Crystal Structure of the Avian Astrovirus Capsid Spike

Abstract

c Astroviruses are small, nonenveloped, single-stranded RNA viruses that cause diarrhea in a wide variety of mammals and birds. On the surface of the viral capsid are globular spikes that are thought to be involved in attachment to host cells. To understand the basis of species specificity, we investigated the structure of an avian astrovirus capsid spike and compared it to a previously reported human astrovirus capsid spike structure. Here we report the crystal structure of the turkey astrovirus 2 (TAstV-2) capsid surface spike domain, determined to 1.5-Å resolution, and identify three conserved patches on the surface of the spike that are candidate avian receptor-binding sites. Surprisingly, the overall TAstV-2 capsid spike structure is unique, with only distant structural similarities to the human astrovirus capsid spike and other viral capsid spikes. There is an absence of conserved putative receptor-binding sites between the human and avian spikes. However, there is evidence for carbohydrate-binding sites in both human and avian spikes, and studies with human astrovirus 1 (HAstV-1) suggest a minor role in infection for chondroitin sulfate but not heparin. Overall, our structural and functional studies provide new insights into astrovirus host cell entry, species specificity, and evolution.A stroviruses are small, nonenveloped RNA viruses that are associated with diarrhea in a wide variety of species. The Astroviridae family is composed of two genera, Mamastrovirus and Avastrovirus, that infect mammalian and avian species, respectively. Members of the Mamastrovirus genus include human astroviruses, which are one of the leading causes of gastroenteritis in children under the age of 2 years, immunocompromised people, and the elderly (1-3). Infection with human astrovirus serotype 1 (HAstV-1) is most commonly detected, although eight canonical serotypes of HAstV (HAstV-1 to -8) and several noncanonical human genogroups have been isolated with various frequencies (4, 5). In poultry, infections with members of the Avastrovirus genus have been shown to cause disease, growth defects, and mortality in a wide variety of poultry, including chickens, turkeys, and ducks (6-10).Astroviruses possess single-stranded, positive-sense RNA genomes containing three open reading frames: ORF1a, ORF1b, and ORF2. These encode the viral nonstructural proteins, the viral RNA-dependent RNA polymerase, and the viral capsid protein, respectively (11-16). The astrovirus capsid is responsible for attachment and entry into host cells (15,17). The capsid is a multidomain protein with a conserved N-terminal region and a highly variable C-terminal region (18-22). 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). The acidic domain is removed from the virion by host cell caspases (24, 25), leaving only the spikes projecting from the capsid ...

Ecological Drivers of Virus Evolution: Astrovirus as a Case Study

“…Clathrin-dependent pathways appear to be used in Caco-2 cells (18). Crystal structure analysis of the capsid spike domain (the primary binding site) demonstrates drastic differences in the morphology of avastroviruses and mamastroviruses, suggesting a potential host limitation, yet the presence of human seroconversion against turkey astroviruses reveals their immunogenicity when incidental hosts are exposed (15,19). The disparity of astrovirus growth rates in different mammalian cell lines indicates these viruses may use multiple receptors or binding sites, making it difficult to discern the susceptible individuals in an environment if there is a mixture of species.…”

Type I Interferon Response Limits Astrovirus Replication and Protects against Increased Barrier Permeability In Vitro and In Vivo

“…HAstV-1 was propagated in Caco2 cells, and the titers of the virus were determined on Caco2 cells by the fluorescent-focus assay (focus-forming units [FFU]) as previously described (20). Briefly, 2 ϫ 10 4 cells were seeded into 96-well plates, and after 3 days, the cells were inoculated with 10-fold dilutions of HAstV-1 in MEM containing 0.3% bovine serum albumin (BSA) for 1 h at 37°C, at which time the virus was replaced with MEM containing 0.3% BSA.…”

A trivalent vaccine candidate against hepatitis E virus, norovirus, and astrovirus

“…As a result, the P domains can elicit neutralizing antibodies against infection of the three viruses [20–22], respectively, and therefore are attractive targets for subunit vaccine development against the three viruses [20,23]. Like those on the viral capsid, heterologous expressions of the P domain proteins by Escherichia coli self-assemble into P domain dimers [10,13,24–26]. …”