Novel N-terminal amino acid sequence required for retention of a hepatitis B virus glycoprotein in the endoplasmic reticulum

Formation of semi-enveloped particles as a unique feature of a hepatitis B virus PreS1 deletion mutant

BACKGROUND

Naturally occurring variants with deletions or mutations in the C-terminal PreS1 domain from hepatitis B virus (HBV) chronically infected patients have been shown to promote HBsAg retention, inhibit HBsAg secretion and change the extracellular appearance of PreS1-containing HBV particles (filaments and virions).

AIMS

To study the impact of N-terminal deletion in preS1 domain on viral secretion and morphogenesis.

METHODS

An HBV mutant with 15 amino acids (aa 25-39) deletion in N-terminal preS1 was isolated. Intracellular and extracellular HBsAg were quantified by Western blot. Subcellular HBsAg distribution was analysed by confocal laser scanning microscopy. The viral morphology was characterised by sucrose density gradient ultracentrifugation, Western blot, electron microscopy, HBV mixed ELISA and HBV particle gel essay.

RESULTS

Expression of this mutant genome released higher amounts of HBsAg in the form of shorter filaments. A significant fraction of semi-enveloped virions was observed in the supernatant that has been unprecedented so far. Stepwise insertion of aa 25-31, aa 32-39 and aa 25-39 increased the length of filaments. The rescue of aa 25-31 and aa 25-39 drastically reduced the amounts of extracellular HBsAg and semi-enveloped virions, while such effects could not be observed after insertion of aa 32-39, arguing against a simple spacer function of this region. The deletion and rescued mutants do not differ in subcellular HBsAg distribution and colocalisation with ER, Golgi and multivesicular bodies markers arguing against differences in release pathways.

CONCLUSION

N-terminal PreS1-domain (aa 25-31) determines HBsAg secretion and triggers proper assembly of PreS1-containing particles.

Posttranslational modifications and secretion efficiency of immunogenic hepatitis B virus L protein deletion variants

BACKGROUND

Subviral particles of hepatitis B virus (HBV) composed of L protein deletion variants with the 48 N-terminal amino acids of preS joined to the N-terminus of S protein (1-48preS/S) induced broadly neutralizing antibodies after immunization of mice with a Semliki Forest virus vector. A practical limitation for use as vaccine is the suboptimal secretion of such particles. The role of the N-terminal preS myristoylation in the cellular retention of full-length L protein is described controversially in the literature and the relation of these data to the truncated L protein was unknown. Thus, we studied the effect of preS myristoylation signal suppression on 1-48preS/S secretion efficiency, glycosylation and subcellular distribution.

FINDINGS

The findings are that 1-48preS/S is secreted, and that removal of the N-terminal myristoylation signal in its G2A variant reduced secretion slightly, but significantly. The glycosylation pattern of 1-48preS/S was not affected by the removal of the myristoylation signal (G2A mutant) but was different than natural L protein, whereby N4 of the preS and N3 of the S domain were ectopically glycosylated. This suggested cotranslational translocation of 1-48preS in contrast to natural L protein. The 1-48preS/S bearing a myristoylation signal was localized in a compact, perinuclear pattern with strong colocalization of preS and S epitopes, while the non-myristoylated mutants demonstrated a dispersed, granular cytoplasmic distribution with weaker colocalization.

CONCLUSIONS

The large deletion in 1-48preS/S in presence of the myristoylation site facilitated formation and secretion of protein particles with neutralizing preS1 epitopes at their surface and could be a useful feature for future hepatitis B vaccines.

Removing N-terminal sequences in pre-S1 domain enhanced antibody and B-cell responses by an HBV large surface antigen DNA vaccine

Although the use of recombinant hepatitis B virus surface (HBsAg) protein vaccine has successfully reduced global hepatitis B infection, there are still a number of vaccine recipients who do not develop detectable antibody responses. Various novel vaccination approaches, including DNA vaccines, have been used to further improve the coverage of vaccine protection. Our previous studies demonstrated that HBsAg-based DNA vaccines could induce both humoral and CMI responses in experimental animal models. However, one form of the the HBsAg antigen, the large S antigen (HBs-L), expressed by DNA vaccine, was not sufficiently immunogenic in eliciting antibody responses. In the current study, we produced a modified large S antigen DNA vaccine, HBs-L(T), which has a truncated N-terminal sequence in the pre-S1 region. Compared to the original HBs-L DNA vaccine, the HBs-L(T) DNA vaccine improved secretion in cultured mammalian cells and generated significantly enhanced HBsAg-specific antibody and B cell responses. Furthermore, this improved HBsL DNA vaccine, along with other HBsAg-expressing DNA vaccines, was able to maintain predominantly Th1 type antibody responses while recombinant HBsAg protein vaccines produced in either yeast or CHO cells elicited mostly Th2 type antibody responses. Our data indicate that HBsAg DNA vaccines with improved immunogenicity offer a useful alternative choice to recombinant protein-based HBV vaccines, particularly for therapeutic purposes against chronic hepatitis infection where immune tolerance led to poor antibody responses to S antigens.

Envelopment of the hepatitis B virus nucleocapsid

The hepatitis B virus (HBV) is an enveloped DNA virus with an icosahedral capsid replicating via reverse transcription. The crystal structure of the capsid is known. It has a diameter of 36 nm and is formed by one protein species (C protein). The viral envelope contains three different coterminal proteins (S, M, and L proteins) spanning the membrane several times. These proteins are not only released from infected cells as components of the viral envelope but in 10,000-fold excess as subviral lipoprotein particles with a diameter of 22 nm containing no capsid. Assembly of the capsid occurs in the cytosol and results in packaging of a 3.5 kb RNA molecule together with viral and cellular factors. This newly formed capsid cannot be enveloped. Rather, synthesis of the viral DNA genome in the lumen of the capsid by reverse transcription is required to induce a budding competent state. Envelopment then takes place at an intracellular membrane of the pre-Golgi compartment. The S and the L protein, but not the M protein, is required for this process. The L protein forms two different transmembrane topologies. The isoform exposing the N-terminal part at the cytosolic side of the membrane is essential for budding. In this domain, a 22 amino acid (aa) long linear stretch has been mapped genetically to play a vital role in the morphogenetic process. This domain probably mediates the contact to the capsid. A second matrix domain was mapped to the cytosolic loop of the S protein. A similar genetic approach identified two small areas on the capsid surface, which might interact with the envelope proteins during envelopment.

The juxtamembrane sequence of cytochrome P-450 2C1 contains an endoplasmic reticulum retention signal

The N-terminal signal anchor of cytochrome P-450 2C1 mediates retention in the endoplasmic reticulum (ER) membrane of several reporter proteins. The same sequence fused to the C terminus of the extracellular domain of the epidermal growth factor receptor permits transport of the chimeric protein to the plasma membrane. In the N-terminal position, the ER retention function of this signal depends on the polarity of the hydrophobic domain and the sequence KQS in the short hydrophilic linker immediately following the transmembrane domain. To determine what properties are required for the ER retention function of the signal anchor in a position other than the N terminus, the effect of mutations in the linker and hydrophobic domains on subcellular localization in COS1 cells of chimeric proteins with the P-450 signal anchor in an internal or C-terminal position was analyzed. For the C-terminal position, the signal anchor was fused to the end of the luminal domain of epidermal growth factor receptor, and green fluorescent protein was additionally fused at the C terminus of the signal anchor for the internal position. In these chimeras, the ER retention function of the signal anchor was rescued by deletion of three leucines at the C-terminal side of its hydrophobic domain; however, deletion of three valines from the N-terminal side did not affect transport to the cell surface. ER retention of the C-terminal deletion mutants was eliminated by substitution of alanines for glutamine and serine in the linker sequence. These data are consistent with a model in which the position of the linker sequence at the membrane surface, which is critical for ER retention, is dependent on the transmembrane domain.

Hepatitis B virus assembly is sensitive to changes in the cytosolic S loop of the envelope proteins

Among the three related L, M, and S envelope proteins of the hepatitis B virus (HBV), the L and S polypeptides are required for virion production. Whereas the pivotal function of the pre-S region of L in nucleocapsid envelopment has been established, the contribution of its S domain and the S protein is less clear. In this study, we evaluated the role of the cytosolic S loop, common to L and S, in HBV assembly by performing mutagenesis experiments. To distinguish between the effect of the mutations on either envelope or virion formation, we investigated the ability of the mutants to assemble into secretable subviral empty envelopes and to replace the wild-type proteins in virion maturation, respectively. Virion production was found to be blocked by each of the secretion-competent deletion and substitution mutants SDelta35-39, SDelta40-46, SDelta50-56, and Svarsigma56-59, while an insertion within the loop is tolerated. Surprisingly, single mutations of the arginines terminating the loop had an opposite effect: while a conservative exchange of Arg-73 still allowed virion formation, the same mutation of Arg-79 did not. The critical sequences and/or structural requirements of the cytosolic S loop involved in nucleocapsid envelopment primarily act in the S background. These findings can be related to a model for a synergistical function of both L and S proteins in HBV morphogenesis.

DNA-mediated immunization to hepatitis B virus envelope proteins: preS antigen secretion enhances the humoral response

In order to design optimized DNA vectors as genetic vaccines against infections with the hepatitis B virus (HBV) we investigated if secretion or retention of the viral antigens has an influence on the quality and quantity of the humoral immune response. Intramuscular injection of plasmid DNA encoding the HBV large L envelope protein, known to be retained within host cells, induced only a weak response in mice whereas a vector expressing the secretion-competent small S envelope protein elicited strong and sustained immunity. Immunization with rearranged envelope genes further demonstrated that secretion affects the magnitude of the immune response. In situ expression of modified small and middle envelope genes carrying C-terminally attached epitopes are derived from the preS1 region of L generated high titers of preS1- and preS2-specific antibodies, unless antigen secretion was blocked. Accessibility of preS antigens to B-cells that can be achieved by generating extracellular forms of the envelope proteins is thus critical to elicit humoral responses. Such DNA constructs carrying preS1 determinants are promising candidates for the development of multivalent HBV vaccines.

Electrophoretic analysis of the ribonucleoproteins of hepatitis delta virus

Replication of hepatitis delta virus (HDV) is dependent on delta antigen (deltaAg), an HDV-encoded protein, which binds to HDV RNA and is capable of multimerization. To characterize HDV-specific ribonucleoprotein complexes (RNP) we used electrophoresis into non-denaturing agarose gels followed by northern analysis, to detect HDV RNA, and immunoblot, to detect deltaAg. We studied RNP from three sources: (i) vRNP, disrupted virions obtained from infected woodchuck serum; (ii) sRNP, disrupted particles secreted from transfected cultured cells; and (iii) cRNP, isolated from cells in which HDV genome replication was occurring. sRNP were approximately 28% smaller than vRNP. Treatment of vRNP with aurin tricarboxylic acid disrupted both deltaAg-deltaAg and deltaAg-RNA interactions while vanadyl ribonucleosides released the RNA without causing detectable disruption of the multimeric deltaAg complex. cRNP were smaller and more heterogeneous than vRNP and sRNP, and probably contained host components. The application of these electrophoretic procedures, and especially the use of prior treatments with vanadyl ribonucleoside complexes have provided valuable information on the RNP of HDV, and we expect they should find applicability in RNP studies of other RNA viruses.

Secretion and purification of HCV E1 protein forms as glutathione-S-transferase fusion in the baculovirus insect cell system

We have expressed the E1 protein of Hepatitis C Virus (HCV) in a new recombinant form by using a baculovirus transfer vector directing the expression of proteins fused to the carboxy-terminus of glutathione-S-transferase (GST). The E1 domain was expressed varying at its carboxy terminus in order to retain (GST-E1) or delete (GST-E1b) the C-terminal hydrophobic region that may be involved in membrane association. Following infection with the recombinant virus, GST-E1b was efficiently secreted into the culture media and could be purified in a single step with the minimum of denaturation by glutathione affinity chromatography. The purified product was specifically immunoprecipitated by HCV positive human sera suggesting the maintenance of an immuno-relevant tertiary structure despite removal of the hydrophobic anchor. By contrast, cells infected with a recombinant baculovirus expressing GST-E1 gave a fusion protein with an appropriate molecular weight but also a series of polypeptides of lower molecular weight consistent with cleavage at the C-terminus of E1. GST-E1 was not secreted into the medium and was associated predominantly with the membrane fraction following cell disruption; the lower molecular weight forms were soluble and secreted.

gp180, a protein that binds duck hepatitis B virus particles, has metallocarboxypeptidase D-like enzymatic activity

Duck gp180 was previously identified by its ability to bind to the preS envelope protein of duck hepatitis B virus particles (Kuroki, K. , Cheung, R., Marion, P. L., and Ganem, D. (1994) J. Virol. 68, 2091-2096). Cloning and sequencing of gp180 cDNA revealed that it is a polyprotein with three carboxypeptidase-like domains (Kuroki, K., Eng, F., Ishikawa, T., Turck, C., Harada, F., and Ganem, D. (1995) J. Biol. Chem. 270, 15022-15028). To evaluate enzymatic properties of this protein, a soluble 170-kDa form of the protein (gp170) lacking the C-terminal transmembrane domain and cytoplasmic tail was expressed in a baculovirus system. The purified 170-kDa protein cleaved 5-dimethylaminonaphthalene-1-sulfonyl (dansyl)-Phe-Ala-Arg with a pH optimum of 5.5-6.5. With this substrate at pH 5.5, the 170-kDa protein displayed a Km of 12 microM and a Kcat of 57 s-1. Dansyl-Pro-Ala-Arg and dansyl-Phe-Phe-Arg were cleaved with Km values of 17 and 21 microM, and Kcat values of 57 and 17 s-1, respectively. Constructs containing only the first or second carboxypeptidase domains also showed enzymatic activity. The effects of inhibitors and ions on enzyme activity of gp170 were generally similar to the effects of these compounds on purified bovine carboxypeptidase D. To evaluate the regions within gp180 necessary for binding preS, a series of deletion mutants were expressed in the 293T human kidney cell line. Deletions of the first and second domains, leaving the third domain intact, eliminated carboxypeptidase activity but retained preS binding. Deletion of the third domain eliminated preS binding but not carboxypeptidase activity. These results indicate that the third domain is responsible for preS binding, and this binding does not require carboxypeptidase activity.

Intracellular retention of duck hepatitis B virus large surface protein is independent of preS topology

The mechanism of intracellular retention for the large surface protein (L) of duck hepatitis B virus (DHBV) was analyzed by examination of the transmembrane topologies and secretory properties of a collection of DHBV L mutants and compared with that of human hepatitis B virus (HBV) L. Our results demonstrate that, in contrast to its HBV counterpart, intracellular retention of DHBV L does not depend on the cytosolic disposition of its preS domain. L mutants with either cytosolic or lumenal preS were mostly retained in the absence of the small surface protein (S), whereas coexpression with S resulted in efficient secretion of both topological forms. Coexpression of the wild-type DHBV L with S resulted in efficient incorporation of L into secreted S + L particles, whereas HBV L was partially excluded from secreted particles under the same conditions. We propose that HBV provides L retention even in the presence of an excess of S, by exclusion of molecules with cytosolic preS domains from secreted particles at the stage of their assembly. DHBV lacks such a retention mechanism due to the absence of topological selection in particulate assembly.

Phosphorylation of the hepatitis delta virus antigens

We used two-dimensional electrophoresis (nonequilibrium pH gradient electrophoresis followed by sodium dodecyl sulfate-10% polyacrylamide gel electrophoresis) coupled with 32P labeling and immunoblotting detection with 125I-protein A to detect and quantitate phosphorylation of the large and small forms of the delta antigen (deltaAg-L and deltaAg-S, respectively). Analysis of deltaAg species from the serum and liver of an infected woodchuck as well as deltaAg species expressed in and secreted from transfected Huh7 cells revealed the following. (i) No detectable phosphorylation of deltaAg-S occurred. (ii) In virions from the serum of an infected animal and in the particles secreted from cotransfected cells, none of the deltaAg-L was phosphorylated. (iii) Only in the infected liver and in transfected cells was any phosphorylation detected; it corresponded to a monophosphorylated form of deltaAg-L. Given these results, we carried out serine-to-alanine mutagenesis of the deltaAg-L to determine whether the monophosphorylation was predominantly at a specific site on the unique 19-amino-acid (aa) extension. We mutated each of the two serines, aa 207 and 210, on this extension and also the serine at aa 177. These three mutations had no significant effect on phosphorylation. In contrast, mutagenesis to alanine of the cysteine at aa 211, which normally acts as the acceptor for farnesylation, completely inhibited phosphorylation. Our interpretation is that the site(s) of phosphorylation is probably not in the 19-aa extension unique to deltaAg-L and that phosphorylation of deltaAg-L may depend upon prior farnesylation. The possible significance of the intracellular phosphorylated forms of deltaAg-L is discussed.

A sorting motif localizes the foamy virus glycoprotein to the endoplasmic reticulum

We recently identified an endoplasmic reticulum (ER) retrieval signal-the dilysine motif-in the glycoproteins of all five foamy viruses (FVs) for which sequences were available (P. A. Goepfert, G. Wang, and M. J. Mulligan, Cell 82:543-544, 1995). In the present study, expression of recombinant human FV (HFV) glycoprotein and analyses of oligosaccharide modifications and precursor cleavage indicated that the protein was localized to the ER. HFV glycoproteins encoding seven different dilysine motif mutations were then expressed. The results indicated that disruptions of the dilysine motif resulted in higher levels of forward transport of the HFV glycoprotein from the ER through the Golgi apparatus to the plasma membrane. We conclude that the dilysine motif is responsible for ER sorting of the FV glycoprotein. Signal-mediated ER localization has not previously been described for a retroviral glycoprotein.

Signal-mediated sorting of membrane proteins between the endoplasmic reticulum and the golgi apparatus

Each organelle of the secretory pathway is required to selectively allow transit of newly synthesized secretory and plasma membrane proteins and also to maintain a unique set of resident proteins that define its structural and functional properties. In the case of the endoplasmic reticulum (ER), residency is achieved in two ways: (a) prevention of residents from entering newly forming transport vesicles and (b) retrieval of those residents that escape. The latter mechanism is directed by discrete retrieval motifs: Soluble proteins have a H/KDEL sequence at their carboxy-terminus; membrane proteins have a dibasic motif, either di-lysine or di-arginine, located close to the terminus of their cytoplasmic domain. Recently it was found that di-lysine motifs bind the complex of cytosolic coat proteins, COP I, and that this interaction functions in the retrieval of proteins from the Golgi to the ER. Also discussed are the potential roles this interaction may have in vesicular trafficking.

Yeast expressing hepatitis B virus surface antigen determinants on its surface: implications for a possible oral vaccine

The two major hydrophilic regions of the hepatitis B virus surface antigen (HBsAg) have been expressed in the outer mannoprotein layer of the cell wall of "Bakers Yeast", Saccharomyces cerevisiae, by fusing them between the yeast invertase signal sequence and the yeast alpha-agglutinin carboxyterminal cell wall anchoring sequence. The fusion protein contained most of the preS sequences, including the hepatocyte receptor, and part of the S sequence including the "a" determinant, and was expressed from multiple genomic copies (MIRY) using the constitutive PCK promoter. Immunofluorescence studies showed that the fusion protein was detectable at the cell surface and was stably expressed at a relatively high level. Intraperitoneal immunization of mice revealed a very weak response against the S region, and a high response against yeast itself. It is proposed that increasing the amount of the antigen and reducing the number of native cell wall proteins, might lead to a yeast that is usable as a safe and cheap live oral vaccine.

gp180, a host cell glycoprotein that binds duck hepatitis B virus particles, is encoded by a member of the carboxypeptidase gene family

Duck hepatitis B virus particles bearing the L and S envelope proteins bind a cellular glycoprotein of 180 kDa (gp180) with high affinity and specificity. Binding is mediated by the pre-S region of the L protein and is blocked by neutralizing but not by non-neutralizing monoclonal antibodies to the virus. These and other properties have suggested that gp180 may be a component of the viral entry machinery. Here we report the purification of gp180 from duck liver and the isolation and characterization of cDNA encoding it. DNA sequence analysis of this cDNA indicates that gp180 is a novel member of the basic carboxypeptidase gene family.

Hepatocyte-specific binding of L/S-HBV particles expressed in insect cells

The genome of hepatitis B virus (HBV) codes for three surface antigen proteins. Two of them are essential components of infectious viral particles. Whereas expression of the small (S) antigen led to formation of virus-like particles in different systems so far, secretion of neither the large antigen nor budding of virus-like particles containing both antigens could be observed. Using modified large antigen genes in dual expression vectors we were able to demonstrate secretion of virus-like particles in the baculovirus insect cell system. N-terminal fusion of an insect protein (melittin) derived signal sequence and destruction of the myristylation site resulted in secretion of the large antigen. Particles consisting of about 95% small and 5% large antigen bind specifically to hepatocytes. These pseudovirions could serve as a HBV vaccine and as a useful component of future hepatocyte-specific gene transfer vehicles.

High level expression of hepatitis B virus preS1 peptide in Escherichia coli

PreS1 region gene fragment encoding the N-terminal 56 amino acid (aa) of hepatitis B virus (HBV, adr subtype), which encodes B- and T-cell epitopes and an hepatocyte receptor binding site, was synthesized by PCR and fused to the 3'-end of MalE gene encoding maltose-binding protein (MBP) to yield expression plasmid pMalpreS1-56. The plasmid was introduced into Escherichia coli DH5 alpha and expressed at 37 degrees C under the control of inducible tac promoter. The resulting fusion protein was highly expressed in a soluble form, about 40% of total cellular proteins, but it bound only partially to an amylose column. Therefore, the soluble preS1 fusion protein was purified to near homogeneity by two passages of anion-exchange chromatography followed by gel filtration. The yield of the fusion protein was 70 mg per 1 culture that had been induced by IPTG for 6 h. The purified fusion protein was specifically cleaved by a Factor Xa digestion to release the preS1 peptide, which was then purified by gel filtration to homogeneity. The purity, integrity, antigenicity and immunogenicity of the purified preS1 peptide was confirmed by glycerol-SDS-PAGE, Western analysis, N-terminal amino acid sequencing and an indirect ELISA.

Mapping a region of the large envelope protein required for hepatitis B virion maturation

The hepatitis B virion is a spherical double-shelled particle carrying three surface proteins (large [L], middle [M], and small [S]) in its envelope. All three proteins are translated from a single open reading frame by means of three different in-frame start codons from unspliced mRNAs. This organization defines three protein domains (pre-S1, pre-S2, and S). All three domains together form the L protein, whereas the M protein consists of domains pre-S2 plus S. The L and S proteins are both necessary for virion production, whereas the M protein is dispensable, suggesting an important function of the pre-S1 domain in virion morphogenesis. To investigate this point, we created a series of N-terminal-truncated L mutants and tested their ability to substitute for the wild-type L protein in virion formation. We found that the constructs fell into two classes, (i) N-terminal deletion mutants lacking up to 102 of the 119 amino acids of the pre-S1 domain still allowed virion maturation, showing that the N-terminal 5/6 of the pre-S1 sequence is dispensable for this process. (ii) Mutants lacking 110 or more N-terminal amino acids were unable to substitute for the L protein in virion assembly, although they were stably expressed and secreted as components of subviral 20-nm hepatitis B surface antigen particles. This suggests that a short C-terminal region of pre-S1 is important for virion formation. Like the wild-type L protein, the mutants of the first class were not glycosylated in their pre-S2 domains; however, this site was used for glycosylation in mutants of the second class, similar to that in the M protein. These findings can be related to a model for the function of the L protein in virion maturation.

Expression of large hepatitis B envelope protein mutants using a new expression vector

Aminoterminal deletion mutants of the gene encoding the large hepatitis B surface protein were expressed in COS cells using a new expression vector. The truncated protein showed the same intracellular retention like the wild type protein. The findings show that the secretion block of the protein is not due to its aminoterminal myristylation.

Sorting of protein A to the staphylococcal cell wall

The cell wall of gram-positive bacteria can be thought of as representing a unique cell compartment, which contains anchored surface proteins that require specific sorting signals. Some biologically important products are anchored in this way, including protein A and fibronectin binding protein of Staphylococcus aureus and streptococcal M protein. Studies of staphylococcal protein A and Escherichia coli alkaline phosphatase show that the signal both necessary and sufficient for cell wall anchoring consists of an LPXTGX motif, a C-terminal hydrophobic domain, and a charged tail. These sequence elements are conserved in many surface proteins from different gram-positive bacteria. We propose the existence of a hitherto undescribed sorting mechanism that positions proteins on the surface of gram-positive bacteria.

High-level expression of hepatitis B viral antigens in fibrosing cholestatic hepatitis

The expression of hepatitis B viral antigens was quantified in liver tissue from four transplant recipients with fibrosing cholestatic hepatitis (FCH) and compared with five other transplant recipients who did not develop this syndrome and 30 patients with chronic hepatitis B virus (HBV) infection. As measured by radioimmunoassays, the liver tissue from patients with FCH had significantly greater amounts of both hepatitis B surface antigen (HBsAg) and nucleocapsid antigens than to transplant patients without this syndrome (P less than 0.01) or patients with chronic HBV infection (P less than 0.001). Intrahepatic expression of pre-S1/pre-S2 in FCH was also extensive with a distribution parallel to that of HBsAg. High-level expression of intrahepatic HBsAg and hepatitis B core antigen in the explanted liver was associated with subsequent development of FCH in the liver graft, suggesting that viral/host factors may also be important. This pattern of intrahepatic hepatitis viral antigen expression, by analogy with Chisari's transgenic mice model and Roingeard's HBV-transfected HepG2 cell model, may be the cause of direct hepatocytopathic injury in this condition.

PreS1-specific binding proteins as potential receptors for hepatitis B virus in human hepatocytes

Cellular receptors play an important role in viral pathogenesis. Until now, there has been no reliable information on the receptor(s) for hepatitis B virus (HBV). Therefore, we attempted to identify specific receptors in human hepatocytes using an immunological approach. Anti-idiotypic (Ab2) antibodies were raised in rabbits against our monoclonal antibody (MAb1) F35.25. MAb1 F35.25 (i) recognized the hepatocyte receptor binding site on HBV (located between amino acid residues 21 and 47 of the preS1 sequence) and (ii) blocked the attachment of preS1-positive HBV particles to human hepatocytes. The presence of Ab2 antibodies in rabbit sera was determined by the ability of antisera to inhibit Id (Ab1)/antigen (HBV) recognition. Affinity-purified Ab2 IgGs to F35.25 represented an internal image for the preS1 domain 12-53. Our present studies indicate that Ab2 IgGs to F35.25 (i) recognized the membrane-associated structure of the preS1-specific HBV receptor in a HepG2 cell binding assay, as visualized by immunoenzymatic staining; (ii) strongly bound to a major 35-kDa component and to three other related proteins of 50, 43, and 40 kDa in extracts of HepG2 cells; and (iii) reacted with several soluble and membrane-associated proteins in normal human liver cells. The binding was insensitive to reduction. All preS1 binding proteins were V8 protease sensitive and endoglycosidase H resistant. The 35-kDa species was trypsin resistant and generated a band of 32 kDa by endoglycosidase F treatment. Together, our results suggest that the identified preS1-specific binding proteins may be involved in the putative complex structure of the hepatocyte receptor for HBV.

Targeting of proteins into the peroxisomal matrix

During the last few years much has been learned regarding signals that target proteins into peroxisomes. The emphasis in the near future will undoubtedly shift towards the elucidation of the mechanism of import. The use of mammalian and yeast cells deficient in peroxisome assembly and/or import (Zoeller & Raetz, 1986; Erdmann et al., 1989; Cregg et al., 1990; Morand et al., 1990; Tsukamoto, Yokota & Fujiki, 1990) should provide a handle on the genes (Erdmann et al., 1991; Tsukamoto et al., 1991) involved in these processes. This will have to be coupled with further development of in vitro systems which will permit the dissection of the steps in the translocation of proteins into peroxisomes. Though some progress has been made in the development of such assays (Imanaka et al., 1987; Small et al., 1987, 1988; Miyazawa et al., 1989), the fragility of peroxisomes and the absence of biochemical hallmarks of import (such as protein modifications or proteolytic processing) have hindered progress. Since peroxisomes exist in the form of a reticulum in mammalian cells (Gorgas, 1984), all peroxisome purification schemes (from mammalian cells at least) must undoubtedly rupture the peroxisomes, which then reseal to form vesicular structures. Additionally, the reliance on the latency of catalase alone as a major criterion for the integrity of peroxisomes ignores the fact that many other matrix proteins leak out of peroxisomes at vastly different rates during purification of the organelles (Thompson & Krisans, 1990). In view of these problems, the development of peroxisomal transport assays with semi-intact cells would also constitute an important advance.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutational analysis of hepatitis B surface antigen particle assembly and secretion

Cells infected with hepatitis B virus produce both virions and 20-nm subviral (surface antigen or HBsAg) particles; the latter are composed of viral envelope proteins and host-derived lipid. Although hepatitis B virus encodes three envelope proteins (L, M, and S), all of the information required to produce an HBsAg particle resides within the S protein. This polypeptide spans the bilayer at least twice and contains three hydrophobic regions, two of which are known to harbor topogenic signal sequences that direct this transmembrane orientation. We have examined the effects of mutations in these and other regions of the S protein on particle assembly and export. Lesions in the N terminal signal sequence (signal I) can still insert into the endoplasmic reticulum bilayer but do not participate in any of the subsequent steps in assembly. Deletion of the major internal signal (signal II) completely destabilizes the chain. Deletion of the C-terminal hydrophobic domain results in a stable, glycosylated, but nonsecreted chain. However, when coexpressed with wild-type S protein this mutant polypeptide can be incorporated into particles and secreted, indicating that the chain is still competent for some of the distal steps in particle assembly. The correct transmembrane disposition of the N terminus of the molecule is important for particle formation: addition of a heterologous (globin) domain to this region impairs secretion, but the defect can be corrected by provision of an N-terminal signal sequence that restores the proper topology of this region. The resulting chimeric chain is assembled into subviral particles that are secreted with normal efficiency.

Golgi complex localization of the Punta Toro virus G2 protein requires its association with the G1 protein

The glycoproteins of bunyaviruses accumulate in membranes of the Golgi complex, where virus maturation occurs by budding. In this study we have constructed a series of full length or truncated mutants of the G2 glycoprotein of Punta Toro virus (PTV), a member of the Phlebovirus genus of the Bunyaviridae, and investigated their transport properties. The results indicate that the hydrophobic domain preceding the G2 glycoprotein can function as a translocational signal peptide, and that the hydrophobic domain near the C-terminus serves as a membrane anchor. A G2 glycoprotein construct with an extra hydrophobic sequence derived from the N-terminal NSM region was stably retained in the ER, and was unable to be transported to the Golgi complex. The full-length G2 glycoprotein, when expressed on its own, was transported out of the ER and expressed on the cell surface, whereas the G1 and G2 proteins when expressed together are retained in the Golgi complex. A truncated anchor-minus form of the G2 glycoprotein was found to be secreted into the culture medium, but was retained in the Golgi complex when coexpressed with the G1 glycoprotein. These results indicate that the G2 membrane glycoprotein is a class I membrane protein which does not contain a signal sufficient for Golgi retention, and suggest that its Golgi localization is a result of association with the G1 glycoprotein.

The large surface protein of hepatitis B virus is retained in the yeast endoplasmic reticulum and provokes its unique enlargement

The coding sequences for each of the three envelope proteins of hepatitis B virus (HBV), the major (S), middle (M), and large (L) surface proteins, were expressed in Saccharomyces cerevisiae. Analysis by immunoelectron microscopy of thin sections of yeast cells showed that production of L protein but not of M or S protein provoked morphological changes in the yeast endoplasmic reticulum. A large accumulation of membranous structures connected with the perinuclear cysternae and specifically labeled by a monoclonal antibody directed against the amino-terminal (preS1) sequence of the L protein, was observed. The L protein was post-translationally modified by N- and O-linked glycosylation, indicative of its entry into the yeast secretory pathway and by N-myristoylation of its amino-terminal glycine residue. Deletion of this glycine residue resulted in the synthesis of a nonmyristoylated L protein. Proliferation of the endoplasmic reticulum was comparable in cells producing either the myristoylated or nonmyristoylated L protein, indicating that myristoylation alone is not responsible for the induction of the abnormal membrane morphology.

The C-terminal half of the preS1 region is essential for the secretion of human hepatitis B virus large S protein devoid of the N-terminal retention sequence

A large surface protein of human hepatitis B virus was expressed from a mutated S gene in which both the reported retention sequence (aa 2-19) and the C-terminal half of the preS1 region (aa 66-117) were deleted. This retention sequence-free protein was not secreted from the monkey kidney cell line COS-M6 in transient expression assays. When this large S protein was overexpressed in the presence of the major S protein, the secretion of the latter was severely inhibited. Moreover, the presence of S protein in large abundance did not facilitate the secretion of the mutated large S protein at all, indicating that it was subject to more profound retention than the wild-type large S protein. The findings that, like the wild-type large S protein, this altered protein retained some of the properties of its C-terminal S domain, and that the preS1 region was still on its surface suggested that there was no extensive alteration of the polypeptide folding. The interpretation is favored that the C-terminal half of the preS1 region plays a crucial role in the secretion of the large S protein when its N-terminal retention sequence is not present.

Myristylation of a duck hepatitis B virus envelope protein is essential for infectivity but not for virus assembly

In addition to the major surface (S) protein, the envelope of the duck hepatitis B virus (DHBV) contains a related presurface (preS) protein whose N-terminus bears a covalently attached myristate group. We have explored the functional significance of this modification by examining the replicative potential of a mutant viral genome whose myristylation signal has been inactivated. Following transfection into permissive hepatoma cells, the mutant expresses an unmyristylated preS protein of normal size, immunoreactivity and stability. Cytoplasmic cores containing viral DNA are synthesized, and Dane particles are assembled and exported into the medium. However, the mutant is noninfectious when inoculated into susceptible ducklings. We conclude that myristylation of preS proteins is essential for hepadnaviral infectivity but not for viral assembly; myristylation is most likely required for an early step of the life cycle involving the entry or uncoating of virus particles.

Epitope mapping of the PreS1 domain of the hepatitis B virus large surface protein

The large (L) surface glycoprotein of hepatitis B virus is an important component of the virion envelope derived from translation initiation at the 5' end of the PreS1 domain of the surface antigen open reading frame. Since key roles in virion assembly and infectivity have been postulated for this protein, further understanding of its structure and topology is important. To this end we have mapped the epitopes recognized by a panel of monoclonal antibodies specific for this polypeptide by examining their reactivity with a series of deletion mutants of the PreS1 region expressed in cultured cells. On the basis of this and other techniques, the antibodies fall into two groups mapping to two distinct epitopes spanning residues 27-35 and 72-78, respectively. Immunoprecipitation studies indicate that both regions are exposed on the surface of HBV-encoded particles.

Detection of receptors for hepatitis B virus on cells of extrahepatic origin

Receptors for hepatitis B virus (HBV; subtype adw) were identified on the surface of human hepatoma HepG2 cells in earlier studies. The cell receptor binding site on HBV was assigned to the preS(21-47) region of the preS1 sequence of the envelope protein. Studies presented here show that (1) amino acid residue replacements within the preS(21-47) sequence distinguishing HBV subtypes adw and ayw, preserve the binding capacity of the HBV env protein for HepG2 cell receptors; (2) the inhibition of binding between HepG2 cells and preS1-specific ligands by antibodies is effective only if the subtype specificity of anti-preS1-specific antibodies and of the preS1-specific ligands are matched; (3) receptors for HBV were present on the surface of human cells of nonhepatic origin, including peripheral blood B-lymphocytes, some hematopoietic cell lines of the B-cell lineage, neuroblastoma, amnion, and embryonic carcinoma cell lines. Receptors for HBV on these cells appeared similar to the receptor on HepG2 cells by the following criteria: (a) recognition by antibodies raised against the receptor on HepG2 cells; (b) inhibitory activity of lysates prepared from these cells on the interaction between HepG2 cells and preS1-specific ligands; and (c) the inhibitory effect of lysates from HepG2 cells on the reaction of these cells with HBsAg- and preS(21-47)-cellulose. The presence of receptors for HBV on some cells of extrahepatic origin is in accordance with earlier observations indicating that hepadnaviruses are not strictly hepatotropic.