Crystal Structure of the Core Region of Hantavirus Nucleocapsid Protein Reveals the Mechanism for Ribonucleoprotein Complex Formation.

UNLABELLED: Hantaviruses, which belong to the genus Hantavirus in the family Bunyaviridae, infect mammals, including humans, causing either hemorrhagic fever with renal syndrome (HFRS) or hantavirus cardiopulmonary syndrome (HCPS) in humans with high mortality. Hantavirus encodes a nucleocapsid protein (NP) to encapsidate the genome and form a ribonucleoprotein complex (RNP) together with viral polymerase. Here, we report the crystal structure of the core domains of NP (NPcore) encoded by Sin Nombre virus (SNV) and Andes virus (ANDV), which are two representative members that cause HCPS in the New World. The constructs of SNV and ANDV NPcore exclude the N- and C-terminal portions of full polypeptide to obtain stable proteins for crystallographic study. The structure features an N lobe and a C lobe to clamp RNA-binding crevice and exhibits two protruding extensions in both lobes. The positively charged residues located in the RNA-binding crevice play a key role in RNA binding and virus replication. We further demonstrated that the C-terminal helix and the linker region connecting the N-terminal coiled-coil domain and NPcore are essential for hantavirus NP oligomerization through contacts made with two adjacent protomers. Moreover, electron microscopy (EM) visualization of native RNPs extracted from the virions revealed that a monomer-sized NP-RNA complex is the building block of viral RNP. This work provides insight into the formation of hantavirus RNP and provides an understanding of the evolutionary connections that exist among bunyaviruses. IMPORTANCE: Hantaviruses are distributed across a wide and increasing range of host reservoirs throughout the world. In particular, hantaviruses can be transmitted via aerosols of rodent excreta to humans or from human to human and cause HFRS and HCPS, with mortalities of 15% and 50%, respectively. Hantavirus is therefore listed as a category C pathogen. Hantavirus encodes an NP that plays essential roles both in RNP formation and in multiple biological functions. NP is also the exclusive target for the serological diagnoses. This work reveals the structure of hantavirus NP, furthering the knowledge of hantavirus RNP formation, revealing the relationship between hantavirus NP and serological specificity and raising the potential for the development of new diagnosis and therapeutics targeting hantavirus infection.

Recognition of decay accelerating factor and alpha(v)beta(3) by inactivated hantaviruses: Toward the development of high-throughput screening flow cytometry assays.

Hantaviruses cause two severe diseases in humans: hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS). The lack of vaccines or specific drugs to prevent or treat HFRS and HCPS and the requirement for conducting experiments in a biosafety level 3 laboratory (BSL-3) limit the ability to probe the mechanism of infection and disease pathogenesis. In this study, we developed a generalizable spectroscopic assay to quantify saturable fluorophore sites solubilized in envelope membranes of Sin Nombre virus (SNV) particles. We then used flow cytometry and live cell confocal fluorescence microscopy imaging to show that ultraviolet (UV)-killed SNV particles bind to the cognate receptors of live virions, namely, decay accelerating factor (DAF/CD55) expressed on Tanoue B cells and alpha(v)beta(3) integrins expressed on Vero E6 cells. SNV binding to DAF is multivalent and of high affinity (K(d) approximately 26pM). Self-exchange competition binding assays between fluorescently labeled SNV and unlabeled SNV are used to evaluate an infectious unit-to-particle ratio of approximately 1:14,000. We configured the assay for measuring the binding of fluorescently labeled SNV to Tanoue B suspension cells using a high-throughput flow cytometer. In this way, we established a proof-of-principle high-throughput screening (HTS) assay for binding inhibition. This is a first step toward developing HTS format assays for small molecule inhibitors of viral-cell interactions as well as dissecting the mechanism of infection in a BSL-2 environment.

Oligomerization of hantaviral nucleocapsid protein: charged residues in the N-terminal coiled-coil domain contribute to intermolecular interactions.

The nucleocapsid (N) protein of hantaviruses (family Bunyaviridae) is the most abundant component of the virion; it encapsidates genomic RNA segments and participates in viral genome transcription and replication, as well as in virus assembly. During RNA encapsidation, the N protein forms intermediate trimers and then oligomers via 'head-to-head, tail-to-tail' interactions. In previous work, using Tula hantavirus (TULV) N protein as a model, it was demonstrated that an intact coiled-coil structure of the N terminus is crucial for the oligomerization capacity of the N protein and that the hydrophobic 'a' residues from the second alpha-helix are especially important. Here, the importance of charged amino acid residues located within the coiled-coil for trimer formation and oligomerization was analysed. To predict the interacting surfaces of the monomers, the previous in silico model of TULV coiled-coils was first upgraded, taking advantage of the recently published crystal structure of the N-terminal coiled-coil of the Sin Nombre virus N protein. The results obtained using a mammalian two-hybrid assay suggested that conserved, charged amino acid residues within the coiled-coil make a substantial contribution to N protein oligomerization. This contribution probably involves (i) the formation of interacting surfaces of the N monomers (residues D35 and D38, located at the tip of the coiled-coil loop, and R63 appear particularly important) and (ii) stabilization of the coiled-coil via intramolecular ionic bridging (with E55 as a key player). It is hypothesized that the tips of the coiled-coils are the first to come into direct contact and thus to initiate tight packing of the three structures.

The coiled-coil domain structure of the Sin Nombre virus nucleocapsid protein.

Hantaviruses can cause hemorrhagic fever with a renal syndrome and hantavirus pulmonary syndrome when transmitted to humans. The nucleocapsid protein of hantaviruses encapsidates viral genomic RNA and associates with transcription and replication complexes. Both the amino and carboxy termini of the nucleocapsid protein had been predicted to form trimers prior to the formation of the ribonucleoprotein. Crystal structures of amino-terminal fragments of the nucleocapsid protein showed the formation of intramolecular antiparallel coiled coils, but not intermolecular trimers. Thus, the amino-terminal part of the nucleocapsid protein is probably insufficient to initiate the trimerization of the full-length molecule.

Characterization of the RNA chaperone activity of hantavirus nucleocapsid protein.

Hantaviruses are tripartite negative-sense RNA viruses and members of the Bunyaviridae family. The nucleocapsid (N) protein, encoded by the smallest of the three genome segments (S), has nonspecific RNA chaperone activity. This activity results in transient dissociation of misfolded RNA structures, may be required for facilitating correct higher-order RNA structure, and may function in viral genome replication. We carried out a series of experiments to further characterize the ability of N to dissociate RNA duplexes. As might be expected, N dissociated RNA duplexes but not DNA duplexes or RNA-DNA heteroduplexes. The RNA-destabilizing activity of N is ATP independent, has a pH optimum of 7.5, and has an Mg(2+) concentration optimum of 1 to 2 mM. N protein is unable to unwind the RNA duplexes that are completely double stranded. However, in the presence of an adjoining single-stranded region, helix unwinding takes place in the 3'-to-5' direction through an unknown mechanism. The N protein trimer specifically recognizes and unwinds the terminal panhandle structure in the viral RNA and remains associated with unwound 5' terminus. We suggest that hantaviral nucleocapsid protein has an active role in hantaviral replication by working cooperatively with viral RNA polymerase. After specific recognition of the panhandle structure by N protein, the unwound 5' terminus likely remains transiently bound to N protein, creating an opportunity for the viral polymerase to initiate transcription at the accessible 3' terminus.

Generation of a recombinant cytomegalovirus for expression of a hantavirus glycoprotein.

A cytomegalovirus (CMV) was isolated from its natural host, Peromyscus maniculatus, and was designated Peromyscus CMV (PCMV). A recombinant PCMV was constructed that contained Sin Nombre virus glycoprotein G1 (SNV-G1) fused in frame to the enhanced green fluorescent protein (EGFP) gene inserted into a site homologous to the human CMV UL33 (P33) gene. The recombinant CMV was used for expression and immunization of deer mice against SNV-G1. The results of the study indicate that P. maniculatus could be infected with as few as 10 virus particles of recombinant virus. Challenge of P. maniculatus with either recombinant or wild-type PCMV produced no overt pathology in infected animals. P. maniculatus immunized with recombinant virus developed an antibody response to SNV and EGFP. When rechallenged with recombinant virus, animals exhibited an anamnestic response against SNV. Interestingly, a preexisting immune response against PCMV did not prevent reinfection with recombinant PCMV.

Purification and characterization of the Sin Nombre virus nucleocapsid protein expressed in Escherichia coli.

Sin Nombre virus is a member of the Hantavirus genus, family Bunyaviridae, and is an etiologic agent of hantavirus pulmonary syndrome. The hantavirus nucleocapsid (N) protein plays an important role in the encapsidation and assembly of the viral negative-sense genomic RNA. The Sin Nombre N protein was expressed as a C-terminal hexahistidine fusion in Escherichia coli and initially purified by nickel-affinity chromatography. We developed methods to extract the soluble fraction and to solubilize the remainder of the N protein using denaturants. Maximal expression of protein from native purification was observed after a 1.5-h induction with IPTG (2.4 mg/L). The zwitterionic detergent Chaps did not enhance the yield of native purifications, but increased the yield of protein obtained from insoluble purifications. Both soluble and insoluble materials, purified by nickel-affinity chromatography, were also subjected to Hi Trap SP Sepharose fast-flow (FF) chromatography. Both soluble and insoluble proteins had a similar A(280) profile on the Sepharose FF column, and both suggested the presence of a nucleic acid contaminant. The apparent dissociation constant of the N protein, purified by nickel-affinity and SP Sepharose FF chromatography, and the 5' end of the viral S-segment genome were measured using a filter binding assay. The N protein-vRNA complex had an apparent dissociation constant of 140 nM.