The ORF3 protein of hepatitis E virus interacts with liver-specific alpha1-microglobulin and its precursor alpha1-microglobulin/bikunin precursor (AMBP) and expedites their export from the hepatocyte

Severe acute respiratory syndrome coronavirus Orf3a protein interacts with caveolin

The orf3a (also called X1 or U274) gene is the largest unique open reading frame in the severe acute respiratory syndrome coronavirus genome and has been proposed to encode a protein with three transmembrane domains and a large cytoplasmic domain. Recent work has suggested that the 3a protein may play a structural role in the viral life cycle, although the mechanisms for this remain uncharacterized. Here, the expression of the 3a protein in various in vitro systems is shown, it has been localized to the Golgi region and its membrane topology in transfected cells has been confirmed. Three potential caveolin-1-binding sites were reported to be present in the 3a protein. By using various biochemical, biophysical and genetic techniques, interaction of the 3a protein with caveolin-1 is demonstrated. Any one of the potential sites in the 3a protein was sufficient for this interaction. These results are discussed with respect to the possible roles of the 3a protein in the viral life cycle.

Mutations within potential glycosylation sites in the capsid protein of hepatitis E virus prevent the formation of infectious virus particles

Hepatitis E virus is a nonenveloped RNA virus. However, the single capsid protein resembles a typical glycoprotein in that it contains a signal sequence and potential glycosylation sites that are utilized when recombinant capsid protein is overexpressed in cell culture. In order to determine whether these unexpected observations were biologically relevant or were artifacts of overexpression, we analyzed capsid protein produced during a normal viral replication cycle. In vitro transcripts from an infectious cDNA clone mutated to eliminate potential glycosylation sites were transfected into cultured Huh-7 cells and into the livers of rhesus macaques. The mutations did not detectably affect genome replication or capsid protein synthesis in cell culture. However, none of the mutants infected rhesus macaques. Velocity sedimentation analyses of transfected cell lysates revealed that mutation of the first two glycosylation sites prevented virion assembly, whereas mutation of the third site permitted particle formation and RNA encapsidation, but the particles were not infectious. However, conservative mutations that did not destroy glycosylation motifs also prevented infection. Overall, the data suggested that the mutations were lethal because they perturbed protein structure rather than because they eliminated glycosylation.

Subcellular localization of hepatitis E virus (HEV) replicase

Hepatitis E virus (HEV) is a hepatotropic virus with a single sense-strand RNA genome of approximately 7.2 kb in length. Details of the intracellular site of HEV replication can pave further understanding of HEV biology. In-frame fusion construct of functionally active replicase-enhanced green fluorescent protein (EGFP) gene was made in eukaryotic expression vector. The functionality of replicase-EGFP fusion protein was established by its ability to synthesize negative-strand viral RNA in vivo, by strand-specific anchored RT-PCR and molecular beacon binding. Subcellular co-localization was carried out using organelle specific fluorophores and by immuno-electron microscopy. Fluorescence Resonance Energy Transfer (FRET) demonstrated the interaction of this protein with the 3' end of HEV genome. The results show localization of replicase on the endoplasmic reticulum membranes. The protein regions responsible for membrane localization was predicted and identified by use of deletion mutants. Endoplasmic reticulum was identified as the site of replicase localization and possible site of replication.

Oligomerization of the human immunodeficiency virus type 1 (HIV-1) Vpu protein--a genetic, biochemical and biophysical analysis

Background

The human immunodeficiency virus type 1(HIV-1) is a complex retrovirus and the causative agent of acquired immunodeficiency syndrome (AIDS). The HIV-1 Vpu protein is an oligomeric integral membrane protein essential for particle release, viral load and CD4 degradation. In silico models show Vpu to form pentamers with an ion channel activity.

Results

Using Vpu proteins from a primary subtype C and the pNL4-3 subtype B isolates of HIV-1, we show oligomerization of the full-length protein as well as its transmembrane (TM) domain by genetic, biochemical and biophysical methods. We also provide direct evidence of the presence of Vpu pentamers in a stable equilibrium with its monomers in vitro. This was also true for the TM domain of Vpu. Confocal microscopy localized Vpu to the endoplasmic reticulum and Golgi regions of the cell, as well as to post-Golgi vesicles. In fluorescence resonance energy transfer (FRET) experiments in live cells we show that Vpu oligomerizes in what appears to be either the Golgi region or intracellular vesicles, but not in the ER.

Conclusion

We provide here direct evidence that the TM domain, is critical for Vpu oligomerization and the most favourable channel assembly is a pentamer. The Vpu oligomerization appears to be either the Golgi region or intracellular vesicles, but not in the ER.

The hepatitis E virus Orf3 protein protects cells from mitochondrial depolarization and death

The biology and pathogenesis of hepatitis E virus are poorly understood due to the lack of an in vitro culture or infection models. The viral Orf3 protein activates the cellular mitogen-activated protein kinase pathway and is likely to modulate the host cell environment for efficient viral replication. We screened for cellular genes whose transcription was differentially up-regulated in an Orf3-expressing stable cell line (ORF3/4). The gene for mitochondrial voltage-dependent anion channel (VDAC) was one such candidate. The up-regulation of VDAC in ORF3/4 cells was confirmed by Northern and Western blotting in various cell lines. Transfection of ORF3/4 cells with an ORF3-specific small interfering RNA led to a reduction in VDAC protein levels. VDAC is a critical mitochondrial outer membrane protein, and its overexpression results in apoptosis. Surprisingly, Orf3-expressing cells were protected against staurosporine-induced cell death by preservation of mitochondrial potential and membrane integrity. A small interfering RNA-mediated reduction in Orf3 and VDAC levels also made cells sensitive to staurosporine. Chemical cross-linking showed Orf3-expressing cells to contain higher levels of oligomeric VDAC. These cells also contained higher levels of hexokinase I that directly interacted with VDAC. This interaction is known to preserve mitochondrial potential and prevent cytochrome c release. We report here the first instance of a viral protein promoting cell survival through such a mechanism.

Cytoplasmic localization of the ORF2 protein of hepatitis E virus is dependent on its ability to undergo retrotranslocation from the endoplasmic reticulum

Hepatitis E virus (HEV) is a positive-strand RNA virus that is prevalent in much of the developing world. ORF2 is the major capsid protein of HEV. Although ORF2 is an N-linked glycoprotein, it is abundantly located in the cytoplasm in addition to having membrane and surface localization. The mechanism by which ORF2 protein obtains access to the cytoplasm is unknown. In this report, we prove that initially all ORF2 protein is present in the endoplasmic reticulum and a fraction of it becomes retrotranslocated to the cytoplasm. The ability of ORF2 to be retrotranslocated is dependent on its glycosylation status and follows the canonical dislocation pathway. However, in contrast to general substrates of the dislocation pathway, retrotranslocated ORF2 protein is not a substrate of the 26S proteasome complex and is readily detectable in the cytoplasm in the absence of any protease inhibitor, suggesting that the retrotranslocated protein is stable in the cytoplasm. This study thus defines the pathway by which ORF2 obtains access to the cytoplasm.

Initiation at the third in-frame AUG codon of open reading frame 3 of the hepatitis E virus is essential for viral infectivity in vivo

To determine the initiation strategy of the hepatitis E virus (HEV) open reading frame 3 (ORF3), we constructed five HEV mutants with desired mutations in the ORF1 and ORF2 junction region and tested their levels of in vivo infectivity in pigs. A mutant with a C-terminally truncated ORF3 is noninfectious in pigs, indicating that an intact ORF3 is required for in vivo infectivity. Mutations with substitutions in the first in-frame AUG in the junction region or with the same T insertion at the corresponding position of HEV genotype 4 did not affect the virus infectivity or rescue, although mutations with combinations of the two affected virus recovery efficiency, and a single mutation at the third in-frame AUG completely abolished virus infectivity in vivo, indicating that the third in-frame AUG in the junction region is required for virus infection and is likely the authentic initiation site for ORF3. A conserved double stem-loop RNA structure, which may be important for HEV replication, was identified in the junction region. This represents the first report of using a unique homologous pig model system to study the molecular mechanism of HEV replication and to systematically and definitively identify the authentic ORF3 initiation site.

ORF3 protein of hepatitis E virus is not required for replication, virion assembly, or infection of hepatoma cells in vitro

A subclone of Huh-7 cells that could be relatively efficiently transfected and infected with hepatitis E virus was identified. Following transfection, infectious virus was produced but remained predominantly cell associated. Intracellular virus, recovered by lysis of transfected cells, infected naïve cells. This in vitro-produced virus appeared to be antigenically identical to virus isolated from clinical samples. Lysates from cells transfected with mutant viral genomes unable to synthesize ORF3 protein contained infectious virions that were similar in number, thermostability, and sedimentation characteristics to those in lysates transfected with wild-type viral genomes. Therefore, in contrast to its requirement in vivo, ORF3 protein is not required for infection of Huh-7 cells or production of infectious virus in vitro.

A bicistronic subgenomic mRNA encodes both the ORF2 and ORF3 proteins of hepatitis E virus

Hepatitis E virus replicons containing the neomycin resistance gene expressed from open reading frames (ORFs) 2 and 3 were transfected into Huh-7 cells, and stable cell lines containing functional replicons were selected by constant exposure to G418 sulfate. Northern blot analyses detected full-length replicon RNA and a single subgenomic RNA. This subgenomic RNA, which was capped, initiated at nucleotide 5122 downstream of the first two methionine codons in ORF3 and was bicistronic; two closely spaced methionine codons in different reading frames were used for the initiation of ORF3 and ORF2 translation.

The VP1 protein of human enterovirus 71 self-associates via an interaction domain spanning amino acids 66-297

Enterovirus 71 (EV71) is a major etiological agent of hand, foot, and mouth disease (HFMD). Several recent outbreaks of HFMD in East Asia were associated with neurological complications and numerous deaths. In 2000, an outbreak in Singapore afflicted thousands of children, resulting in four fatal cases from whom EV71 was isolated. The virus possesses four structural proteins VP1, VP2, VP3, and VP4, each of which is involved in forming the pentameric icosahedral structure of the virus. Here we report that the full-length VP1 structural protein of EV71 is capable of self-association. Dimerization of VP1 was tested using the yeast two-hybrid system, fluorescence resonance energy transfer (FRET) analysis, in vitro coupled transcription-translation binding assays, and mammalian cell transcription-translation experiments. Dimerization of various truncated versions of the VP1 protein was also studied by mutational analysis. Systematic deletions of parts of VP1 revealed that the region spanning amino acids 66-132 of VP1 contains the major dimerization domain. However, the region between amino acids 132 and 297 was indispensable, and contributed largely to increasing the strength of the interaction. This ability of EV71 VP1 to self-associate and to participate significantly in forming the characteristic icosahedral capsid strongly enhances the pathogenicity and stability of the virus to withstand the environment of the gastrointestinal tract.

Enhanced alpha1 microglobulin secretion from Hepatitis E virus ORF3-expressing human hepatoma cells is mediated by the tumor susceptibility gene 101

Viruses are known to exploit the host cell machinery for their benefit during different stages of their life cycle within the infected host. One of the major challenges for a virus during the early stages of infection is to escape recognition by the host immune system. Viruses have adopted many novel strategies to evade the host immune response or to create an immune suppressed environment. An earlier study in our laboratory has demonstrated that the ORF3 protein of the hepatitis E virus expedites the secretion of alpha1 microglobulin, an immunosuppressant molecule. Based on this observation, we proposed that enhanced secretion of alpha1 microglobulin may help maintain an immunosuppressed milieu around the infected hepatocyte (Tyagi, S., Surjit, M., Roy, A. K., Jameel, S., and Lal, S. K. (2004) J. Biol. Chem. 279, 29308-29319). In the present study, we discovered that the ability of the ORF3 protein to expedite alpha1 microglobulin secretion is attributed to the PSAP motif present at the C terminus of the former. The ORF3 protein was able to associate with the tumor susceptibility gene 101 (TSG101) through the PSAP motif. Further, a PSAP motif-mutated ORF3 protein was unable to associate with TSG101 and also lost its ability to enhance the secretion of alpha1 microglobulin. In addition, the ORF3 protein was found to associate simultaneously with TSG101 and alpha1 microglobulin because all three of them were co-precipitated as a ternary complex. Finally, a dominant negative mutant of the VPS4 protein was shown to block the enhanced alpha1 microglobulin secretion in ORF3-expressing hepatocytes. These results suggest a mechanism by which the ORF3 protein exploits the endosomal sorting machinery to enhance the secretion of an immunosuppressant molecule (alpha1 microglobulin) from the cultured hepatocytes.

The Nef protein of HIV-1 induces loss of cell surface costimulatory molecules CD80 and CD86 in APCs

The Nef protein of HIV-1 is essential for its pathogenicity and is known to down-regulate MHC expression on infected cell surfaces. We now show that Nef also redistributes the costimulatory molecules CD80 and CD86 away from the cell surface in the human monocytic U937 cell line as well as in mouse macrophages and dendritic cells. Furthermore, HIV-1-infected U937 cells and human blood-derived macrophages show a similar loss of cell surface CD80 and CD86. Nef colocalizes with MHC class I (MHCI), CD80, and CD86 in intracellular compartments, and binds to both mouse and human CD80 and CD86. Some Nef mutants defective in MHCI down-modulation, including one from a clinical isolate, remain capable of down-modulating CD80 and CD86. Nef-mediated loss of surface CD80/CD86 is functionally significant, because it leads to compromised activation of naive T cells. This novel immunomodulatory role of Nef may be of potential importance in explaining the correlations of macrophage-tropism and Nef with HIV-1 pathogenicity and immune evasion.

The 41-amino-acid C-terminal region of the hepatitis E virus ORF3 protein interacts with bikunin, a kunitz-type serine protease inhibitor

Hepatitis E virus (HEV), a human plus-stranded RNA virus, contains three open reading frames (ORF). Of these, ORF1 encodes the viral nonstructural polyprotein, ORF2 encodes the major capsid protein, and ORF3 codes for a phosphoprotein of undefined function. Recently, using the yeast two-hybrid system to screen a human cDNA liver library, we have isolated and characterized AMBP (alpha1-microglobulin/bikunin precursor), which specifically interacts with the ORF3 protein of HEV. The ORF3 protein expedites the processing and secretion of alpha1-microglobulin. When checked individually for interaction, the second processed protein from AMBP, bikunin, strongly interacted with the full-length ORF3 protein. This protein-protein interaction has been validated by immunoprecipitation in both COS-1 and Huh7 cells and by His6 pull-down assays. In dual-labeling immunofluorescent staining, followed by fluorescence microscopy of transfected human liver cells, ORF3 colocalized with endogenously expressed bikunin. Finally, a 41-amino-acid C-terminal region of ORF3 has been found to be responsible for interacting with bikunin. The importance of this virus-host protein-protein interaction, with reference to the viral life cycle, has been discussed.

The open reading frame 3 gene of hepatitis E virus contains a cis-reactive element and encodes a protein required for infection of macaques

An infectious cDNA clone of hepatitis E virus was mutated in order to prevent synthesis of either open reading frame 2 (ORF2) protein or ORF3 protein. HuH-7 cells transfected with an ORF2-null mutant produced ORF3, and those transfected with an ORF3-null mutant produced ORF2. Silent mutations introduced into a highly conserved nucleotide sequence in the ORF3 coding region eliminated the synthesis of both ORF2 and ORF3 proteins, suggesting that it comprised a cis-reactive element. A mutant that was not able to produce ORF3 protein did not produce a detectable infection in rhesus macaques. However, a mutant that encoded an ORF3 protein lacking a phosphorylation site reported to be critical for function was able to replicate its genome in cell culture and to induce viremia and seroconversion in rhesus monkeys, suggesting that phosphorylation of ORF3 protein was not necessary for genome replication or for production of infectious virions.

Human α-1-Microglobulin Is Covalently Bound to Kynurenine-derived Chromophores

Alpha-1-microglobulin carries a set of covalently linked chromophores that give it a peculiar yellow-brown color, fluorescence properties, and both charge and size heterogeneity. In this report it is shown that these features are due to the adducts with the tryptophan metabolite, 3-hydroxykynurenine, and its autoxidation products and that the modification is more pronounced in the protein isolated from urine of hemodialyzed patients. The light yellow amniotic fluid alpha-1-microglobulin acquires the optical properties and charge heterogeneity of the urinary counterpart following incubation with kynurenines. The colored amino acid adducts of urinary and amniotic fluid alpha-1-microglobulins were separated by chromatography after acid hydrolysis and analyzed by mass spectrometry. Human serum albumin samples, native and treated with 3-hydroxykynurenine in the presence of oxygen, were used as a control. The retention times and mass fragmentation products were compared, and a lysyl adduct with hydroxantommathin was identified in the urinary alpha-1-microglobulin and in the modified albumin samples. The more extensive modification of the urinary protein appears to be correlated with uremia, a condition in which the catabolism of tryptophan via the kynurenine pathway is increased, and the consequent rise in the concentration of its derivatives is accompanied by the oxidative processes due to the hemodialysis treatment. The oxidative derivatives of 3-hydroxykynurenine, which are known to act as protein cross-linking agents, are the likely cause of the propensity of urinary alpha-1-microglobulin to form dimers and oligomers. This process, as well as the redox properties of these metabolites, may contribute to the toxic effects of the kynurenine species.