Hepatitis delta antigen expressed by recombinant baculoviruses: comparison of biochemical properties and post-translational modifications between the large and small forms.

Hepatitis delta virus (HDV) encodes only one protein, the hepatitis delta antigen (HDAg). Two forms of HDAg, a large (27 kDa) and a small (24 kDa) one, participate in the various steps of HDV replication. To further understand the properties of HDAg, we have constructed recombinant baculoviruses and expressed both forms of the HDAg in insect cells. The gene encoding HDAg was placed under the control of the polyhedrin promoter of Autographa Californica nuclear polyhedrosis virus (AcNPV) by homologous recombination. When Spodoptera frugiperda (Sf9) cells were infected with the recombinant viruses, both the small HDAg and the large HDAg were expressed at high levels. The HDAgs produced by the recombinants were similar in size and antigenic properties to those of the proteins produced in mammalian hepatoma cell lines. It was also localized exclusively in the nuclei. In addition, both proteins bound to HDV RNA in an in vitro assay. No difference in the RNA-binding affinity was noted between the two forms of HDAg, suggesting that the trans-dominant inhibitory activity of the large HDAg on HDV replication is not due to its competition with the small HDAg for RNA binding. Two RNA-protein complexes could be detected, suggesting either that there are at least two binding sites on the HDV RNA or that HDAg binds to HDV RNA in two multimeric forms. We have further shown that both the large and the small HDAgs are phosphoproteins, with the former having an approximately sixfold higher level of phosphorylation. Finally, it was demonstrated that the large HDAg was isoprenylated, while the small one was not. These differences in post-translational modifications are the first differences in biochemical properties demonstrated between the two forms and may explain the differential effects of the large and small HDAgs on HDV RNA replication and virus packaging.     

Recombinant hepatitis delta antigen from E. coli promotes hepatitis delta virus RNA replication only from the genomic strand but not the antigenomic strand

Hepatitis delta antigen (HDAg) of hepatitis delta virus (HDV) typically consists of two related protein species. The small HDAg (S-HDAg) is a 24-kDa protein of 195 amino acids and the large HDAg (L-HDAg) is a 27-kDa protein with an additional 19 amino acids at its C-terminus. These two proteins have distinct functions in the HDV life cycle. We have developed conditions for expressing S-HDAg and L-HDAg in E. coli as soluble proteins to facilitate large-scale purification. These proteins were purified to homogeneity and shown to be biologically active. Transfection of the purified recombinant S-HDAg together with HDV genomic RNA resulted in viral RNA replication. Surprisingly, the purified S-HDAg could not initiate replication from the antigenomic-sense HDV RNA, even though the latter led to RNA replication when transfected with an mRNA encoding the S-HDAg. These results suggest that initiation of HDV RNA synthesis from the antigenomic RNA may require a form of HDAg that is modified in mammalian cells; in contrast, RNA synthesis from the genomic RNA could be initiated by the recombinant S-HDAg from E. coli. Interestingly, the purified L-HDAg appeared as multiple protein species, including one corresponding to S-HDAg, probably as a result of degradation. The partially proteolyzed L-HDAg also initiated HDV RNA replication under the same conditions. These results add to the mounting evidence that genomic- and antigenomic-strand HDV RNA syntheses are carried out by different mechanisms.     

Histone H1e interacts with small hepatitis delta antigen and affects hepatitis delta virus replication

Hepatitis delta virus (HDV) encodes two isoforms of delta antigens (HDAgs). The small form of HDAg (SHDAg) is required for HDV RNA replication, while the large form of HDAg (LHDAg) is required for viral assembly. Using tandem affinity purification method combined with mass spectrometry, we found that linker histone H1e bound to SHDAg. The binding domain of SHDAg to histone H1e was mapped to the N-terminal 67 amino acids. Oligomerization of SHDAg was required for its interaction with histone H1e. LHDAg barely bound to histone H1e and was masked at N-terminus. The binding domain of histone H1e to SHDAg was mapped to its central globular domain. HDV replication was inhibited by N- or C-terminal deletion mutants of histone H1e and was rescued by wild-type histone H1e. We conclude that histone H1e plays a significant role in HDV replication through forming protein complex with SHDAg.     

Electron microscopy of ribonucleic acid in nuclear particulate aggregates of hepatitis D using nuclease-gold complexes

The ultrastructural localization of hepatitis delta antigen (HDAg) and ribonucleic acid (RNA) was investigated by immunoperoxidase electron microscopy and by enzyme electron microscopy of RNase-gold complexes on liver biopsies from seven patients with hepatitis D. HDAg was localized mainly in the nucleus and sometimes in the cytoplasm of hepatocytes. Ultrastructurally, intranuclear HDAg was found on nuclear particulate structures measuring 20 to 30 nm in diameter. Intranuclear RNA visualized with gold particles was found in high amounts in the nucleolus, to a small extent in the chromatin area, and also on nuclear particulate structures. These findings suggest that intranuclear aggregates of irregular granular particulate structures in hepatitis D are the internal component of hepatitis delta virus (HDV) particles in blood.     

The small delta antigen of hepatitis delta virus is an acetylated protein and acetylation of lysine 72 may influence its cellular localization and viral RNA synthesis

Hepatitis delta virus (HDV) is a single-stranded RNA virus that encodes two viral nucleocapsid proteins named small and large form hepatitis delta antigen (S-HDAg and L-HDAg). The S-HDAg is essential for viral RNA replication while the L-HDAg is required for viral assembly. In this study, we demonstrated that HDAg are acetylated proteins. Metabolic labeling with [(3)H]acetate revealed that both forms of HDAg could be acetylated in vivo. The histone acetyltransferase (HAT) domain of cellular acetyltransferase p300 could acetylate the full-length and the N-terminal 88 amino acids of S-HDAg in vitro. By mass spectrometric analysis of the modified protein, Lys-72 of S-HDAg was identified as one of the acetylation sites. Substitution of Lys-72 to Arg caused the mutant S-HDAg to redistribute from the nucleus to the cytoplasm. The mutant reduced viral RNA accumulation and resulted in the earlier appearance of L-HDAg. These results demonstrated that HDAg is an acetylated protein and mutation of HDAg at Lys-72 modulates HDAg subcellular localization and may participate in viral RNA nucleocytoplasmic shuttling and replication.     

Heterogeneity of hepatitis delta antigen

Hepatitis delta antigen (HDAg) is the only known protein encoded by the hepatitis delta virus (HDV). Two HDAg species of different sizes have been detected in the sera and livers of the infected humans, chimpanzees, and woodchucks, even though only one RNA species was previously identified in most of the HDV strains. To study HDAg heterogeneity, we took advantage of the fact that a single base mutation at nucleotide 1015 (C to U), which results in an amber termination codon in the HDAg open reading frame (ORF), eliminates a unique Ncol restriction enzyme site. We screened various HDV cDNA clones and detected sequence heterogeneity of the HDAg-coding region on the basis of the presence or absence of the Ncol site. Five delta hepatitis patients were examined. In every patient, two types of HDAg-coding sequence were detected at nucleotide 1015: one which contains a C and results in an ORF encoding a delta antigen of 214 amino acids, and the other which possesses a U and results in an amber termination codon and a truncated HDAg species of 195 amino acids. The in vitro translation products of these two ORFs comigrated with the two HDAg species from the patient's plasma on SDS polyacrylamide gels. Polymerase chain reaction (PCR) amplification of the HDV RNA from some patients' sera and subsequent sequencing showed several additional mutations in the HDAg-coding region. These mutations are independent of the C or U nucleotide change at the site of the amber termination codon.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for replication of hepatitis delta virus RNA in hepatocyte nuclei after in vivo infection

Examination of a naturally infected human liver and experimentally infected chimpanzee and woodchuck livers by in situ hybridization showed that hepatitis delta virus (HDV) RNA was restricted to hepatocytes. Genomic RNA was 20-30 times more abundant than antigenomic RNA and was predominantly single-stranded while antigenomic RNA was predominantly double-stranded. In acute delta hepatitis, viral RNA was a more reliable marker of virus infection in single cells than hepatitis delta antigen (HDAg) while in chronic hepatitis both markers were usually present in the same cell. In all cases, viral antigen and RNA were localized predominantly to the nuclei of infected cells. Thus, replication of HDV RNA is closely associated with HDAg expression at the cellular and intracellular level and it is likely that this new class of defective animal RNA viruses replicates in the nucleus of the infected cells.     

Sequence conservation and divergence of hepatitis delta virus RNA

The complete RNA sequence of the hepatitis delta virus (HDV) obtained from the Nauru Island in the Pacific was determined by cDNA cloning and amplification by polymerase chain reaction (PCR). The sequence showed 14-17% divergence from the two known HDV RNA sequences. There are three highly conserved domains: the region around the autocatalytic cleavage site of the genomic RNA (nucleotides 659 to 772), the region around the autocatalytic cleavage site of the antigenomic-sense RNA (nucleotides 847 to 966), and the region around the middle one-third domain of the open reading frame (ORF) encoding the hepatitis delta antigen on the antigenomic RNA (nucleotides 1267 to 1347). The two autocatalytic activities are required for the cleavage and ligation of HDV RNA during RNA replication. The third conserved domain codes for the RNA-binding domain of HDAg, which specifically interacts with HDV RNA. Three nucleotide changes within the genomic catalytic sequence are present but did not alter the catalytic cleavage activity of the HDV RNA. Microheterogeneity of the RNA sequences was also detected. One of these occurred within the coding region of the delta antigen, creating an amber termination codon in some of the RNA species. Thus, this HDV strain contains two different RNA species, one of which encodes a delta antigen of 214 amino acids and the other 195 amino acids. These two protein species were detected by immunoblotting of the patient's plasma. In contrast to other HDV strains, only three ORFs capable of encoding more than 100 amino acids each are present in this HDV RNA. We recommend that oligonucleotides complementary to the highly conserved sequences should be used as primers for PCR in clinical detection assays of hepatitis delta virus infection.     

Hepatitis delta virus RNA, protein synthesis and associated cytotoxicity in a stably transfected cell line

A trimer of hepatitis delta virus (HDV) cDNA in a retrovirus expression vector was transfected into subclone of the PLC/PRF/5 human hepatoma cell line, and a stable cell line (H1 delta 9) was clonally selected that supported the synthesis of both genomic and antigenomic sense HDV RNA. The H1 delta 9 cell line also expressed hepatitis delta antigen (HDAg) in cell nuclei in three distinct morphological patterns, including patterns typically seen in HDV-infected livers. HDAg expression was restricted to the smaller (p24) of the two HDAg-associated polypeptides in early passages of the H1 delta 9 cell line, but continuous passage of the cells resulted in increasing of expression of the larger (p27) HDAg-specific polypeptide. Passage of the H1 delta 9 cells also led to sustained expression of monomeric HDV RNA and a reduction in the levels of dimeric- and trimeric-HDV RNA. This was accompanied by an attenuation of virus-related cytotoxicity which was a feature of early cell passage numbers. HDV RNA replication in these cells was resistant to actinomycin D suggesting that replication was not dependent on continued expression from the transfected HDV cDNA and thus was likely to be self-sustaining.     

乙型肝炎病毒在丁型肝炎病毒包装中的作用机理研究

用含ＨＢＶ全基因和ＨＢＶ大、中、小分子表面蛋白基因的真核细胞表达质粒（ＣＭＶ－ＨＢＶ、ＣＭＶ－ＬＳ、ＣＭＶ－ＭＳ、ＣＭＶ－Ｓ）分别与含ＨＤＶ ｃＤＮＡ三聚体的重组质粒共转染ＣＨＯ细胞。转染后３天在上述４种转染细胞内及培养上清中均检出了ＨＤＶ ＲＮＡ和ＨＤＡｇ表明上述４种转染细胞的培养上清中均有ＨＤＶ病毒颗粒的包装和分泌。提示：ＨＤＶ病毒的包装可能仅需ＨＢＶ Ｓ 基因及其小分子表面蛋白的辅助。     

Evolution rate of hepatitis delta virus RNA isolated in Taiwan

The complete RNA sequences of hepatitis delta viruses (HDV) isolated at 3 years apart from a chronic delta hepatitis patient in Taiwan were determined. The sequence analysis showed an overall evolution rate of 3.18 × 10^?3 substitutions/nucleotide/year. The evolution rates in different parts of HDV RNA varied. The hypervariable region evolved faster (4.55 × 10^?3 substi-tutions/nucleotide/year) than the hepatitis delta antigen (HDAg)-coding region (2.60 × 10^?3 substitutions/nucleotide/year) and the autocata-lytic region (1.11 × 10^?3 substitutions/nucleotide/ year). These data are compatible with the previous finding that the hypervariable region is more divergent than the HDAg-coding region and the au-tocatalytic regions among the HDV isolates from different geographic areas. No substitution was found in the four previously identified conserved domains of HDV RNA, further confirming their functional importance in viral replication. The evolution rate of this HDV RNA is higher than that determined from the partial RNA sequences of two Japanese HDV isolates and similar to that found in a Lebanon isolate. Further, it was found that this HDV RNA retained the same microheterogeneities at 15 nucleotide positions detected in the RNA 3 years earlier. It is concluded that HDV RNA in patients' serum is extremely heterogeneous, and that the nucleotide substitutions in certain nucleotide positions likely have conferred evolutionary advantages for HDV. Viral sequence evolution is a possible mechanism for chronic HDV infection. © 1994 Wiley-Liss, Inc.     

Molecular Phylogenetic Analysis of Iranian HDV Complete Genome

Hepatitis D (delta) virus (HDV) is a subviral pathogen agent and a satellite of Hepatitis B virus. Three distinct genotypes are described for HDV; genotype I is distributed worldwide but other genotypes appear to be more restricted geographically. In the present study, the entire nucleotide sequence of an HDV isolate from an Iranian patient (IR-1) was obtained using twelve pairs of primers to amplify six overlapping fragments covering the whole HDV genome by RT-nested PCR. Phylogenetic and pairwise alignments were done on this new isolate to determine IR-1 position among other isolates. Our results indicate that IR-1 contains 1676 nucleotides encoding 214 a.a. of the hepatitis delta antigen (HDAg). This new isolate belongs to genotype I with most sequence similarity to an Italian HDV isolate (92.6%). At amino acid level, predicted HDAg sequence of IR-1 revealed the most homology with those of Italian and Lebanese isolates. Data analysis confirmed genetic variability and heterogeneity of the HDV species isolated from different geographical areas.     

丁型病毒性肝炎的原位分子杂交及免疫组化研究

利用原位分子杂交和免疫组化方法对１４２例乙型肝炎活检肝组织进行丁型肝炎病毒ＲＮＡ及其抗原的定位研究。２８／１４２例丁型肝炎病毒标记阳性。其中慢性重症型６例；慢性活动性１７例；慢性持续性５例。慢性活动性乙肝重叠丁型肝炎病毒感染组发生早期肝硬变的比例明显高于无重叠感染组（Ｐ＜０．０５）。２８例丁型肝炎病毒感染肝组织１９例ＨＢｃＡｇ阳性，并以核浆型为主，提示活动性ＨＢＶ复制与ＨＤＶ感染的正相关性，两者相加作用导致肝损害加重并加速发展为肝纤维化。ＨＤＶＲＮＡ在肝细胞内大量蓄积，ＨＤＡｇ在碎屑状坏死边缘肝细胞或气球样变肝细胞内呈浆膜型分布，提示ＨＤＶ直接细胞毒在丁型肝炎发病学中的作用。     

Binding of the polypyrimidine tract-binding protein-associated splicing factor (PSF) to the hepatitis delta virus RNA

The hepatitis delta virus (HDV) has a very limited protein coding capacity and must rely on host proteins for its replication. A ribonucleoprotein complex was detected following UV cross-linking between HeLa nuclear proteins and an RNA corresponding to the right terminal stem-loop domain of HDV genomic RNA. Mass spectrometric analysis of the complex revealed the polypyrimidine tract-binding protein-associated splicing factor (PSF) as a novel HDV RNA-interacting protein. Co-immunoprecipitation demonstrated the interaction between HDV RNA and PSF both in vitro in HeLa nuclear extract and in vivo within HeLa cells containing both polarities of the HDV genome. Analysis of the binding of various HDV-derived RNAs to purified, recombinant PSF further confirmed the specificity of the interaction and revealed that PSF directly binds to the terminal stem-loop domains of both polarities of HDV RNA. Our findings provide evidence of the involvement of a host mRNA processing protein in the HDV life cycle.     

Specific interaction between the hepatitis delta virus RNA and glyceraldehyde 3-phosphate dehydrogenase: an enhancement on ribozyme catalysis

Replication of hepatitis delta virus (HDV) RNA occurs in the nuclei of infected cells. The replication is mediated by cellular factors containing an RNA polymerase II-like enzyme activity through a double rolling-circle mechanism and is regulated by delta antigens. In this study, UV cross-linking experiments were carried out to examine interactions between HDV RNA and proteins present in HeLa nuclear extract. Cellular proteins with molecular mass of 23 (p23), 36 (p36), 38 (p38), and 58 (p58) kDa bound to full-length HDV RNA of both genomic and antigenomic strands. Deletion analysis on the antigenomic strand mapped the interacting domain within a 79-nucleotide fragment but not at the ends of the rod-shaped viral RNA structure. The specificity of the RNA-protein interactions was demonstrated by competition experiments and the specific HDV RNA-binding proteins were purified through column chromatography. Electrophoresis mobility shift assay with the purified fractions demonstrated that the interaction between p36 and HDV RNA was relatively stable even in the presence of 0.5 M NaCl. Biochemical analysis including protein microsequencing identified the p36 as glyceraldehyde 3-phosphate dehydrogenase (GAPDH). RNase footprinting indicated that the UC-rich domain between nucleotides 379 and 414 of the HDV antigenomic RNA was involved in the GAPDH binding. Functional studies further demonstrated an enhancing effect of GAPDH on the ribozyme activity of HDV antigenomic RNA. In addition, in the presence of HDV RNA cellular GAPDH relocalized from the cytoplasm to the nucleus where HDV replication occurs. These results suggest that GAPDH is involved in the replication of HDV.