Serine protease of pestiviruses: determination of cleavage sites.

The single-stranded genomic RNA of pestiviruses is of positive polarity and encompasses one large open reading frame of about 4,000 codons. The resulting polyprotein is processed co- and posttranslationally by virus-encoded and host cell proteases to give rise to the mature viral proteins. A serine protease residing in the nonstructural (NS) protein NS3 (p80) has been shown to be essential for the release of the NS proteins located downstream of NS3. In this report the NS3 serine protease-dependent cleavage sites for bovine viral diarrhea virus (BVDV) strain CP7 are described. Proteins used for analysis were generated in Escherichia coli or in eukaryotic cells by the use of the T7 vaccinia virus system. The N termini of NS4A, NS4B, NS5A, and NS5B were determined by protein sequencing. Analysis of the data obtained showed that leucine at P1 is the only position conserved for all cleavage sites. At P1' alanine is found at the NS4A-NS4B site, whereas serine resides at this position at the NS3-NS4A, NS4B-NS5A, and NS5A-NS5B cleavage sites. For all cleavage sites the amino acids found at P1 and P1' are conserved for different genotypes of pestiviruses, despite the high degree of sequence variation found between these viruses. It is therefore assumed that the cleavage sites determined for BVDV CP7 are representative of those for all pestiviruses.     

Bovine viral diarrhea virus NS3 serine proteinase: polyprotein cleavage sites, cofactor requirements, and molecular model of an enzyme essential for pestivirus replication.

Members of the Flaviviridae encode a serine proteinase termed NS3 that is responsible for processing at several sites in the viral polyproteins. In this report, we show that the NS3 proteinase of the pestivirus bovine viral diarrhea virus (BVDV) (NADL strain) is required for processing at nonstructural (NS) protein sites 3/4A, 4A/4B, 4B/5A, and 5A/5B but not for cleavage at the junction between NS2 and NS3. Cleavage sites of the proteinase were determined by amino-terminal sequence analysis of the NS4A, NS4B, NS5A, and NS5B proteins. A conserved leucine residue is found at the P1 position of all four cleavage sites, followed by either serine (3/4A, 4B/5A, and 5A/5B sites) or alanine (4A/4B site) at the P1' position. Consistent with this cleavage site preference, a structural model of the pestivirus NS3 proteinase predicts a highly hydrophobic P1 specificity pocket. trans-Processing experiments implicate the 64-residue NS4A protein as an NS3 proteinase cofactor required for cleavage at the 4B/5A and 5A/5B sites. Finally, using a full-length functional BVDV cDNA clone, we demonstrate that a catalytically active NS3 serine proteinase is essential for pestivirus replication.     

Characterization of the hepatitis C virus-encoded serine proteinase: determination of proteinase-dependent polyprotein cleavage sites.

Processing of the hepatitis C virus (HCV) H strain polyprotein yields at least nine distinct cleavage products: NH2-C-E1-E2-NS2-NS3-NS4A-NS4B-NS5A-NS5B-CO OH. As described in this report, site-directed mutagenesis and transient expression analyses were used to study the role of a putative serine proteinase domain, located in the N-terminal one-third of the NS3 protein, in proteolytic processing of HCV polyproteins. All four cleavages which occur C terminal to the proteinase domain (3/4A, 4A/4B, 4B/5A, and 5A/5B) were abolished by substitution of alanine for either of two predicted residues (His-1083 and Ser-1165) in the proteinase catalytic triad. However, such substitutions have no observable effect on cleavages in the structural region or at the 2/3 site. Deletion analyses suggest that the structural and NS2 regions of the polyprotein are not required for the HCV NS3 proteinase activity. NS3 proteinase-dependent cleavage sites were localized by N-terminal sequence analysis of NS4A, NS4B, NS5A, and NS5B. Sequence comparison of the residues flanking these cleavage sites for all sequenced HCV strains reveals conserved residues which may play a role in determining HCV NS3 proteinase substrate specificity. These features include an acidic residue (Asp or Glu) at the P6 position, a Cys or Thr residue at the P1 position, and a Ser or Ala residue at the P1' position.     

Activity of purified hepatitis C virus protease NS3 on peptide substrates.

The protease domain of the hepatitis C virus (HCV) protein NS3 was expressed in Escherichia coli, purified to homogeneity, and shown to be active on peptides derived from the sequence of the NS4A-NS4B junction. Experiments were carried out to optimize protease activity. Buffer requirements included the presence of detergent, glycerol, and dithiothreitol, pH between 7.5 and 8.5, and low ionic strength. C- and N-terminal deletion experiments defined a peptide spanning from the P6 to the P4' residue as a suitable substrate. Cleavage kinetics were subsequently measured by using decamer P6-P4' peptides corresponding to all intermolecular cleavage sites of the HCV polyprotein. The following order of cleavage efficiency, in terms of kcat/Km, was determined: NS5A-NS5B > NS4A-NS4B >> NS4B-NS5A. A 14-mer peptide containing residues 21 to 34 of the protease cofactor NS4A (Pep4A 21-34), when added in stoichiometric amounts, was shown to increase cleavage rates of all peptides, the largest effect (100-fold) being observed on the hydrolysis of the NS4B-NS5A decamer. From the kinetic analysis of cleavage data, we conclude that (i) primary structure is an important determinant of the efficiency with which each site is cleaved during polyprotein processing, (ii) slow cleavage of the NS4B-NS5A site in the absence of NS4A is due to low binding affinity of the enzyme for this site, and (iii) formation of a 1:1 complex between the protease and Pep4A 21-34 is sufficient and required for maximum activation.     

Substrate determinants for cleavage in cis and in trans by the hepatitis C virus NS3 proteinase.

Processing of the hepatitis C virus polyprotein is accomplished by a series of cotranslational and posttranslational cleavages mediated by host cell signalases and two virally encoded proteinases. Of these the NS3 proteinase is essential for processing at the NS3/4A, NS4A/4B, NS4B/5A, and NS5A/5B junctions. Processing between NS3 and NS4A occurs in cis, implying an intramolecular reaction mechanism, whereas cleavage at the other sites can also be mediated in trans. Sequence analysis of the amino termini of mature cleavage products and comparisons of amino acid residues around the scissile bonds of various hepatitis C virus isolates identified amino acid residues which might contribute to substrate specificity and processing efficiency: an acidic amino acid at the P6 position, a Thr or Cys at the P1 position, and a Ser or Ala at the P1' position. To study the importance of these residues for NS3-mediated cleavage we have undertaken a mutational analysis using an NS3'-5B polyprotein expressed by recombinant vaccinia viruses in mammalian cells. For all NS3-dependent cleavage sites P1 substitutions had the most drastic effects on cleavage efficiency, showing that amino acid residues at this position are the most critical substrate determinants. Since less drastic effects were found for substitutions at the P1' position, these residues appear to be less important for proper cleavage. For all cleavage sites the P6 acidic residue was dispensable, suggesting that it is not essential for substrate recognition and subsequent cleavage. Analysis of a series of mutations at the NS3/4A site revealed great flexibility for substitutions compared with more stringent requirements at the trans cleavage sites. On the basis of these results we propose a model in which processing in cis is determined primarily by polyprotein folding, whereas cleavage in trans is governed not only by the structure of the polyprotein but also by specific interactions between the proteinase and the polyprotein substrate at or around the scissile bond.     

Mutational analysis of the octapeptide sequence motif at the NS1-NS2A cleavage junction of dengue type 4 virus.

We have previously shown that proper processing of dengue type 4 virus NS1 from the NS1-NS2A region of the viral polyprotein requires a hydrophobic N-terminal signal and the downstream NS2A. Results from deletion analysis indicate that a minimum length of eight amino acids at the C terminus of NS1 is required for cleavage at the NS1-NS2A junction. Comparison of this eight-amino-acid sequence with the corresponding sequences of other flaviviruses suggests a consensus cleavage sequence of Met/Leu-Val-Xaa-Ser-Xaa-Val-Xaa-Ala. Site-directed mutagenesis was performed to construct mutants of NS1-NS2A that contained a single amino acid substitution at different positions of the consensus cleavage sequence or at the immediate downstream position. Three to eight different substitutions were made at each position. A total of 50 NS1-NS2A mutants were analyzed for their cleavage efficiency relative to that of the wild-type dengue type 4 virus sequence. As predicted, nearly all substitutions at positions P1, P3, P5, P7, and P8, occupied by conserved amino acids, yielded low levels of cleavage, with the exception that Pro or Ala substituting for Ser (P5) was tolerated. Substitutions of an amino acid at the remaining positions occupied by nonconserved amino acids generally yielded high levels of cleavage. However, some substitutions at nonconserved positions were not tolerated. For example, substitution of Gly or Glu for Gln (P4) and substitution of Val or Glu for Lys (P6) each yielded a low level of cleavage. Overall, these data support the proposed cleavage sequence motif deduced by comparison of sequences among the flaviviruses. This study also showed that in addition to the eight-amino-acid sequence, the amino acid immediately following the NS1-NS2A cleavage site plays a role in cleavage.     

Probing the substrate specificity of hepatitis C virus NS3 serine protease by using synthetic peptides.

We probed the substrate specificity of a recombinant noncovalent complex of the full-length hepatitis C virus (HCV) NS3 serine protease and NS4A cofactor, using a series of small synthetic peptides derived from the three trans-cleavage sites of the HCV nonstructural protein sequence. We observed a distinct cleavage site preference exhibited by the enzyme complex. The values of the turnover number (k(cat)) for the most efficient NS4A/4B, 4B/5A, and 5A/5B peptide substrates were 1.6, 11, and 8 min(-1), respectively, and the values for the corresponding Michaelis-Menten constants (Km) were 280, 160, and 16 microM, providing catalytic efficiency values (k(cat)/Km) of 92, 1,130, and 8,300 M(-1) s(-1). An alanine-scanning study for an NS5A/5B substrate (P6P4') revealed that P1 Cys and P3 Val were critical. Finally, substitutions at the scissile P1 Cys residue by homocysteine (Hcy), S-methylcysteine (Mcy), Ala, S-ethylcysteine (Ecy), Thr, Met, D-Cys, Ser, and penicillamine (Pen) produced progressively less efficient substrates, revealing a stringent stereochemical requirement for a Cys residue at this position.     

Characterization of the trans-activation-responsive element of the parvovirus H-1 P38 promoter

The parvovirus early protein NS1 positively regulates the expression of the P38 promoter for the viral capsid protein gene. We have examined the trans-activation of P38 by NS1 by using fusions of P38 to the reporter gene, chloramphenicol acetyltransferase (cat). Maximal trans-activation requires a small 5' cis element (tar) between -137 and -116. The tar element has activity in both orientations when 5' to the P38 promoter, but no activity has been detected 3' to the promoter. The wild-type P38 has a biphasic response to NS1 depending on the dosage of the NS1-expressing plasmid. Promoters lacking the tar also have a biphasic response that is reduced about 10-fold, and they can be inhibited by larger doses of the NS1 plasmid. Heterologous promoters from other viruses and the Harvey-ras oncogene promoter are inhibited by NS1. Truncated and internally deleted versions of NS1 lose the trans-activation, but some of them retain the inhibitory properties. Thus transactivation can be uncoupled from inhibition. The tar element has shown no activity with the heterologous simian virus 40 early promoter. In contrast, the P38 promoter responds to a heterologous enhancer, but the enhanced promoter loses activity to trans-activation by NS1. In summary, the P38 tar element has some of the properties of an enhancer with a high preference for a 5' position and a stringent requirement for the P38 promoter.     

Functional characterization of the cleavage specificity of the sapovirus chymotrypsin-like protease

Sapovirus is a positive-stranded RNA virus with a translational strategy based on processing of a polyprotein precursor by a chymotrypsin-like protease. So far, the molecular mechanisms regulating cleavage specificity of the viral protease are poorly understood. In this study, the catalytic activities and substrate specificities of the predicted forms of the viral protease, the 3C-like protease (NS6) and the 3CD-like protease-polymerase (NS6-7), were examined in vitro. The purified NS6 and NS6-7 were able to cleave synthetic peptides (15 to 17 residues) displaying the cleavage sites of the sapovirus polyprotein, both NS6 and NS6-7 proteins being active forms of the viral protease. High-performance liquid chromatography and subsequent mass spectrometry analysis of digested products showed a specific trans cleavage of peptides bearing Gln-Gly, Gln-Ala, Glu-Gly, Glu-Pro, or Glu-Lys at the scissile bond. In contrast, peptides bearing Glu-Ala or Gln-Asp at the scissile bond (NS4-NS5 and NS5-NS6, or NS6-NS7 junctions, respectively) were resistant to trans cleavage by NS6 or NS6-7 proteins, whereas cis cleavage of the Glu-Ala scissile bond of the NS5-NS6 junction was evidenced. Interestingly, the presence of a Phe at position P4 overruled the resistance to trans cleavage of the Glu-Ala junction (NS5-NS6), whereas substitutions at the P1 and P2′ positions altered the cleavage efficiency. The differential cleavage observed is supported by a model of the substrate-binding site of the sapovirus protease, indicating that the P4, P1, and P2′ positions in the substrate modulate the cleavage specificity and efficiency of the sapovirus chymotrypsin-like protease.     

Hepatitis C virus-encoded NS2-3 protease: cleavage-site mutagenesis and requirements for bimolecular cleavage.

Cleavage at the 2/3 site of hepatitis C virus (HCV) is thought to be mediated by a virus-encoded protease composed of the region of the polyprotein encoding NS2 and the N-terminal one-third of NS3. This protease is distinct from the NS3 serine protease, which is responsible for downstream cleavages in the nonstructural region. Site-directed mutagenesis of residues surrounding the 2/3 cleavage site showed that cleavage is remarkably resistant to single-amino-acid substitutions from P5 to P3' (GWRLL decreases API). The only mutations which dramatically inhibited cleavage were the ones most likely to alter the conformation of the region, such as Pro substitutions at the P1 or P1' position, deletion of both amino acids at P1 and P1', or simultaneous substitution of multiple Ala residues. Cotransfection experiments were done to provide additional information on the polypeptide requirements for bimolecular cleavage. Polypeptides used in these experiments contained amino acid substitutions and/or deletions in NS2 and/or the N-terminal one-third of NS3. Polypeptides with defects in either NS2 or the N-terminal portion of NS3 but not both were cleaved when cotransfected with constructs expressing intact versions of the defective region. Cotransfection experiments also showed that certain defective NS2-3 constructs partially inhibited cleavage of wild-type polypeptides. Although these results show that inefficient cleavage can occur in a bimolecular reaction, they suggest that both molecules must contribute a functional subunit to allow formation of a protease which is capable of cleavage at the 2/3 site. This reaction may resemble the cis cleavage thought to occur at the 2/3 site during processing of the wild-type HCV polyprotein.     

Specificity of the hepatitis C virus NS3 serine protease: effects of substitutions at the 3/4A, 4A/4B, 4B/5A, and 5A/5B cleavage sites on polyprotein processing.

Cleavage at four sites (3/4A, 4A/4B, 4B/5A, and 5A/5B) in the hepatitis C virus polyprotein requires a viral serine protease activity residing in the N-terminal one-third of the NS3 protein. Sequence comparison of the residues flanking these cleavage sites reveals conserved features including an acidic residue (Asp or Glu) at the P6 position, a Cys or Thr residue at the P1 position, and a Ser or Ala residue at the P1' position. In this study, we used site-directed mutagenesis to assess the importance of these and other residues for NS3 protease-dependent cleavages. Substitutions at the P7 to P2' positions of the 4A/4B site had varied effects on cleavage efficiency. Only Arg at the P1 position or Pro at P1' substantially blocked processing at this site. Leu was tolerated at the P1 position, whereas five other substitutions allowed various degrees of cleavage. Substitutions with positively charged or other hydrophilic residues at the P7, P3, P2, and P2' positions did not reduce cleavage efficiency. Five substitutions examined at the P6 position allowed complete cleavage, demonstrating that an acidic residue at this position is not essential. Parallel results were obtained with substrates containing an active NS3 protease domain in cis or when the protease domain was supplied in trans. Selected substitutions blocking or inhibiting cleavage at the 4A/4B site were also examined at the 3/4A, 4B/5A, and 5A/5B sites. For a given substitution, a site-dependent gradient in the degree of inhibition was observed, with a 3/4A site being least sensitive to mutagenesis, followed by the 4A/4B, 4B/5A, and 5A/5B sites. In most cases, mutations abolishing cleavage at one site did not affect processing at the other serine protease-dependent sites. However, mutations at the 3/4A site which inhibited cleavage also interfered with processing at the 4B/5A site. Finally, during the course of these studies an additional NS3 protease-dependent cleavage site has been identified in the NS4B region.     

Efficient transactivation of the minute virus of mice P38 promoter requires upstream binding of NS1.

The P38 promoter of the autonomous parvovirus minute virus of mice is strongly transactivated by the nonstructural protein NS1, a sequence-specific DNA-binding protein. In the context of the complete viral genome, the only unique cis-acting signals required for P38 transactivation by NS1 are the proximal Sp1 site and the TATA element. In the absence of additional upstream sequences, a dependence upon the NS1 binding site within the transactivation response region is observed. Addition of synthetic NS1 binding sites to transactivation response region deletion mutants can restore the ability of NS1 to transactivate P38, and NS1 transactivation has been directly correlated to its ability to bind upstream of the P38 promoter.     

Decreased interferon-alpha production and impaired regulatory function of plasmacytoid dendritic cells induced by the hepatitis C virus NS 5 protein

pDC are known to produce large amount of IFN-alpha/beta in response to viruses, and act as a major link between the innate and adaptive immune response. This study concentrated on the interaction of human peripheral blood derived pDC with HCV NS3, NS4, and NS5 proteins, and their maturation, cytokine secretion and functional properties. It was shown that HCV NS5 interferes with CD40L induced maturation of pDC as indicated by decreased expression of CD83 and CD86 markers. CpG ODN stimulated HCV NS3 and NS5 treated pDC showed decreased production of IFN-alpha. In the case of NS3, IFN-alpha production was reduced to 126 pg/ml as compared to 245 pg/ml in controls (P < 0.01), and with NS5, IFN-alpha production was reduced to 92 pg/ml as compared to 238 pg/ml in controls (P < 0.05). In the presence of HCV NS5, the T cell stimulatory capacity of pDC was impaired, as indicated by decreased proliferation of T cells, and decreased production by the T cells of IFN-gamma, which were down to 86 pg/ml as compared to 260 pg/ml in controls (P < 0.05). These results suggest that HCV NS5 impairs pDC function and is in agreement with several other in vivo studies indicating decreased numbers of, and dysfunctional pDC, in chronic HCV infected patients.     

Efficacy of nasal strip and furosemide in mitigating EIPH in Thoroughbred horses.

The purpose of this investigation was to study the effects of an equine nasal strip (NS), furosemide (Fur), and a combination of both (NS + Fur) on exercise-induced pulmonary hemorrhage (EIPH) at speeds corresponding to near-maximal effort. Five Thoroughbreds (526 +/- 25 kg) were run on a flat treadmill from 7 to 14 m/s in 1 m x s(-1) x min(-)1 increments every 2 wk (treatment order randomized) under control (Con), Fur (1 mg/kg iv 4 h prior), NS, or NS + Fur conditions. During each run, pulmonary arterial (Ppa) and esophageal (Pes) pressures were measured. Severity of EIPH was quantified via bronchoalveolar lavage (BAL) 30 min postrun. Furosemide (Fur and NS + Fur trials) reduced peak Ppa approximately 7 mmHg compared with Con (P < 0.05) whereas NS had no effect (P > 0.05). Maximal Pes swings were not different among groups (P > 0.05). NS significantly diminished EIPH compared with the Con trial [Con, 55.0 +/- 36.2; NS, 30.8 +/- 21.8 x 10(6) red blood cells (RBC)/ml BAL fluid; P < 0.05]. Fur reduced EIPH to a greater extent than NS (5.2 +/- 3.0 x 10(6) RBC/ml BAL; P < 0.05 vs. Con and NS) with no additional benefit from NS + Fur (8.5 +/- 4.2 x 10(6) RBC/ml BAL; P > 0.05 vs. Fur, P < 0.05 vs. Con and NS). In conclusion, although both modalities (NS and Fur) were successful in mitigating EIPH, neither abolished EIPH fully as evaluated via BAL. Fur was more effective than NS in constraining the severity of EIPH. The simultaneous use of both interventions appears to offer no further gain with respect to reducing EIPH.     

Hepatitis C virus NS5A protein modulates template selection by the RNA polymerase in in vitro system

Hepatitis C virus (HCV) NS5A phosphoprotein is a component of virus replicase. Here we demonstrate that in vitro unphosphorylated NS5A protein inhibits HCV RNA-dependent RNA polymerase (RdRp) activity in polyA-oligoU system but has little effect on synthesis of viral RNA. The phosphorylated casein kinase (CK) II NS5A protein causes the opposite effect on RdRp in each of these systems. The phosphorylation of NS5A protein with CKII does not affect its affinity to the HCV RdRp and RNA. The NS5A phosphorylation with CKI does not change the RdRp activity. Herein we report evidence that the NS5A prevents template binding to the RdRp.

Structured summary

MINT-6803697: CKI (uniprotkb:P97633) phosphorylates (MI:0217) NS5A (uniprotkb:P26662) by protein kinase assay (MI:0424)MINT-6803713: CKII (uniprotkb:P67870) phosphorylates (MI:0217) NS5A (uniprotkb:P26662) by protein kinase assay (MI:0424)     

Purified NS2B/NS3 serine protease of dengue virus type 2 exhibits cofactor NS2B dependence for cleavage of substrates with dibasic amino acids in vitro

Dengue virus type 2 NS3, a multifunctional protein, has a serine protease domain (NS3pro) that requires the conserved hydrophilic domain of NS2B for protease activity in cleavage of the polyprotein precursor at sites following two basic amino acids. In this study, we report the expression of the NS2B-NS3pro precursor in Escherichia coli as a fusion protein with a histidine tag at the N terminus. The precursor was purified from insoluble inclusion bodies by Ni(2+) affinity and gel filtration chromatography under denaturing conditions. The denatured precursor was refolded to yield a purified active protease complex. Biochemical analysis of the protease revealed that its activity toward either a natural substrate, NS4B-NS5 precursor, or the fluorogenic peptide substrates containing two basic residues at P1 and P2, was dependent on the presence of the NS2B domain. The peptide with a highly conserved Gly residue at P3 position was 3-fold more active as a substrate than a Gln residue at this position. The cleavage of a chromogenic substrate with a single Arg residue at P1 was NS2B-independent. These results suggest that heterodimerization of the NS3pro domain with NS2B generates additional specific interactions with the P2 and P3 residues of the substrates.     

Interaction with a ubiquitin-like protein enhances the ubiquitination and degradation of hepatitis C virus RNA-dependent RNA polymerase

To identify potential cellular regulators of hepatitis C virus (HCV) RNA-dependent RNA polymerase (NS5B), we searched for cellular proteins interacting with NS5B protein by yeast two-hybrid screening of a human hepatocyte cDNA library. We identified a ubiquitin-like protein, hPLIC1 (for human homolog 1 of protein linking intergrin-associated protein and cytoskeleton), which is expressed in the liver (M. F. Kleijnen, A. H. Shih, P. Zhou, S. Kumar, R. E. Soccio, N. L. Kedersha, G. Gill, and P. M. Howley, Mol. Cell 6: 409-419, 2000). In vitro binding assays and in vivo coimmunoprecipitation studies confirmed the interaction between hPLIC1 and NS5B, which occurred through the ubiquitin-associated domain at the C terminus of the hPLIC1 protein. As hPLICs have been shown to physically associate with two E3 ubiquitin protein ligases as well as proteasomes (Kleijnen et al., Mol. Cell 6: 409-419, 2000), we investigated whether the stability and posttranslational modification of NS5B were affected by hPLIC1. A pulse-chase labeling experiment revealed that overexpression of hPLIC1, but not the mutant lacking the NS5B-binding domain, significantly shortened the half-life of NS5B and enhanced the polyubiquitination of NS5B. Furthermore, in Huh7 cells that express an HCV subgenomic replicon, the amounts of both NS5B and the replicon RNA were reduced by overexpression of hPLIC1. Thus, hPLIC1 may be a regulator of HCV RNA replication through interaction with NS5B.     

Kinetic and structural analyses of hepatitis C virus polyprotein processing.

Recombinant vaccinia viruses were used to study the processing of hepatitis C virus (HCV) nonstructural polyprotein precursor. HCV-specific proteins and cleavage products were identified by size and by immunoprecipitation with region-specific antisera. A polyprotein beginning with 20 amino acids derived from the carboxy terminus of NS2 and ending with the NS5B stop codon (amino acids 1007 to 3011) was cleaved at the NS3/4A, NS4A/4B, NS4B/5A, and NS5A/5B sites, whereas a polyprotein in which the putative active site serine residue was replaced by an alanine remained unprocessed, demonstrating that the NS3-encoded serine-type proteinase is essential for cleavage at these sites. Processing of the NS3'-5B polyprotein was complex and occurred rapidly. Discrete polypeptide species corresponding to various processing intermediates were detected. With the exception of NS4AB-5A/NS5A, no clear precursor-product relationships were detected. Using double infection of cells with vaccinia virus recombinants expressing either a proteolytically inactive NS3'-5B polyprotein or an active NS3 proteinase, we found that cleavage at the NS4A/4B, NS4B/5A, and NS5A/5B sites could be mediated in trans. Absence of trans cleavage at the NS3/4A junction together with the finding that processing at this site was insensitive to dilution of the enzyme suggested that cleavage at this site is an intramolecular reaction. The trans-cleavage assay was also used to show that (i) the first 211 amino acids of NS3 were sufficient for processing at all trans sites and (ii) small deletions from the amino terminus of NS3 selectively affected cleavage at the NS4B/5A site, whereas more extensive deletions also decreased processing efficiencies at the other sites. Using a series of amino-terminally truncated substrate polyproteins in the trans-cleavage assay, we found that NS4A is essential for cleavage at the NS4B/5A site and that processing at this site could be restored by NS4A provided in cis (i.e., together with the substrate) or in trans (i.e., together with the proteinase). These results suggest that in addition to the NS3 proteinase, NS4A sequences play an important role in HCV polyprotein processing.     

Construction of a genetic switch for inducible trans-activation of gene expression in eucaryotic cells.

The cotransfection of selectable marker genes and the gene for the nonstructural proteins NS1 and NS2 of the autonomous parvovirus H-1 failed to produce cell lines that constitutively expressed NS1. A plasmid, pP38NS1cat, was constructed that expressed the NS1-NS2 gene from the H-1 P38 coat protein promoter in place of the natural P4 promoter. The P38 promoter is constitutively weak and is trans-activated by NS1. Stable cell lines were isolated that contained pP38NS1cat that was constitutively silent, but inducible with exogenous NS1 by superinfection or by treatment with sodium butyrate. The cells that were induced for this self-stimulatory genetic circuit did not remain in the culture, suggesting that expression of NS1-NS2 is cytotoxic or that the expression is not sustained. The properties of these cell lines and an example of the construction of a cell line inducible for expression of the viral coat protein gene and the bacterial gene for chloramphenicol acetyltransferase (cat) are described.