Structure of the hepatitis E virus-like particle suggests mechanisms for virus assembly and receptor binding

Characterization of self-assembled virus-like particles of rat hepatitis E virus generated by recombinant baculoviruses

Hepatitis E virus (HEV) is a causative agent of hepatitis E. Recently, a novel hepatitis E-like virus was isolated from Norway rats in Germany. However, the antigenicity, pathogenicity and epidemiology of this virus are unclear because of the lack of a cell-culture system in which to grow it. In this study, an N-terminally truncated ORF2 protein was expressed in insect Tn5 cells using a recombinant baculovirus expression system and a large amount of 53 kDa protein was expressed and efficiently released into the supernatant. Electron microscopic analyses of the purified 53 kDa protein revealed that the protein self-assembled into two types of empty HEV-like particles (rat HEVLPs). The smaller rat HEVLPs were estimated to be 24 nm in diameter, which is similar to the size of genotype G1, G3 and G4 HEVLPs. The larger rat HEVLPs were estimated to measure 35 nm in diameter, which is similar to the size of native rat HEV particles. An ELISA to detect antibodies was established using rat HEVLPs as the antigens, which demonstrated that rat HEVLPs were cross-reactive with G1, G3 and G4 HEVs. Detection of IgG and IgM antibodies was performed by examination of 139 serum samples from wild rats trapped in Vietnam, and it was found that 20.9 % (29/139) and 3.6 % (5/139) of the samples were positive for IgG and IgM, respectively. In addition, rat HEV RNA was detected in one rat serum sample that was positive for IgM. These results indicated that rat HEV is widespread and is transmitted among wild rats.

Crystal structure of the human astrovirus capsid spike

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 = 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 basis for the neutralization and genotype specificity of hepatitis E virus

Hepatitis E virus (HEV) causes acute hepatitis in humans, predominantly by contamination of food and water, and is characterized by jaundice and flu-like aches and pains. To date, no vaccines are commercially available to prevent the disease caused by HEV. Previously, we showed that a monoclonal antibody, 8C11, specifically recognizes a neutralizing conformational epitope on HEV genotype I. The antibody 8C11 blocks the virus-like particle from binding to and penetrating the host cell. Here, we report the complex crystal structure of 8C11 Fab with HEV E2s(I) domain at 1.9 Å resolution. The 8C11 epitopes on E2s(I) were identified at Asp(496)-Thr(499), Val(510)-Leu(514), and Asn(573)-Arg(578). Mutations and cell-model assays identified Arg(512) as the most crucial residue for 8C11 interaction with and neutralization of HEV. Interestingly, 8C11 specifically neutralizes HEV genotype I, but not the other genotypes. Because HEV type I and IV are the most abundant genotypes, to understand this specificity further we determined the structure of E2s(IV) at 1.79 Å resolution and an E2s(IV) complex with 8C11 model was generated. The comparison between the 8C11 complexes with type I and IV revealed the key residues that distinguish these two genotypes. Of particular interest, the residue at amino acid position 497 at the 8C11 epitope region of E2s is distinct among these two genotypes. Swapping this residue from one genotype to another inversed the 8C11 reactivity, demonstrating the essential role played by amino acid 497 in the genotype recognition. These studies may lead to the development of antibody-based drugs for the specific treatment against HEV.

Efficient cell culture systems for hepatitis E virus strains in feces and circulating blood

Attempts have been made to propagate hepatitis E virus (HEV) in primary hepatocyte culture and various other cultured cells. However, the replication ability of HEV recovered from culture media remains extremely low. Recently, efficient culture systems have been established in PLC/PRF/5 (hepatocellular carcinoma) and A549 (lung cancer) cell lines for HEV strains of genotypes 3 and 4 in our laboratory. They originated in fecal extracts from patients containing HEV RNA in extremely high-titers (10(7)  copies/ml), and named the JE03-1760F (genotype 3) and HE-JF5/15F (genotype 4) strains, respectively. HEV RNA in culture supernatants reached 10(8)  copies/ml in titer, and were transmitted successively through many passages. An infectious HEV cDNA clone (pJE03-1760F/wt) was constructed that has replication activity comparable to that of the wild-type JE03-1760F in feces. The ORF3 protein is indispensable for shedding HEV particles from cells in the reverse genetics system. HEV recovered from culture media, as well as circulating HEV, possess ORF3 proteins on the surface and are covered with cellular membranes, and therefore, ORF2 epitopes are buried in these particles. In contrast, HEV excreted into feces are naked nucleocapsids without a lipid layer or surface expression of the ORF3 protein. HEV in sera of patients with acute hepatitis E can infect and replicate in PLC/PRF/5 and A549 cells, with efficiency comparable to the circulating HEV RNA levels. High-efficiency cell culture systems for infectious viruses, thus developed, are expected to open up a new era and resolve many mysteries in the epidemiology, molecular biology, and treatment of HEV.

Structure of hepatitis E viral particle

Hepatitis E is acute hepatitis caused by infection of hepatitis E virus (HEV) via a fecal-to-oral or zoonotic route. HEV is a small, non-enveloped virus containing positive strand RNA as a genome. Recently, the three-dimensional structures of the HEV-like particles and spike domain protruded from the surface of the particle expressed by recombinant baculovirus or bacteria have been revealed. Based on these reports, the structural features of the HEV capsid subunit and viral particle are reviewed to give insights to the mechanisms underlying the particle assembly, antigenicity, host cell attachment and native virion packaging.

Three amino acid mutations (F51L, T59A, and S390L) in the capsid protein of the hepatitis E virus collectively contribute to virus attenuation

Hepatitis E virus (HEV) is an important but extremely understudied human pathogen, and the mechanisms of HEV replication and pathogenesis are largely unknown. We previously identified an attenuated genotype 3 HEV mutant (pSHEV-1) containing three unique amino acid mutations (F51L, T59A, and S390L) in the capsid protein. To determine the role of each of these mutations, we constructed three HEV single mutants (rF51L, rT59A, and rS390L) which were all found to be replication competent in Huh7 liver cells. To determine the pathogenicities of the mutants, we utilized the specific-pathogen-free (SPF) pig model for HEV and a unique inoculation procedure that bypasses the need for propagating infectious HEV in vitro. A total of 60 pigs were intrahepatically inoculated, via an ultrasound-guided technique, with in vitro-transcribed full-length capped RNA transcripts from the infectious clones of each single mutant, the pSHEV-1 triple mutant, wild-type pSHEV-3, or phosphate-buffered saline (PBS) buffer (n = 10). The results showed that the F51L mutation partially contributed to virus attenuation, whereas the T59A and S390L mutations resulted in more drastic attenuation of HEV in pigs, as evidenced by a significantly lower incidence of viremia, a delayed appearance and shorter duration of fecal virus shedding and viremia, and lower viral loads in liver, bile, and intestinal content collected at three different necropsy times. The results indicate that the three mutations in the capsid protein collectively contribute to HEV attenuation. This study has important implications for developing a modified live-attenuated vaccine against HEV.

Hepatitis E virus cell culture models

Early studies reported the propagation of hepatitis E virus (HEV) in either primary hepatocytes or several established cell lines, but replication was inefficient. Efficient cell culture systems for HEV in PLC/PRF/5 and A549 cells have recently been established, using inoculum of fecal suspensions with high HEV loads, originally obtained from patients with genotype 3 HEV (the JE03-1760F strain, 2.0×10(7) copies/ml) or genotype 4 HEV (the HE-JF5/15F strain, 1.3×10(7) copies/ml), and many generations were successfully propagated in serial passages of culture supernatant. In addition, a full-length infectious cDNA clone (pJE03-1760F/wt) of the JE03-1760F strain was constructed, which can replicate efficiently in PLC/PRF/5 and A549 cells. A derivative ORF3-deficient mutant revealed that the ORF3 protein of HEV is responsible for virion egress from infected cells and is present on the surface of released HEV particles, which is associated with lipids. Various HEV strains with high loads of ≥10(5) copies/ml in circulating blood were also propagated efficiently in PLC/PRF/5 and A549 cells. This paper reviews the road map toward the development of efficient cell culture systems for a wide variety of HEV strains and introduces the current knowledge on virion egress obtained by cell culture models.

Molecular virology of hepatitis E virus

This review details the molecular virology of the hepatitis E virus (HEV). While replicons and in vitro infection systems have recently become available, a lot of information on HEV has been generated through comparisons with better-studied positive-strand RNA viruses and through subgenomic expression of viral open reading frames. These models are now being verified with replicon and infection systems. We provide here the current knowledge on the HEV genome and its constituent proteins--ORF1, ORF2 and ORF3. Based on the available information, we also modify the existing model of the HEV life cycle.

Analysing complete genome sequence of swine hepatitis E virus (HEV), strain CHN-XJ-SW13 isolated from Xinjiang, China: putative host range, and disease severity determinants in HEV

Hepatitis E is a worldwide public health problem, particular in areas where hygiene conditions are poor. Hepatitis E virus (HEV) has at least four genotypes: genotypes 1 and 2 exclusively infect human; while genotypes 3 and 4, are considered to be a zoonotic agent, infecting both humans and animals. This study was aimed at determining why genotype 3 and 4 HEV strains isolated from swine are able to cross species borders, whereas genotype1 and 2 strains isolated from humans are not. The full length genome of the swine HEV isolate CHN-XJ-SW13 was amplified as overlapping fragments using reverse-transcription-nested polymerase chain reaction (RT-nPCR) and rapid amplification of cDNA ends (RACE). The sequence of CHN-XJ-SW13 was compared with those of 90 HEV strains covering genotype 1-4 retrieved from GenBank. Possible regions of the viral genome, specifying the host range of HEV or associated with the severity of hepatitis E disease, were then screened for with the aid of the ALIGNX sequences alignment software package. The CHN-XJ-SW13 swine HEV isolate was determined to be a novel subtype of genotype 4, whose sequence provided several valuable clues for tracing the sources of human HEV infection. 25 specific nucleotide positions were identified to possibly being involved specifying the host range of HEV or determining the severity of hepatitis E disease.

Hepatitis E: Historical, contemporary and future perspectives

Hepatitis E was suspected for the first time in 1980 during a waterborne epidemic of acute hepatitis in Kashmir, India. In the 30 years since then, a small virus with single-stranded RNA genome has been identified as the cause of this disease and named as hepatitis E virus (HEV). The virus has four genotypes; of these, genotypes 1 and 2 are known to infect only humans, whereas genotypes 3 and 4 primarily infect other mammals, particularly pigs, but occasionally cause human disease. In highly-endemic areas, the disease occurs in epidemic and sporadic forms, caused mainly by infection with genotype 1 or 2 virus, acquired through the fecal-oral route, usually through contaminated water supplies. The disease is characterized by particularly severe course and high mortality among pregnant women. In persons with pre-existing chronic liver disease, HEV superinfection can present as acute-on-chronic liver disease. In low-endemic regions, sporadic cases of locally-acquired HEV infection are reported; these are caused mainly by genotype 3 or 4 HEV acquired possibly through zoonotic transmission from pigs, wild boars or deer. In these areas, chronic infection with genotype 3 HEV, which may progress to liver cirrhosis, has been reported among immunosuppressed persons. Two subunit vaccines containing recombinant truncated capsid proteins of HEV have been shown to be highly effective in preventing the disease; however, these are not yet commercially available. These vaccines should be of particular use in groups that are at high risk of HEV infection and/or of poor outcome.

Intergenotypic chimeric hepatitis E viruses (HEVs) with the genotype 4 human HEV capsid gene in the backbone of genotype 3 swine HEV are infectious in pigs

Genotypes 1 and 2 hepatitis E virus (HEV) infect only humans whereas genotypes 3 and 4 HEV infect both humans and pigs. To evaluate the mechanism of cross-species HEV infection between humans and swine, in this study we constructed five intergenotypic chimeric viruses and tested for their infectivity in vitro and in pigs. We demonstrated that chimeric viruses containing the ORF2 capsid gene either alone or in combination with its adjacent 5' junction region (JR) and 3' noncoding region (NCR) from a genotype 4 human HEV in the backbone of a genotype 3 swine HEV are replication-competent in Huh7 cells and infectious in HepG2/C3A cells and in pigs, and thus supporting the hypothesis that genotypes 3 and 4 human HEV are of swine origin. However, chimeric viruses containing the JR+ORF2+3' NCR of genotypes 3 or 4 HEV in the backbone of genotype 1 human HEV failed to infect pigs, suggesting that other genomic regions such as 5' NCR and ORF1 may also be involved in HEV cross-species infection. The results from this study provide the first experimental evidence of the exchangeability of the capsid gene between genotype 3 swine HEV and genotype 4 human HEV, and have important implications for understanding the mechanism of HEV cross-species infection.

Hepatitis E virus replication involves alternating negative- and positive-sense RNA synthesis

Hepatitis E virus (HEV) is the major cause of epidemic hepatitis and many outbreaks of sporadic hepatitis. The virus responsible has a single-stranded, positive-sense RNA. Its replication and the regulatory process involved therein are poorly understood. Much of the HEV biology studied has been done by using full-length capped RNA transcripts (replicons) and transient transfections in cell cultures. We investigated replicon replication using negative-sense strand-specific molecular beacons in live cell imaging, and quantifying intracellular viral RNA using strand-specific real-time PCR every 2 h until 24 h post-transfection. A graph of the copy numbers of both positive- and negative-sense RNA at the different time points was plotted. This showed a temporal separation and alternating cycles of negative- and positive-sense RNA formation. As a control, a dysfunctional replicase mutant (GDD→GAA) was used, which showed no increase in copy number. The live cell imaging corroborated the quantitative data, in that the maximal amount of negative-sense RNA was observed at 8 h post-transfection. The real-time-PCR copy-number analysis of the subgenome showed the presence of a single subgenomic RNA. Using fluorescent protein genes mCherry and EGFP fused in-frame to ORF2 and ORF3 in separate constructs and immunofluorescence, we showed the formation of both proteins pORF2 and pORF3 from a single subgenomic RNA. Our study demonstrated cyclical bursts of virus replication and the role of subgenomic RNA in the HEV life cycle.

Spatial configuration of hepatitis E virus antigenic domain

Hepatitis E virus (HEV) is a human pathogen that causes acute hepatitis. When an HEV capsid protein containing a 52-amino-acid deletion at the C terminus and a 111-amino-acid deletion at the N terminus is expressed in insect cells, the recombinant HEV capsid protein can self-assemble into a T=1 virus-like particle (VLP) that retains the antigenicity of the native HEV virion. In this study, we used cryoelectron microscopy and image reconstruction to show that anti-HEV monoclonal antibodies bind to the protruding domain of the capsid protein at the lateral side of the spikes. Molecular docking of the HEV VLP crystal structure revealed that Fab224 covered three surface loops of the recombinant truncated second open reading frame (ORF2) protein (PORF2) at the top part of the spike. We also determined the structure of a chimeric HEV VLP and located the inserted B-cell tag, an epitope of 11 amino acids coupled to the C-terminal end of the recombinant ORF2 protein. The binding site of Fab224 appeared to be distinct from the location of the inserted B-cell tag, suggesting that the chimeric VLP could elicit immunity against both HEV and an inserted foreign epitope. Therefore, the T=1 HEV VLP is a novel delivery system for displaying foreign epitopes at the VLP surface in order to induce antibodies against both HEV and the inserted epitope.

Structure of hepatitis E virion-sized particle reveals an RNA-dependent viral assembly pathway

Hepatitis E virus (HEV) induces acute hepatitis in humans with a high fatality rate in pregnant women. There is a need for anti-HEV research to understand the assembly process of HEV native capsid. Here, we produced a large virion-sized and a small T=1 capsid by expressing the HEV capsid protein in insect cells with and without the N-terminal 111 residues, respectively, for comparative structural analysis. The virion-sized capsid demonstrates a T=3 icosahedral lattice and contains RNA fragment in contrast to the RNA-free T=1 capsid. However, both capsids shared common decameric organization. The in vitro assembly further demonstrated that HEV capsid protein had the intrinsic ability to form decameric intermediate. Our data suggest that RNA binding is the extrinsic factor essential for the assembly of HEV native capsids.

Homology model and potential virus-capsid binding site of a putative HEV receptor Grp78

P239, a truncated construct of the hepatitis E virus (HEV) ORF2 protein, has been proven able to bind with a chaperone, Grp78, in both an in vitro co-immune precipitation test and an in vivo cell model. We previously solved the crystal structure of E2s--the C-terminal domain of p239 involved in host interactions. In the present study, we built a 3D structure of Grp78 using homology modeling methods, and docked this molecule with E2s using the Zdockpro module of the InsightII software package. The modeled Grp78 structure was deemed feasible by profile 3D evaluation and molecular dynamic simulations. The docking result consists of six clusters of distinct complexes and C035 was selected as the most reasonable. The interacting interface of the predicted complex is comprised of the Grp78 linker region and nucleotide binding domain along with the E2s groove region and surrounding loops. Using energy, hydrogen bond and solvent accessible surface analyses, we identified a series of key residues that may be involved in the Grp78:E2s interaction. By comparing with the known structure of the Hsp70:J complex, we further concluded that the interaction of Grp78 and E2s could interrupt binding of Grp78 with the J domain, and in turn diminish or even eliminate the binding ability of the Grp78 substrate binding domain. The predicted series of key residues also provides clues for further research that should improve our understanding of the fundamental molecular mechanisms of HEV infection.

Release of genotype 1 hepatitis E virus from cultured hepatoma and polarized intestinal cells depends on open reading frame 3 protein and requires an intact PXXP motif

Hepatitis E virus genotype 1 strain Sar55 replicated in subcloned Caco-2 intestinal cells and Huh7 hepatoma cells that had been transfected with in vitro transcribed viral genomes, and hepatitis E virions were released into the culture medium of both cell lines. Virus egress from cells depended on open reading frame 3 (ORF3) protein, and a proline-rich sequence in ORF3 was important for egress from cultured cells and for infection of macaques. Both intracellular ORF3 protein accumulation and virus release occurred at the apical membrane of polarized Caco-2 cells. ORF3 protein and lipids were intimately associated with virus particles produced in either cell line; ORF2 epitopes were masked in these particles and could not be immunoprecipitated with anti-ORF2.

Hepatitis E virus: a zoonosis adapting to humans

Hepatitis E virus (HEV) infection is gaining global attention, not only because of the increasing burden of the disease in low endemicity countries, in terms of morbidity and mortality rates, but also due to recent advances in the molecular virology and epidemiology of this emerging pathogen. HEV infection spread can be described as the evolution of a zoonosis towards an established human infection. As known from other viruses, such as the human immunodeficiency virus or the influenza viruses, crossing the species barriers from animals to humans is a recurrent phenomenon. Albeit slow at the beginning, once the virus has adapted to humans, the person-to-person spread can proceed very quickly. Although an optimal cell culture system for HEV is not yet available, outstanding progress has been made with the in vitro expression of HEV-like particles. These new tools have fostered new research to understand the molecular, structural and immunological aspects of human HEV infection. Although some promising data from Phase II vaccine trials are available, recent discoveries will certainly open new avenues for HEV-specific prophylaxis and therapy.

Recent advances in Hepatitis E virus

Hepatitis E virus (HEV), the causative agent of hepatitis E, belongs to the family Hepeviridae. At least four major genotypes of HEV have been recognized: genotypes 1 and 2 are restricted to humans and associated with epidemics in developing countries, whereas genotypes 3 and 4 are zoonotic and infect humans and several other animals in both developing and industrialized countries. Besides humans, strains of HEV have been genetically identified from swine, chickens, sika deer, mongeese, and rabbits. The genome of HEV consists of three open reading frames (ORFs): ORF1 codes for nonstructural proteins, ORF2 codes for capsid protein, and ORF3 codes for a small multifunctional protein. The ORF2 and ORF3 proteins are translated from a single bicistronic mRNA and overlap each other but neither overlaps ORF1. The recent determination of the 3D crystal structure of the HEV capsid protein should facilitate the development of vaccines and antivirals. The identification and characterization of animal strains of HEV from pigs and chickens and the demonstrated ability of cross-species infection by swine HEV raise public health concerns for zoonosis. Accumulating evidence indicated that hepatitis E is a zoonotic disease and pigs and more likely other animal species are reservoirs for HEV. This article provides an overview of the recent advances in hepatitis E and its causative agent, including nomenclature and genomic organization, gene expression and functions, 3D structure of the virions, changing perspectives on higher mortality during pregnancy and chronic hepatitis E, animal reservoirs, zoonotic risk, food safety, and novel animal models.

Hepatitis E Virus (HEV) strains in serum samples can replicate efficiently in cultured cells despite the coexistence of HEV antibodies: characterization of HEV virions in blood circulation

We recently developed a cell culture system for hepatitis E virus (HEV) in PLC/PRF/5 and A549 cells, using fecal specimens from HEV-infected patients. Since transfusion-associated hepatitis E has been reported, we examined PLC/PRF/5 and A549 cells for the ability to support replication of HEV in various serum samples obtained from 23 patients with genotype 1, 3, or 4 HEV. HEV progenies emerged in culture media of PLC/PRF/5 cells, regardless of the coexistence of HEV antibodies in serum but dependent on the load of HEV inoculated (31% at 2.0 x 10(4) copies per well and 100% at >or=3.5 x 10(4) copies per well), and were successfully passaged in A549 cells. HEV particles in serum, with or without HEV antibodies, banded at a sucrose density of 1.15 to 1.16 g/ml, which was markedly lower than that for HEV particles in feces, at 1.27 to 1.28 g/ml, and were nonneutralizable by immune sera in this cell culture system. An immuno-capture PCR assay of HEV virions treated with or without detergent indicated that HEV particles in serum are associated with lipids and HEV ORF3 protein, similar to those in culture supernatant. By immunoprecipitation, it was found that >90% of HEV particles in the circulation exist as free virions not complexed with immunoglobulins, despite the coexistence of HEV antibodies. These results suggest that our in vitro cell culture system can be used for propagation of a wide variety of HEV strains in sera from various infected patients, allowing extended studies on viral replication specific to different HEV strains.

Application of truncated immunodominant polypeptide from hepatitis E virus (HEV) ORF2 in an assay to exclude nonspecific binding in detecting anti-HEV immunoglobulin M

The diagnosis of recent hepatitis E virus (HEV) infection depends on serologic testing for anti-HEV IgM; however, false-positive results may occur. In the present study, we cloned the ORF2 fragment of genotype 4 HEV and demonstrated that a subregion covering amino acids 459 to 607 in ORF2 forms the immunodominant B-cell epitopes, as it does in genotype 1 viruses. Truncation of several residues from either the N or C terminus of the polypeptide abolished the reactivity of anti-HEV from naturally infected persons. By the combination of high reactivity of the immunodominant polypeptide and poor reactivity of the truncated polypeptide, we established an indirect enzyme-linked immunosorbent assay (ELISA) to detect anti-HEV IgM. In this assay, all 37 sera that were HEV RNA positive reacted with the immunodominant polypeptide but not with the truncated one, and none of 159 sera from healthy persons reacted with either of the polypeptides. In retesting of 117 sera that originally tested positive for anti-HEV IgM, using a Genelabs kit, only 34 were positive and 83 were negative. Western blot analyses and other experiments strongly indicated that these 83 discordant sera were negative for anti-HEV IgM. Furthermore, among the 117 sera, 5 reacted with both the immunodominant and truncated polypeptides, with comparable optical densities at 450 nm. However, their reactivity was demonstrated to result from nonspecific binding. Together, the data indicate that the poor reactivity of a truncated ORF2 polypeptide can be used to exclude nonspecific binding in the detection of anti-HEV IgM.

Heparan sulfate proteoglycans are required for cellular binding of the hepatitis E virus ORF2 capsid protein and for viral infection

The hepatitis E virus (HEV), a nonenveloped RNA virus, is the causative agent of hepatitis E. The mode by which HEV attaches to and enters into target cells for productive infection remains unidentified. Open reading frame 2 (ORF2) of HEV encodes its major capsid protein, pORF2, which is likely to have the determinants for virus attachment and entry. Using an approximately 56-kDa recombinant pORF2 that can self-assemble as virus-like particles, we demonstrated that cell surface heparan sulfate proteoglycans (HSPGs), specifically syndecans, play a crucial role in the binding of pORF2 to Huh-7 liver cells. Removal of cell surface heparan sulfate by enzymatic (heparinase) or chemical (sodium chlorate) treatment of cells or competition with heparin, heparan sulfate, and their oversulfated derivatives caused a marked reduction in pORF2 binding to the cells. Syndecan-1 is the most abundant proteoglycan present on these cells and, hence, plays a key role in pORF2 binding. Specificity is likely to be dictated by well-defined sulfation patterns on syndecans. We show that pORF2 binds syndecans predominantly via 6-O sulfation, indicating that binding is not entirely due to random electrostatic interactions. Using an in vitro infection system, we also showed a marked reduction in HEV infection of heparinase-treated cells. Our results indicate that, analogous to some enveloped viruses, a nonenveloped virus like HEV may have also evolved to use HSPGs as cellular attachment receptors.