Structural relationships between the surface antigens of ground squirrel hepatitis virus and human hepatitis B virus.

Several physical, chemical, and serological properties of surface antigen particles from ground squirrel hepatitis virus (GSHsAg) and human hepatitis B virus (HBsAg) were compared. GSHsAg and HBsAg particles were purified from positive sera by gel chromatography and isopycnic centrifugation. Both antigens consisted mainly of spherical particles with an average diameter of approximately 20 nm and a buoyant density in CsCl of approximately 1.19 g/ml. Their UV absorption spectra indicated the presence of more tryptophane than tyrosine and the absence of detectable nucleic acid. GSHsAg was found to contain two major polypeptides of approximately 23,000 and 27,000 daltons, with electrophoretic migration rates distinctly faster than those of the two major polypeptides of HBsAg particles. After radiolabeling of purified antigen preparations with Bolton-Hunter reagent, the two major polypeptides of GSHsAg showed almost identical tryptic peptide maps. The tryptic peptide map of the major polypeptide from GSHsAg contained 13 of 37 spots also present in the map of the major HBsAg polypeptide, and 13 of 27 spots in the map of the major HBsAg polypeptide were also present in the map of the major GSHsAg polypeptide. This suggests considerable sequence homology between the major surface antigen polypeptides of the two viruses. However, there was only a weak serological cross-reactivity between antigens of the two viruses. Using an anti-HBs-containing serum with a relatively strong cross-reactivity, GSHsAg was found to consist of at least two antigenically different subspecies. The more strongly cross-reacting from had a slightly higher buoyant density than the other antigenic form.     

Antigenic and structural relationships of the surface antigens of hepatitis B virus, ground squirrel hepatitis virus, and woodchuck hepatitis virus.

The surface antigens of human hepatitis B (HBsAg), ground squirrel hepatitis (GSHsAg), and woodchuck hepatitis (WHsAg) viruses were compared serologically, and their major polypeptides were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and tryptic peptide mapping. Results showed that both GSHsAg and WHsAg are antigenically cross-reactive, that their major pairs of polypeptides have identical mobilities on sodium dodecyl sulfate gels, and that the major polypeptides of GSHsAg and WHsAg migrate faster in sodium dodecyl sulfate-polyacrylamide gel electrophoresis than do the corresponding bands of HBsAg. The peptide maps of the major (P-22) surface antigen polypeptides of GSHsAg and WHsAg showed that they shared over half of their spots. Peptide mapping of HBsAg subtypes indicated a close relationship between the major polypeptides (P-24) of adw and adr and a more distal relationship to ayw. Only about 25% of the spots shared by the combined HBsAg subtypes were also found in the peptide maps of GSHsAg and WHsAg, indicating at least some structural homology among the major polypeptides of the human and animal virus surface antigen particles. This is also reflected in the serological cross-reactivity among HBsAg, GSHsAg, and WHsAg. Further, the detection of ground squirrel and woodchuck antigens by Ausria II radioimmunoassay, combined with peptide mapping data indicating the common origin of these viruses, suggests that the common a determinant is shared by each and is restricted to approximately 25% of the sequences in their major polypeptides.     

Nonoverlapping antigenic sites of woodchuck hepatitis virus surface antigen and their cross-reactivity with ground squirrel hepatitis virus and hepatitis B virus surface antigens.

Five nonoverlapping antigenic sites (sites I through V) of woodchuck hepatitis virus surface antigen were identified with competitive binding assays involving monoclonal antibodies. Site I contributed to cross-reactions among surface antigens of hepatitis B-like viruses infecting woodchucks, ground squirrels, and humans. At least three distinct sites (sites I, II, and III) are responsible for cross-reactions between woodchuck and ground squirrel hepatitis virus surface antigens. Sites IV and V of woodchuck hepatitis virus surface antigen are not major cross-reactive sites, suggesting that these elicit virus-specific antibodies. There were no cross-reactions with duck hepatitis B virus surface antigen.     

Biological characterization of acute infection with ground squirrel hepatitis virus.

Ground squirrel hepatitis virus (GSHV) is a small DNA virus, structurally and antigenically related to the human hepatitis B virus, which occurs naturally among certain wild populations of ground squirrels (P. L. Marion et al., Proc. Natl. Acad. Sci. U.S.A. 77:2941-2945, 1980). Serum from naturally infected animals was used to transmit GSHV in the laboratory by parenteral inoculation of susceptible squirrels. Sixty percent of recipient animals developed viral surface antigenemia after a latent period of 2 to 3 months; three of these animals have remained viremic for over 9 months. Like hepatitis B virus, GSHV demonstrates marked hepatotropism, with viral DNA detected in significant quantities only in the liver, where an average of 6 X 10(2) to 6 X 10(3) viral DNA molecules per cell were found by molecular hybridization. However, histological signs of liver injury after acute infection are minimal. In contrast to infection of its natural host, parenteral administration of GSHV to rats, mice, guinea pigs, and hamsters did not result in demonstrable antigenemia, suggesting that the host range of GSHV, like that of hepatitis B virus, is narrow.     

Characterization and classification of virus particles associated with hepatitis A. II. Type and configuration of nucleic acid.

Virus particles banding at 1.34 g/ml in CsCl and sedimenting at 160S in sucrose gradients were isolated from fecal specimens of patients suffering from hepatitis. In the presence of 4 M urea and about 90% formamide, these particles released linear nucleic acid molecules of the kinked appearance characteristic of single-stranded RNA or single-stranded DNA. They could be distinguished from the nucleic acid of phage lambda added to the preparation as a marker for double-stranded configuration. Experiments in which the virus particles under investigation were incubated at pH 12.9 at 50 degrees C for 30 min revealed that their nucleic acid molecules were hydrolyzed as readily as the RNA genome of poliovirus type 2 analyzed in parallel. Both the single-stranded DNA of phage phiX174 and that of parvovirus LuIII, however, proved unaffected by this treatment, and the double-stranded DNA of phage lambda was denatured to single-stranded molecules. It was concluded, therefore, that the virus of human hepatitis A contains a linear genome of single-stranded RNA and has to be classified with the picornaviruses.     

Characterization and classification of virus particles associated with hepatitis A. III. Structural proteins

Hepatitis A virus was purified from fecal samples collected at various times in the incubation period of patients with naturally acquired hepatitis A. The proteins of particles banding at around 1.34 g/ml in CsCl and sedimenting at about 160S were radioiodinated in vitro and separated by electrophoresis on polyacrylamide gels in the presence of 0.1% sodium dodecyl sulfate and 8 M urea. Under these conditions, the capsid proteins resolved into four polypeptides with molecular weights of approximately 31,000, 24,500, 21,000, and 9,000, respectively. A fifth protein of about 40,000 daltons in size and assumed to be equivalent to the precursor polypeptide VP0 of the picornaviruses was present in particles sedimenting at only 150 to 155S and banding at around 1.33 g/ml in CsCl. The physicochemical characteristics of these particles are consistent with those of the provirion structures of picornaviruses. In several of the fecal samples, these particles represented a considerable fraction of all particles present. The significance of this finding with respect to the antigenicity of hepatitis A antigen extracted from stool specimens is discussed.     

Formation of intracellular particles by hepatitis B virus large surface protein.

Hepatitis B virus small surface protein is synthesized as a transmembrane protein of the rough endoplasmic reticulum (RER) and then buds into the lumen in the form of subviral particles that are secreted. The closely related large surface protein is also targeted to the RER but is retained in a pre-Golgi compartment and cannot be secreted. It has been assumed that the large surface protein remains as a transmembrane RER protein and hence cannot form particles, possibly because of binding to a host factor on the cytosolic face of the RER membranes. We have reexamined this question and found the following results. (i) The retained large surface protein is associated not with RER but, rather, with a more distal compartment. (ii) Electron microscopy reveals intravesicular 20-nm particles, similar to those formed by the small surface protein. (iii) The large surface protein colocalizes with and binds to calnexin, an ER chaperone protein. Therefore, our results indicate that the large surface protein is capable of budding and forming particles, and hence its intracellular retention cannot be attributed to a cytosolic factor. We interpret the data as evidence that the large surface protein is retained by virtue of interacting with calnexin, a component of what is considered the quality control mechanism of the ER.     

Characterization of the major duck hepatitis B virus core particle protein.

The amino acid composition of the major duck hepatitis B virus (DHBV) core particle proteins was determined. The results of this analysis indicated that cores are composed of a single major protein that initiates translation from the second available AUG in the DHBV core gene. Proteins isolated from core particles purified from the cytoplasm of DHBV-infected duck hepatocytes exhibited heterogeneity in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, independent of the stage of viral DNA maturation. Incubation of native cores with alkaline phosphatase removed this heterogeneity, indicating that phosphorylation of external amino acids was responsible. Core protein isolated from mature DHBV purified from serum of infected animals did not display heterogeneity, suggesting a possible role for dephosphorylation in virus maturation.     

Ground squirrel hepatitis virus DNA: molecular cloning and comparison with hepatitis B virus DNA

Ground squirrel hepatitis virus (GSHV) shares many ultrastructural antigenic, molecular, and biological features with hepatitis B virus (HBV) of humans, indicating that they are members of the same virus group. Both viruses contain small circular DNA molecules which are partially single stranded. Here, we ligated an endonuclease EcoRI digest of GSHV DNA with EcoRI-cleaved plasmid vector pBR322 and cloned recombinant plasmids in Escherichia coli C600. Two cloned recombinants were characterized. One (pGS2) was found to contain only part of the GSHV genome, and the other (pGS11) was found to contain the entire viral DNA. A restriction endonuclease cleavage map of the GSHV insert in pGS11 and the locations of certain physical features of the virion DNA were determined. The relative positions of the single-stranded region, the unique 5' end of the short DNA strand, and the unique nick in the long DNA strand in GSHV DNA were found to be the same as those previously described for HBV DNA. Hybridization with an HBV [32P]DNA probe containing the apparent coding sequence for the major polypeptide of HBV surface antigen and a probe containing the putative coding sequence for the major polypeptide of the HBV core revealed specific homology with different restriction fragments of GSHV DNA. The two homologous regions had approximately the same locations relative to the single-stranded region, the 5' end of the short strand, and the nick in the long strand in the two viral DNAs. These results suggest that in both viruses the genes for the major HBV surface antigen and core polypeptides have the same locations relative to unique physical features of the viral DNAs.     

A cell surface protein that binds avian hepatitis B virus particles.

We have identified a 180-kDa cellular glycoprotein (gp180) that binds with high affinity to duck hepatitis B virus (DHBV) particles. The protein was detected by coprecipitating labeled duck hepatocyte proteins with virions or recombinant DHBV envelope proteins, using nonneutralizing monoclonal antibodies to the virion envelope. Binding of gp180 requires only the pre-S region of the viral large envelope protein, since recombinant fusion proteins bearing only this region efficiently coprecipitate gp180. The DHBV-gp180 interaction is blocked by two independent neutralizing monoclonal antibodies. The protein is found on both internal and surface membranes of the cell, and the species distribution of gp180 binding activity mirrors the known host range of DHBV infection. Functional gp180 is expressed in a wide variety of tissues in susceptible ducks.     

Effect of core protein phosphorylation by protein kinase C on encapsidation of RNA within core particles of hepatitis B virus.

Phosphorylation of core particles derived either from hepatitis B viruses or from livers of hepatitis B-infected individuals has been long recognized, but the nature and function of the phosphorylating enzyme remained unknown. By immunoblotting with a monoclonal antibody, we have now detected protein kinase C within the liver-derived core particles. To study the significance of the encapsidated protein kinase C for the viral life cycle, we established an in vitro assembly system consisting of Escherichia coli-expressed core protein, protein kinase C, and in vitro-synthesized hepatitis B virus RNA. Phosphorylation of the core protein led to a reduced RNA encapsidation capacity of the core particles. Furthermore, RNA and protein kinase C competed for their target sequence, which is the carboxy-terminal arginine-rich domain of the core protein. This finding implies that phosphorylation of the nucleic acid binding site in the core protein occurs within the particles after encapsidation of protein kinase C, pregenomic RNA, and viral polymerase at a later step during viral genome maturation.     

Virion DNA of ground squirrel hepatitis virus: structural analysis and molecular cloning.

The structure of the encapsidated DNA genome of ground squirrel hepatitis virus (GSHV) has been examined by restriction endonuclease cleavage, nucleic acid hybridization, and molecular cloning. GSHV virion DNA is a relaxed circular molecule of approximately 3,200 bases in length; most molecules harbor an extensive single-stranded region which is largely confined to one-half of the genome. The full-length viral DNA strand is covalently bound to protein. The single-stranded region can be repaired in vitro by the action of the endogenous virion polymerase, exogenously added DNA polymerase from avian myeloblastosis virus, or both. Restriction enzyme cleavage of viral DNA from different isolates demonstrated that multiple variants of GSHV exist in nature. The genomes of two such strains have been cloned in Escherichia coli, and their physical maps have been determined. Nucleic acid hybridization studies revealed that the strains share sequence homology with the DNA of human hepatitis B virus. Regions homologous to the coding regions for the surface and core antigens of human hepatitis B virus have been localized on the GSHV chromosome. Molecular cloning experiments have also led to the identification of a region of the viral genome which is altered in a procaryotic host.     

Antibodies to pre-S and X determinants arise during natural infection with ground squirrel hepatitis virus.

The DNA sequence of the ground squirrel hepatitis virus (GSHV) genome predicts the existence of several proteins in addition to the major surface (S) and core antigens. These include the pre-S1 and pre-S2 proteins, initiated at sites within the open reading frame preceding and continuous with the coding region for the S gene product, and the X protein, the putative product of an independent reading frame. Using an antibody directed against a peptide predicted by codons 130 to 143 of the pre-S1 reading frame, we identified a 43-kilodalton product of the pre-S1 coding region in preparations of GSHV surface antigen purified from the sera of infected animals. In addition, by immunoprecipitation of S- and pre-S-specific in vitro translation products with ground squirrel sera obtained after GSHV infection, we determined that antibodies arise to both S and pre-S determinants. The antibody response to pre-S includes, in some cases, reactivity to pre-S1-specific domains and is not always associated with an anti-S response. Similarly, by production of the viral X gene product in vitro followed by immunoprecipitation with ground squirrel sera, we showed that antibodies to this viral gene product also arise during infection, indicating that X antigenic determinants are synthesized during viral infection and are recognized by the host immune system.     

Characterization and classification of virus particles associated with hepatitis A. I. Size, density, and sedimentation.

Virus-like particles were purified from stools of patients in an epidemic of hepatitis A in Germany. When reference MS-1 chimpanzee pre-inoculation and convalescent sera were used, the close serological relationship of the purified particles to well-known isolates of hepatitis A could be established. On the other hand, the physicochemical characteristics of the particles were determined in parallel to the characteristics of a marker parvovirus (LuIII) and a marker picornavirus (poliovirus type 2). It could be shown that the majority of the hepatitis A-associated particles band at 1.34 g/ml in CsCl and, like poliovirus, sediment at about 160S. In addition, a distinct hepatitis A antigen was observed, which banded at 1.305 g/ml and sedimented between 50 and 90S. A further component accumulated in the density range of between 1.38 and 1.44 g/ml. However, it seemed to be rather labile. Upon reisolation from CsCl and sedimentation in sucrose, it resolved into a 160S, a 90 to 100S, and a 50S form. The size of the 160S particles (27 to 29 nm) could be readily distinguished from that of the parvovirus (22 to 24 nm). It is concluded, therefore, that hepatitis A-associated virus particles are more likely to be classified with the picornaviruses than with the parvoviruses.     

Upstream sequences and cap proximity in the regulation of polyadenylation in ground squirrel hepatitis virus.

The polyadenylation signal of mammalian hepadnaviruses is unusual in that its hexanucleotide element is the variant UAUAAA rather than AAUAAA. This signal functions inefficiently and must be augmented by multiple activator elements located in the upstream 400 nucleotides (nt) to promote efficient processing. Here we characterize one of these upstream elements, termed PS2, in the ground squirrel hepatitis virus. PS2 is located within the 107 nt 5' to the UAUAAA and raises the efficiency of polyadenylation by this signal from < 10% to 50 to 60%. It can function independently of the more 5' activator elements and conversely is not required for their function. Its action is orientation dependent, and a predicted stem-loop structure within the element is not necessary for its activity. PS2 is the sole upstream element that maps within the terminal redundancy of viral genomic RNA. Thus, it is present, together with the UAUAAA, at both the 5' and 3' ends of this RNA. During genomic RNA synthesis, the poly(A) signals in the 5' repeat are bypassed, while those in the 3' copy are used. The ability of PS2 to function independently of the other, more upstream activators suggests that the absence of the latter elements from the 5' redundancy is insufficient to account for bypass of the 5' poly(A) site, as we had earlier proposed. Rather, the short distance from the cap site to the UAUAAA at the 5' end of genomic RNA actively suppresses its use, as this suppression can be experimentally relieved by increasing this distance to 230 to 400 nt.     

Nucleotide sequence of an infectious molecularly cloned genome of ground squirrel hepatitis virus.

We have determined the complete nucleotide sequence of an infectious cloned genome of ground squirrel hepatitis virus (GSHV), a nonpathogenic member of the hepadnavirus group. The genome is 3,311 base pairs long and contains the major open reading frames described for the related human and woodchuck hepatitis B viruses (HBV and WHV, respectively). These reading frames include genes for the major structural proteins (the surface and core antigens), unassigned open reading frames (A and B), the longer of which is presumed to encode the viral DNA polymerase, and an open reading frame preceding and continuous with the surface antigen gene. The arrangement of these open reading frames is similar to that encountered in the genomes of HBV and WHV: all of the reading frames are encoded on the same strand, they are positioned in the same fashion with respect to each other, and a large portion (at least 51%) of the genome can be translated in two reading frames. Comparisons of the predicted translational products of the three mammalian hepadnaviruses reveal 78% amino acid homology between the proteins of GSHV and WHV and 43% homology between those of GSHV and HBV. In addition, a perfect direct repeat of 10 to 11 base pairs, separated by ca. 46 to 223 base pairs, is present in the three mammalian viruses and in duck hepatitis B virus; the position of the repeats near the 5' termini of the two strands of virion DNA suggests a role in viral replication.     

Woodchuck hepatitis virus is a more efficient oncogenic agent than ground squirrel hepatitis virus in a common host.

Chronic infection with hepatitis B viruses (hepadnaviruses) is a major cause of hepatocellular carcinoma (HCC), but the incubation time varies from 1 to 2 years to several decades in different host species infected with indigenous viruses. To discern the influence of viral and host factors on the kinetics of induction of HCC, we exploited the recent observation that ground squirrel hepatitis virus (GSHV) is infectious in woodchucks (C. Seeger, P. L. Marion, D. Ganem, and H. E. Varmus, J. Virol. 61:3241-3247, 1987) to compare the pathogenic potential of GSHV and woodchuck hepatitis virus (WHV) in chronically infected woodchucks. Chronic GSHV infection in woodchucks produces mild to moderate portal hepatitis, similar to that observed in woodchucks chronically infected with WHV. However, HCC developed in GSHV carriers about 18 months later than in WHV carriers. Thus, although both viruses are oncogenic in woodchucks, GSHV and WHV differ in oncogenic determinants that can affect the kinetics of appearance of HCC in chronically infected animals.     

Nuclear export and import of human hepatitis B virus capsid protein and particles

It remains unclear what determines the subcellular localization of hepatitis B virus (HBV) core protein (HBc) and particles. To address this fundamental issue, we have identified four distinct HBc localization signals in the arginine rich domain (ARD) of HBc, using immunofluorescence confocal microscopy and fractionation/Western blot analysis. ARD consists of four tight clustering arginine-rich subdomains. ARD-I and ARD-III are associated with two co-dependent nuclear localization signals (NLS), while ARD-II and ARD-IV behave like two independent nuclear export signals (NES). This conclusion is based on five independent lines of experimental evidence: i) Using an HBV replication system in hepatoma cells, we demonstrated in a double-blind manner that only the HBc of mutant ARD-II+IV, among a total of 15 ARD mutants, can predominantly localize to the nucleus. ii) These results were confirmed using a chimera reporter system by placing mutant or wild type HBc trafficking signals in the heterologous context of SV40 large T antigen (LT). iii) By a heterokaryon or homokaryon analysis, the fusion protein of SV40 LT-HBc ARD appeared to transport from nuclei of transfected donor cells to nuclei of recipient cells, suggesting the existence of an NES in HBc ARD. This putative NES is leptomycin B resistant. iv) We demonstrated by co-immunoprecipitation that HBc ARD can physically interact with a cellular factor TAP/NXF1 (Tip-associated protein/nuclear export factor-1), which is known to be important for nuclear export of mRNA and proteins. Treatment with a TAP-specific siRNA strikingly shifted cytoplasmic HBc to nucleus, and led to a near 7-fold reduction of viral replication, and a near 10-fold reduction in HBsAg secretion. v) HBc of mutant ARD-II+IV was accumulated predominantly in the nucleus in a mouse model by hydrodynamic delivery. In addition to the revised map of NLS, our results suggest that HBc could shuttle rapidly between nucleus and cytoplasm via a novel TAP-dependent NES.     

Analysis of integrated ground squirrel hepatitis virus and flanking host DNA in two hepatocellular carcinomas.

We cloned the integrated ground squirrel hepatitis B virus (GSHV) sequences from two hepatomas showing a single viral insertion. The GSHV inserts shared structural features with integrated DNAs of other hepadnaviruses. Insertional activation of a cellular gene appears unlikely: the integrated GSHV sequences lacked the known viral enhancers and were not expressed in the tumors, and we found no evidence for the presence of a gene at the integration site. Our results, together with those earlier studies, suggest that GSHV does not behave as an extensive insertional mutagen, in sharp contrast with the closely related woodchuck hepatitis virus. GSHV may thus cause carcinogenesis by more indirect mechanisms, as does the human hepatitis B virus.