVR of AstV is believed to form the capsid spike (Dong et al, 2011) and to control binding to cell receptors as neutralising epitopes have been mapped inside this capsid portion. This region is subjected to impressive diversification across type-1 to -8 HAstVs, with the aa identity ranging from 44% to 69% (Sanchez-Fauquier et al, 1994;Wang et al, 2001).…”

“…The mature infectious virion contains three predominant protein species derived from the N-terminal domain and from the HVR after intra-and extra-cellular processing (Bass and Qiu, 2000;Sanchez-Fauquier et al, 1994). Mapping of neutralising monoclonal antibodies (Sanchez-Fauquier et al, 1994) and structural analysis have revealed that AstV capsid spike is formed by dimerisation of a polypeptide spanning the HVR and a putative binding receptor site with affinity for polysaccharide molecules has been predicted in the spike structure (Dong et al, 2011).…”

Crystal Structure of the Avian Astrovirus Capsid Spike

“…The capsid is a multidomain protein with a conserved N-terminal region and a highly variable C-terminal region (18)(19)(20)(21)(22). The conserved N-terminal region, comprising the basic, S, and P1 domains, encompasses the viral genome and forms the shell of the capsid (20,23). The highly variable C-terminal region comprises the spike (also called the P2 domain) and acidic domains (23).…”

Astrovirus Pathogenesis

“…Astrovirus has been shown to increase epithelial cell permeability upon infection of Caco-2 cells in vitro, possibly due to a reduction in actin fibers [30], reduction of occludin from the junctional complex [30], and redistribution of e-cadherin 24 h post-infection (hpi; Figure 2). sulfate partially blocked HAstV-8 infectivity of Caco-2 cells [66]. However, pretreatment of Caco-2 cells with chondroitinase and heparinase did not significantly reduce HAstV-1 infection in the same cells [63], suggesting that there may be strain-and/or genotype-specific differences.…”

“…Crystallization of the human HAstV-8 and turkey astrovirus type 2 (TAstV-2) capsid and spike proteins [62][63][64][65][66] showed conserved polysaccharide binding motifs [65]. The addition of heparin, heparin sulfate, or dextran sulfate Viruses 2017, 9, 22 4 of 10 partially blocked HAstV-8 infectivity of Caco-2 cells [66]. However, pretreatment of Caco-2 cells with chondroitinase and heparinase did not significantly reduce HAstV-1 infection in the same cells [63], suggesting that there may be strain-and/or genotype-specific differences.…”