Structure of the hepatitis A virion: peptide mapping of the capsid region

Downstream processing of cell culture-derived virus particles

Manufacturing of cell culture-derived virus particles for vaccination and gene therapy is a rapidly growing field in the biopharmaceutical industry. The process involves a number of complex tasks and unit operations ranging from selection of host cells and virus strains for the cultivation in bioreactors to the purification and formulation of the final product. For the majority of cell culture-derived products, efforts focused on maximization of bioreactor yields, whereas design and optimization of downstream processes were often neglected. Owing to this biased focus, downstream procedures today often constitute a bottleneck in various manufacturing processes and account for the majority of the overall production costs. For efficient production methods, particularly in sight of constantly increasing economic pressure within human healthcare systems, highly productive downstream schemes have to be developed. Here, we discuss unit operations and downstream trains to purify virus particles for use as vaccines and vectors for gene therapy.

Phospholipid-model membrane interactions with branched polypeptide conjugates of a hepatitis A virus peptide epitope

To establish correlation between structural properties (charge, composition, and conformation) and membrane penetration capability, the interaction of epitope peptide-carrier constructs with phospholipid model membranes was studied. For this we have conjugated a linear epitope peptide, (110)FWRGDLVFDFQV(121) (110-121), from VP3 capside protein of the Hepatitis A virus with polylysine-based branched polypeptides with different chemical characteristics. The epitope peptide elongated by one Cys residue at the N-terminal [C(110-121)] was attached to poly[Lys-(DL-Ala(m)()-X(i)())] (i < 1, m approximately 3), where x = ø(AK), Ser (SAK), or Glu (EAK) by the amide-thiol heterobifunctional reagent, 3-(2-pyridyldithio)propionic acid N-hydroxy-succinimide ester. The interaction of these polymer-[C(110-121)] conjugates with phospholipid monolayers and bilayers was studied using DPPC and DPPC/PG (95/5 mol/mol) mixture. Changes in the fluidity of liposomes induced by these conjugates were detected by using two fluorescent probes 1,6-diphenyl-1,3, 5-hexatriene (DPH) and sodium anilino naphthalene sulfonate (ANS). The binding of conjugates to the model membranes was compared and the contribution of the polymer component to these interactions were evaluated. We found that conjugates with polyanionic/EAK-[C(110-121)] or polycationic/SAK-[C(110-121)], AK-[C(110-121)]/character were capable to form monomolecular layers at the air/water interface with structure dependent stability in the following order: EAK-[C(110-121)] > SAK-[C(110-121)] > AK-[C(110-121)]. Data obtained from penetration studies into phospholipid monolayers indicated that conjugate insertion is more pronounced for EAK-[C(110-121)] than for AK-[C(110-121)] or SAK-[C(110-121)]. Changes in the fluorescence intensity and in polarization of fluorescent probes either at the polar surface (ANS) or within the hydrophobic core (DPH) of the DPPC/PG liposomes suggested that all three conjugates interact with the outer surface of the bilayer. Marked penetration was documented by a significant increase of the transition temperature only with the polyanionic compound/EAK-[C(110-121)]. Taken together, we found that the binding/penetration of conjugates to phospholipid model membranes is dependent on the charge properties of the constructs. Considering that the orientation and number of VP3 epitope peptides attached to branched polypeptides were almost identical, we can conclude that the structural characteristics (amino acid composition, charge, and surface activity) of the carrier have a pronounced effect on the conjugate-phospholipid membrane interaction. These observations suggest that the selection of polymer carrier for epitope attachment might significantly influence the membrane activity of the conjugate and provide guidelines for adequate presentation of immunogenic peptides to the cells.

Excretion of hepatitis A virus (HAV) in adults: comparison of immunologic and molecular detection methods and relationship between HAV positivity and infectivity in tamarins

Fecal excretion of hepatitis A virus (HAV) in 18 patients with HAV infection was evaluated by enzyme immunoassay (EIA) to detect viral antigen and by reverse transcription-PCR amplification followed by ethidium bromide staining (PCR-ETBr) or nucleic acid hybridization (PCR-NA) to detect viral genetic material. A gradation of sensitivity was observed in the detection of virus by the three methods. In persons who had detectable virus, serial stool samples were found to be positive by EIA for up to 24 days after the peak elevation of liver enzymes. Viral genetic material could be detected by PCR-ETBr for up to 34 days and by PCR-NA for up to 54 days after the peak elevation of liver enzymes. After intravenous inoculation of tamarins with stool suspensions categorized as highly reactive for HAV (positive by EIA, PCR-ETBr, and PCR-NA), moderately reactive (positive by PCR-ETBr and PCR-NA), or weakly reactive (positive by PCR-NA), only tamarins infected with highly reactive stool suspensions (EIA positive) developed HAV infection. We conclude that positivity of stool specimens for HAV by PCR-ETBr or PCR-NA indicates a lower potential for infectivity, compared to that of EIA-positive stools.

Anti-hepatitis A virus antibody response elicited in mice by different forms of a synthetic VP1 peptide

Peptide VP1 (11-25) of the capsid of hepatitis A virus was synthesized by the Fmoc-polyamide solid phase method, and administered to mice in different forms: (1) free, (2) encapsulated in multilamellar liposomes, (3) coupled to keyhole limpet hemocyanin (KHL), and (4) incorporated into a tetrameric branched lysine core. The highest anti-VP1 peptide responses were generated by synthetic peptides entrapped into liposomes and coupled to KLH. No anti-HAV response was generated with the free peptide, while all the other forms induced both anti-HAV and HAV-neutralizing antibodies. Maximum neutralization indices were observed in ascites from mice treated with liposome-entrapped and KLH peptides.

Effect of postexposure vaccination in a chimpanzee model of hepatitis A virus infection

Passive transfer of antibodies to hepatitis A virus (HAV) in immune globulin (IG) effectively prevents hepatitis A when given after exposure, but does not provide lasting protection from infection. Hepatitis A vaccines have been shown to generate quickly levels of antibody equivalent to those found after IG administration. The effect of hepatitis A vaccine in preventing infection following fecal-oral exposure was evaluated in a chimpanzee model of HAV infection. Two animals were vaccinated 1 and 3 days, respectively following inoculation and two inoculated animals served as unprotected controls. Of the two immunized animals, one had no evidence of HAV infection, while the other had an attenuated infection with no evidence of virus shedding. These results suggest that while postexposure hepatitis A vaccination may be infection permissive, it attenuates disease expression and prevents virus shedding.

Antibody response to nonstructural proteins of hepatitis A virus following infection

The nonstructural proteins of hepatitis A virus (HAV), produced during active virus replication, are alternative antigens that could be used to differentiate disease from inactivated vaccine-induced antibodies. An assay based on immune precipitation of proteins translated from transcripts of the P2 region of viral cDNA was used to evaluate the development of antibodies after natural infection or vaccination. Antibodies against P2 proteins were found in all sera from clinical cases of hepatitis A following the acute phase. Chimpanzees vaccinated with inactivated or cell-adapted HAV had no detectable antibodies against P2 products, either before or after wild type virus challenge. A serosurvey of sera positive for total anti-HAV (HAVAB, Abbott Laboratories, North Chicago) suggested that some individuals had no detectable antibodies to the P2 antigen by immune precipitation. These results were attributed to the lower sensitivity of the immunoprecipitation assay, since antibodies to capsid proteins, as measured by immunoprecipitation, were also not detected in most of these sera.

Synthesis of immunogenic hepatitis A virus particles by recombinant baculoviruses

Recombinant baculoviruses were constructed which contained the hepatitis A virus (HAV) open reading frame (ORF) under the control of the polyhedrin promoter. Northern blot analysis with an HAV-specific oligonucleotide probe demonstrated a single transcript large enough to include the HAV ORF in Spodoptera frugiperda cells infected with these recombinants. Immunoblots revealed a 220 kDa protein representing the HAV polyprotein. In addition, proteins which co-migrated with HAV capsid proteins, and several proteins of intermediate size were present, consistent with processing intermediates. HAV antigen was present in cells infected with the recombinant baculoviruses when assessed by solid-phase radioimmunoassay. This HAV antigen had a buoyant density in caesium chloride gradients similar to HAV empty capsids, and elicited HAV neutralizing antibodies in mice.

Identification of precursors of structural proteins VP1 and VP2 of hepatitis A virus

The morphogenetic pathway of hepatitis A virus (HAV), classified as a member of the enteroviruses within the Picornaviridae, still remains obscure and seems to differ considerably from that of poliovirus, the most studied representative of this genus. In order to elucidate the precursor/product relationship of HAV structural proteins, subviral particles, which represent more than 50% of the viral antigen produced in infected cells, were separated from mature virions and their polypeptide pattern was analyzed by polyacrylamide gel electrophoresis and immunoblotting using monospecific antisera. Whereas mature virions are composed of viral proteins VP1, VP2, and VP3, subviral particles contained VP0 and smaller polypeptides instead of VP2. Comparison of proteins of different strains of HAV showed that VP0 of strain HAS-15 migrated slower than that of strains MBB or GBM. During the course of the infectious cycle, VP0 accumulated and only small portions were converted to VP2 supporting earlier observations that encapsidation of RNA with concomitant cleavage of VP0 is rate-limiting, leaving a large amount of viral antigen in premature particles. Similar to VP0, accumulation of VP1 was observed and two immunologically related precursor proteins, p38 and p36, were found during the course of infection. Immunological characterization of p38 using antisera directed to the N-terminus of VP1 and to synthetic peptides located at the presumptive C- and N-termini of 2A suggests that p38 is VP1 delta 2A carrying 45 N-terminal amino acids of the P2-region.

Characterization of a genetic variant of human hepatitis A virus

Human isolates of hepatitis A (HAV) are a single serotype; however, recent genetic surveys using limited nucleotide sequencing have provided evidence that more than one genotype is responsible for HAV infection in different parts of the world (Jansen et al. [1990]: Proc Natl Acad Sci USA 87:2867-2871; Robertson et al. [1991] J Infect Dis 163:286-292). One of these genotypes was originally isolated from Panamanian owl monkeys (strain PA21), but has subsequently been found associated with human cases of HAV from Sweden in 1979 (H-122) and the United States of America in 1976 (GA76). The nucleic acid sequence of the exposed capsid polypeptide region of GA76 differs from other human HAV sequences by approximately 20%, yet differs by only 2.4% when compared with P1 sequence of the PA21 strain. The 20% nucleic acid variability between GA76 and other human HAV results in limited amino acid changes (3%), while a comparison with PA21 revealed only four homologous amino acid substitutions within VP2, VP3, and VP1 polypeptides. HAV infected stool specimens from Nepal and northern India during 1989 and 1990 were found to contain virus whose genetic makeup was related to the PA21 and GA76 isolates. This genotype of HAV appears to be circulating in some parts of the world where HAV is hyperendemic, and is a potential cause of hepatitis A infection within a susceptible population.

Protease digestion of hepatitis A virus: disparate effects on capsid proteins, antigenicity, and infectivity

High concentrations of either trypsin or chymotrypsin caused nearly complete cleavage of capsid protein VP2 of hepatitis A virus but did not significantly reduce the infectivity, thermostability, or antigenicity of the virus. Chymotrypsin also had a lesser effect on VP1. These findings indicate the presence of a protease-accessible VP2 surface site which neither contributes significantly to the dominant antigenic site nor plays a role in the attachment of the virus to putative cell receptors.

Characterization of hepatitis A virus capsid proteins with antisera raised to recombinant antigens

The capsid proteins of hepatitis A virus (HAV) were expressed as fusion proteins of beta-galactosidase in E. coli using the expression vector lambda gt11. Four fusion proteins were stably expressed and used to immunize rabbits to obtain mono-specific antisera. The antisera were unable to neutralize viral infectivity or react with HAV by radioimmunoassay. Three of the antisera were able to recognize HAV antigens in infected BS-C-1 cells by immunofluorescence and denatured capsid proteins by immunoblot analysis. The antisera were used to investigate the migration of the capsid proteins in gels by immunoblot analysis using standard SDS-PAGE conditions and in gels containing urea. The migration of VP1 and VP3 correlated with their molecular weights predicted from the nucleotide sequence and was consistent in either the presence or absence of urea. However, VP2 migrated with an apparent molecular weight significantly higher than the predicted value and, in gels containing urea, migrated as a doublet. It is proposed that the upper band of this doublet represents VP0, the proteolytic precursor of VP2 and VP4. The relative molecular mass (Mr) of VP4 was estimated to be less than 1 kDa, which is substantially lower than the 2.5 kDa predicted from the nucleotide sequence.

Immunologic priming with recombinant hepatitis A virus capsid proteins produced in Escherichia coli

Hepatitis A virus capsid proteins (VP0, VP3, and VP1) have been synthesized in Escherichia coli for use in antigenic and immunogenic analyses. Rabbits immunized with each of these individual recombinant capsid proteins developed a rapid neutralizing antibody response when subsequently challenged with a subimmunogenic dose of whole virus.

Recombinant proteins VP1 and VP3 of hepatitis A virus prime for neutralizing response

Six overlapping genomic regions of capsid proteins VP1 and VP3 of hepatitis A virus (HAV) inserted into the expression vectors pBD or pUR respectively expressed beta-galactosidase-HAV fusion proteins. The recombinant proteins were poorly soluble so they were difficult to detect by human anti-HAV sera in radioimmunoassay, but the fusion proteins dissolved in sodium dodecyl sulfate reacted with human and rabbit anti-HAV-positive sera in immunoblots. Antisera against VP1 and VP3 recombinant proteins reacted with the respective structural proteins of HAV in immunoblots. Two recombinant proteins, one including the first 120 amino acids of the N-terminus of VP1 and the other containing all of VP1 except for the first 60 N-terminal amino acids, induced a transient neutralizing antibody response in rabbits. Antisera directed against other regions of VP1 and VP3 neither neutralized viral infectivity nor recognized native virus in a competitive radioimmunoassay. However, when immunized animals were challenged with a sub-immunogenic dose of HAV, all animals responded with stable virus-neutralizing antibodies.

Recovery of hepatitis A virus from a water supply responsible for a common source outbreak of hepatitis A

An outbreak of hepatitis A occurred in a north Georgia trailer park served by a private well. Of 18 residents who were serosusceptible to hepatitis A virus (HAV), 16 (89%) developed hepatitis A. Well water samples were collected 3 months after illness onset in the index case and 28 days after illness onset in the last trailer park resident. Hepatitis A virus antigen (HAVAg) was detected in the samples by enzyme immunoassay from three of the five cell lines following two 30-day passages and from a fourth cell line following a third passage of 21 days.

Identification of virus components in circulating immune complexes isolated during hepatitis A virus infection

Circulating immune complexes were isolated by conglutinin affinity chromatography during the course of hepatitis A virus infection in a chimpanzee. Characterization of circulating immune complexes showed that most of the hepatitis A virus-specific antibody was IgM, that IgG was present and that C3d and fibronectin were also present. Hepatitis A virus capsid polypeptides were identified in the circulating immune complexes and polypeptides in the molecular weight range of 63 to 67 kDa having immunological determinants common to both C3d and hepatitis A virus. Hepatitis A virus-RNA was detected in these circulating immune complexes using the polymerase chain reaction for in vitro amplification of nucleic acid and suggests the circulating immune complexes contain intact virus.

Structure of the hepatitis A virion: identification of potential surface-exposed regions

Iodination of highly purified hepatitis A (HAV) virus results in the selective labeling of two viral polypeptides, which are identified as the the VP 1 and VP 2 capsid polypeptides. Based upon the kinetics of labeling, the exposed region of VP 1 appears to be more accessible to iodination, although the ultimate proportion of label present within VP 1 and VP 2 is approximately equal. By utilizing iodinated whole virions, isolated VP 1, VP 2, and the tryptic digest derived from VP 1 and VP 2, binding by heterologous anti-160 S antibody indicated that a significant portion of the antibodies was directed against an epitope on VP 2 that was not affected by denaturation. Identification of the regions exposed for iodination on these two polypeptides was accomplished by tryptic digestion of the isolated polypeptides followed by characterization of the iodinated tryptic peptide by gel filtration and reverse-phase chromatography. The results indicate that tyrosine 100 on VP 2 and a large tryptic peptide composed of amino acids 222 through 260 on VP 1 which contains four tyrosine residues are two regions that are surface-exposed on these molecules.

Appearance of immune complexes during experimental hepatitis A infection in chimpanzees

Circulating immune complexes (CICs) were detected during the course of experimental hepatitis A virus (HAV) infection in 8 of 9 chimpanzees. In all cases, the predominant class of antibody detected in the CIC was IgM. The appearance of IgM-CIC usually preceded the onset of liver enzyme elevations, and in all instances, the appearance of IgM-CIC correlated with the presence of IgM anti-HAV. Six of 8 animals tested had significant depression of C3 concentrations during the course of infection, and this depression occurred at the peak of CIC activity. Immunohistologic studies demonstrated granular deposits of IgM localized in sinusoidal cells during peak of IgM-CIC activity. IgM-CICs appear to be a fairly consistent finding during HAV infection and probably represent the viremic phase of the disease. However, they do not appear to mediate hepatocellular injury by direct action on hepatocytes.

Restricted replication of hepatitis A virus in cell culture: encapsidation of viral RNA depletes the pool of RNA available for replication

The replication of hepatitis A virus (HAV) in BS-C-1 cells was examined under single-cycle growth conditions by using strand-specific probes for detection of viral RNA species. No measurable lag phase was demonstrated between accumulation of positive-strand HAV RNA and production of infectious virions, indicating that replication of virion RNA is rate limiting for the production of infectious virus. Intracellular viral RNA was further analyzed by using 2 M LiCl to fractionate the insoluble nonvirion 35S RNA and replicative intermediates (RI) from the soluble virions and double-stranded replicative forms, in conjunction with sucrose density gradient ultracentrifugation to separate the different forms of viral RNA. Throughout the productive phase of HAV infection, 95 to 97% of positive-strand HAV RNA was soluble in 2 M LiCl and was shown to be contained in mature virions. Of the LiCl-insoluble HAV RNA, more than 99% was positive-stranded 35S RNA, whereas 0.4% was negative stranded and had the sedimentation and partial RNase resistance characteristics of RI. The pattern of RNA accumulation in HAV-infected cells is thus very different from that seen in poliovirus-infected cells, where large pools of RI and mRNA are produced before RNA is sequestered into mature virions. The results of this study suggest that encapsidation of positive-strand HAV RNA inhibits transcription at all times during the growth cycle, thereby reducing the pool of replicating RNA and the final yield of infectious HAV.

Queensland fruit fly virus, a probable member of the Picornaviridae

A picornavirus was isolated from various life stages of the Queensland fruit fly, Dacus tryoni. This virus, Queensland fruit fly virus (QFFV) has virions with a diameter of 30 nm and a sedimentation coefficient of 178 S. One third of the particles in preparations were empty capsids or natural top component (NTC) with a sedimentation coefficient of 95 S. The buoyant density (rho) of virions and NTC in CsCl was 1.34 and 1.30 g/ml respectively; small amounts of a dense component (rho = 1.45 g/ml) were also detected. The capsid contained three major protein species of molecular weight (mol.wt.) 41,700, 36,500, and 31,300, in approximately equimolar proportions. NTC contained three major species of mol. wt. 44,700, 41,700, and 31,300. The nucleic acid present only in the bottom component virions was RNA and comprised about 30% of the particle weight and had a mol. wt of 2.88 kd, contained a poly(A) tract, and had a base ratio: G = 20; A = 32; C = 15; U = 33. The mol. wt. of the virion was estimated to be approximately equal to 9.5 kd. When virions were heated at 56 degrees C and above, they converted into artificial top component (ATC), which had the same protein composition as the virion when analysed by SDS-PAGE. In immunodiffusion tests the virions and NTC were indistinguishable, but a minor difference in antigenicity was detected between the virions and ATC. Virions were stable between pH 3 and 9 inclusive, and between 5 and 7 in the presence of 0.14 M NaCl. Immunodiffusion tests showed that QFFV was serologically unrelated to a range of picornaviruses as well as an unclassified virus isolated from the Mediterranean fruit fly, Ceratitis capitata. The data show that QFFV is probably a member of the Picornaviridae, genus Enterovirus.

Immunoreactivity of human and rabbit antisera to hepatitis A virus

Rabbit antibodies produced by immunization with complete hepatitis A virions (HAV) recognized all the viral structural proteins and neutralized HAV infectivity in cell culture. Rabbit antibodies to chromatographically purified individual viral proteins and to synthetic peptides representing epitopes on the structural viral protein VP1 neither recognized whole virus nor neutralized infectivity, indicating that native epitopes on the virus surface are necessary for virus recognition and neutralization. Human anti-HAV-positive sera of the acute and convalescent phase of disease recognized and neutralized viral particles. Analysis of the immunoreactivity of these human sera in immunoblot showed that the IgM antibody preferentially recognizes the structural viral proteins VP0 and VP3 of HAV, whereas IgA and IgG antibodies reacted more strongly with VP1.

Prediction of secondary structure, spatial organization and distribution of antigenic determinants for hepatitis A virus proteins

On the basis of the secondary structure calculations from the known amino acid sequence we came to the conclusion that hepatitis A virus capsid proteins have the typical antiparallel beta-sheet bilayer structure. The predicted secondary structure of the HAV proteins can be well aligned with those of the poliovirus (type 1 Mahoney) and human rhinovirus (type 14). It enabled us to use the X-ray structure of the PV-1M and HRV-14 proteins as a template and then, firstly, to localize the positions of alpha and beta regions in the architecture of the HAV protein molecules and, secondly, to discover the amino acid homologies of the secondary structure regions aligned. The obtained model of the three-dimensional structure for HAV proteins helped us to indicate the exposed regions of the polypeptide chains and to pinpoint the potential neutralizing antigenic sites.

The entire nucleotide sequence of the genome of human hepatitis A virus (isolate MBB)

Hepatitis A virus (HAV) is an important human pathogen causing hepatitis, with high incidence in developed as well as in developing countries. No vaccines are available. In order to determine the primary structure of the HAV genome, we have prepared cDNAs from viral RNA and cloned these into plasmid pBR322. These clones were used to determine the entire nucleotide sequence of the HAV RNA by rapid sequencing methods. We have compared this sequence of 7470 bases to known partial sequences, and one complete sequence of HAV RNA which were obtained recently from different strains of HAV. It is hoped that a comparison of sequence data from different isolates will help in the elucidation of the unusual growth pattern of HAV. In addition, it might provide helpful information about the immunological determinants that elicit the antibody response to infection.

Development of a plaque assay for a cytopathic, rapidly replicating isolate of hepatitis A virus

Most hepatitis A virus (HAV) replication in cell culture has been reported to be nonlytic and relatively slow. A rapidly replicating isolate of strain HM-175 from persistently infected, serially passed cell cultures (pHM-175) was found to induce a cytopathic effect. This observation allowed the development of a classic plaque assay for pHM-175 in FRhK-4 cells. The plaques were neutralized by polyclonal and monoclonal antisera to HAV.

Complete nucleotide sequence of wild-type hepatitis A virus: comparison with different strains of hepatitis A virus and other picornaviruses

The complete nucleotide sequence of wild-type hepatitis A virus (HAV) HM-175 was determined. The sequence was compared with that of a cell culture-adapted HAV strain (R. Najarian, D. Caput, W. Gee, S.J. Potter, A. Renard, J. Merryweather, G.V. Nest, and D. Dina, Proc. Natl. Acad. Sci. USA 82:2627-2631, 1985). Both strains have a genome length of 7,478 nucleotides followed by a poly(A) tail, and both encode a polyprotein of 2,227 amino acids. Sequence comparison showed 624 nucleotide differences (91.7% identity) but only 34 amino acid differences (98.5% identity). All of the dipeptide cleavage sites mapped in this study were conserved between the two strains. The sequences of these two HAV strains were compared with the partial sequences of three other HAV strains. Most amino acid differences were located in the capsid region, especially in VP1. Whereas changes in amino acids were localized to certain portions of the genome, nucleotide differences occurred randomly throughout the genome. The most extensive nucleotide homology between the strains was in the 5' noncoding region (96% identity for cell culture-adapted strains versus wild type; greater than 99% identity among cell culture-adapted strains). HAV proteins are less homologous with those of any other picornavirus than the latter proteins are when compared with each other. When the sequences of wild-type and cell culture-adapted HAV strains are compared, the nucleotide differences in the 5' noncoding region and the amino acid differences in the capsid region suggest areas that may contain markers for cell culture adaptation and for attenuation.

Characterization of hepatitis A virus structural proteins

HAV particles isolated from infected cells banded at buoyant densities of 1.42, 1.32, and 1.20 g/ml, and distinctive protein patterns were established by gel electrophoresis and reverse phase high performance liquid chromatography. The relatively higher amounts of p30 in particles with lower buoyant densities suggest that this protein is VP0 and is part of the immature picornavirion. The protein elution profiles obtained by HPLC were virtually identical for all the HAV strains examined but differed from those of other picornaviruses. The N-terminal amino acid sequence of VP1 and VP2 was determined and aligned to the nucleotide sequence. Sequencing VP0 and VP3 was not possible, probably because the amino termini are blocked. VP1, VP3, and VP0 induced specific antibodies in rabbits.