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Structure of a pentameric virion-associated fiber with a potential role in Orsay virus entry to host cells.

ABSTRACT: Despite the wide use of Caenorhabditis elegans as a model organism, the first virus naturally infecting this organism was not discovered until six years ago. The Orsay virus and its related nematode viruses have a positive-sense RNA genome, encoding three proteins: CP, RdRP, and a novel ? protein that shares no homology with any other proteins. ? can be expressed either as a free ? or a CP-? fusion protein by ribosomal frameshift, but the structure and function of both ? and CP-? remain unknown. Using a combination of electron microscopy, X-ray crystallography, computational and biophysical analyses, here we show that the Orsay ? protein forms a ~420-Å long, pentameric fiber with an N-terminal ?-helical bundle, a ?-stranded filament in the middle, and a C-terminal head domain. The pentameric nature of the ? fiber has been independently confirmed by both mass spectrometry and analytical ultracentrifugation. Recombinant Orsay capsid containing CP-? shows protruding long fibers with globular heads at the distal end. Mutant viruses with disrupted CP-? fibers were generated by organism-based reverse genetics. These viruses were found to be either non-viable or with poor infectivity according to phenotypic and qRT-PCR analyses. Furthermore, addition of purified ? proteins to worm culture greatly reduced Orsay infectivity in a sequence-specific manner. Based on the structure resemblance between the Orsay CP-? fiber and the fibers from reovirus and adenovirus, we propose that CP-? functions as a cell attachment protein to mediate Orsay entry into worm intestine cells.

SUBMITTER: Fan Y

PROVIDER: S-EPMC5344674 | biostudies-literature | 2017 Feb

REPOSITORIES: biostudies-literature

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Structure of a pentameric virion-associated fiber with a potential role in Orsay virus entry to host cells.

Fan Yanlin Y Guo Yusong R YR Yuan Wang W Zhou Ying Y Holt Matthew V MV Wang Tao T Demeler Borries B Young Nicolas L NL Zhong Weiwei W Tao Yizhi J YJ

PLoS pathogens 20170227 2

Despite the wide use of Caenorhabditis elegans as a model organism, the first virus naturally infecting this organism was not discovered until six years ago. The Orsay virus and its related nematode viruses have a positive-sense RNA genome, encoding three proteins: CP, RdRP, and a novel δ protein that shares no homology with any other proteins. δ can be expressed either as a free δ or a CP-δ fusion protein by ribosomal frameshift, but the structure and function of both δ and CP-δ remain unknown. ...[more]

PMID: 28241071

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Project description:Fiber proteins are commonly found in eukaryotic and prokaryotic viruses, where they play important roles in mediating viral attachment and host cell entry. They typically form trimeric structures and are incorporated into virions via noncovalent interactions. Orsay virus, a small RNA virus which specifically infects the laboratory model nematode Caenorhabditis elegans, encodes a fibrous protein δ that can be expressed as a free protein and as a capsid protein-δ (CP-δ) fusion protein. Free δ has previously been demonstrated to facilitate viral exit following intracellular expression; however, the biological significance and prevalence of CP-δ remained relatively unknown. Here, we demonstrate that Orsay CP-δ is covalently incorporated into infectious particles, the first example of any attached viral fibers known to date. The crystal structure of δ(1-101) (a deletion mutant containing the first 101 amino acid [aa] residues of δ) reveals a pentameric, 145-Å long fiber with an N-terminal coiled coil followed by multiple β-bracelet repeats. Electron micrographs of infectious virions depict particle-associated CP-δ fibers with dimensions similar to free δ. The δ proteins from two other nematode viruses, Le Blanc and Santeuil, which both specifically infect Caenorhabditis briggsae, were also found to form fibrous molecules. Recombinant Le Blanc δ was able to block Orsay virus infection in worm culture and vice versa, suggesting these two viruses likely compete for the same cell receptor(s). Thus, we propose that while CP-δ likely mediates host cell attachment for all three nematode viruses, additional downstream factor(s) ultimately determine the host specificity and range of each virus.IMPORTANCE Viruses often have extended fibers to mediate host cell recognition and entry, serving as promising targets for antiviral drug development. Unlike other known viral fibers, the δ proteins from the three recently discovered nematode viruses are incorporated into infectious particles as protruding fibers covalently linked to the capsid. Crystal structures of δ revealed novel pentameric folding repeats, which we term β-bracelets, in the intermediate shaft region. Based on sequence analysis, the β-bracelet motif of δ is conserved in all three nematode viruses and could account for ∼60% of the total length of the fiber. Our study indicated that δ plays important roles in cell attachment for this group of nematode viruses. In addition, the tightly knitted β-bracelet fold, which presumably allows δ to survive harsh environments in the worm gut, could be applicable to bioengineering applications given its potentially high stability.

Project description:UnlabelledThe pollination services provided by the western honeybee (Apis mellifera) are critical for agricultural production and the diversity of wild flowering plants. However, honeybees suffer from environmental pollution, habitat loss, and pathogens, including viruses that can cause fatal diseases. Israeli acute bee paralysis virus (IAPV), from the family Dicistroviridae, has been shown to cause colony collapse disorder in the United States. Here, we present the IAPV virion structure determined to a resolution of 4.0 Å and the structure of a pentamer of capsid protein protomers at a resolution of 2.7 Å. IAPV has major capsid proteins VP1 and VP3 with noncanonical jellyroll β-barrel folds composed of only seven instead of eight β-strands, as is the rule for proteins of other viruses with the same fold. The maturation of dicistroviruses is connected to the cleavage of precursor capsid protein VP0 into subunits VP3 and VP4. We show that a putative catalytic site formed by the residues Asp-Asp-Phe of VP1 is optimally positioned to perform the cleavage. Furthermore, unlike many picornaviruses, IAPV does not contain a hydrophobic pocket in capsid protein VP1 that could be targeted by capsid-binding antiviral compounds.ImportanceHoneybee pollination is required for agricultural production and to sustain the biodiversity of wild flora. However, honeybee populations in Europe and North America are under pressure from pathogens, including viruses that cause colony losses. Viruses from the family Dicistroviridae can cause honeybee infections that are lethal, not only to individual honeybees, but to whole colonies. Here, we present the virion structure of an Aparavirus, Israeli acute bee paralysis virus (IAPV), a member of a complex of closely related viruses that are distributed worldwide. IAPV exhibits unique structural features not observed in other picorna-like viruses. Capsid protein VP1 of IAPV does not contain a hydrophobic pocket, implying that capsid-binding antiviral compounds that can prevent the replication of vertebrate picornaviruses may be ineffective against honeybee virus infections.

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Structure of a pentameric virion-associated fiber with a potential role in Orsay virus entry to host cells.

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Structure of a pentameric virion-associated fiber with a potential role in Orsay virus entry to host cells.

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