Expression of potyviral polyproteins in transgenic plants reveals three proteolytic activities required for complete processing

Simultaneous accumulation of multiple viral coat proteins from a TEV-NIa based expression vector

We previously described an expression cassette that relies on the tobacco etch virus (TEV) nuclear inclusion a (NIa) protease and leads to the coordinated accumulation of multiple proteins through self processing of a polyprotein [21]. However, low levels of proteins accumulated when the full-length protease was encoded within the polyprotein [22]. Studies were conducted to evaluate whether the disruption of NIa nuclear localization would affect the levels of proteins produced via the cassette. Modifications comprised either removal of its nuclear localization signals (NLSs), removal of the VPg domain (which includes the NLSs), and fusion to the 6 kDa protein, previously demonstrated to be a viral cytoplasmic anchor [28]. In in vitro translation reactions and in vivo protoplast experiments the modified NIa retained sequence-specific proteolysis. Moreover, the removal of the NLSs correlated with an increase in GUS reporter accumulation. The modified cassette, pPRO10, led to the synthesis of up to three viral coat protein (CPs) in addition to NIa. However, the accumulation of proteins in protoplasts depended upon the position of the CP coding sequence within the cassette as well as on the stability of the protein.

RNA binding activity of NIa proteinase of tobacco etch potyvirus

The C-terminal domain of NIa protein (NIaPro) from tobacco etch potyvirus (TEV) is a sequence-specific proteinase required for processing of the viral polyprotein. This proteinase also interacts with NIb, the TEV RNA-dependent RNA polymerase. NIaPro and two NIaPro-containing polyproteins (NIa and 6/NIa) were analyzed from extracts of recombinant Escherichia coli. Using RNA-protein blot and UV-crosslinking assays, NIaPro and the NIaPro-containing polyproteins were shown to possess RNA-binding activity. NIaPro bound nonspecifically to several RNAs, including plus- and minus-strands of the TEV 5' and 3' noncoding regions. Saturation binding data obtained using the UV-crosslinking assay were consistent with a possible cooperative RNA-binding activity of NIaPro. In addition, the RNA-binding activities of NIaPro and full-length NIa protein were similar. Based on its RNA-binding activity and other known functions, NIaPro or a NIaPro-containing polyprotein is proposed to serve one or more direct roles during TEV RNA synthesis.

Regulation of photomorphogenesis by expression of mammalian biliverdin reductase in transgenic Arabidopsis plants

The photoregulatory activity of the phytochrome photoreceptor requires the synthesis and covalent attachment of the linear tetrapyrrole prosthetic group phytochromobilin. Because the mammalian enzyme biliverdin IX alpha reductase (BVR) is able to functionally inactivate phytochromobilin in vitro, this investigation was undertaken to determine whether BVR expression in transgenic plants would prevent the synthesis of functionally active phytochrome in vivo. Here, we show that plastid-targeted, constitutive expression of BVR in Arabidopsis yields plants that display aberrant photomorphogenesis throughout their life cycle. Photobiological and biochemical analyses of three transgenic BVR lines exhibiting a 25-fold range of BVR expression established that the BVR-dependent phenotypes are light dependent, pleiotropic, and consonant with the loss of multiple phytochrome activities. Chlorophyll accumulation in BVR-expressing transgenic plants was particularly sensitive to increased light fluence rates, which is consistent with an important role for phytochrome in light tolerance. Under blue light, transgenic BVR plants displayed elongated hypocotyls but retained phototropic behavior and the ability to fully deetiolate. Directed BVR expression may prove to be useful for probing the cellular and developmental basis of phytochrome-mediated responses and for selective control of individual aspects of light-mediated plant growth and development.

Regulation of photomorphogenesis by expression of mammalian biliverdin reductase in transgenic Arabidopsis plants

The photoregulatory activity of the phytochrome photoreceptor requires the synthesis and covalent attachment of the linear tetrapyrrole prosthetic group phytochromobilin. Because the mammalian enzyme biliverdin IX alpha reductase (BVR) is able to functionally inactivate phytochromobilin in vitro, this investigation was undertaken to determine whether BVR expression in transgenic plants would prevent the synthesis of functionally active phytochrome in vivo. Here, we show that plastid-targeted, constitutive expression of BVR in Arabidopsis yields plants that display aberrant photomorphogenesis throughout their life cycle. Photobiological and biochemical analyses of three transgenic BVR lines exhibiting a 25-fold range of BVR expression established that the BVR-dependent phenotypes are light dependent, pleiotropic, and consonant with the loss of multiple phytochrome activities. Chlorophyll accumulation in BVR-expressing transgenic plants was particularly sensitive to increased light fluence rates, which is consistent with an important role for phytochrome in light tolerance. Under blue light, transgenic BVR plants displayed elongated hypocotyls but retained phototropic behavior and the ability to fully deetiolate. Directed BVR expression may prove to be useful for probing the cellular and developmental basis of phytochrome-mediated responses and for selective control of individual aspects of light-mediated plant growth and development.

Mutations in the region encoding the central domain of helper component-proteinase (HC-Pro) eliminate potato virus X/potyviral synergism

Coinfection of tobacco plants with potato virus X (PVX) and any of several members of the potyvirus group causes a synergistic disease characterized by a dramatic increase in symptom severity correlated with a 3- to 10-fold increase in the accumulation of PVX in the first systemically infected leaves. We have recently shown that PVX/potyviral synergistic disease is mediated by expression of potyviral 5'-proximal sequences encoding P1, helper component-proteinase (HC-Pro), and a fraction of P3 (termed P1/HC-Pro sequence). Here we report the effect of mutations in this potyviral sequence on the induction of synergistic disease. Three transgenic tobacco lines expressing the tobacco etch potyvirus (TEV) P1/HC-Pro sequence with mutations within the P1 coding region were not impaired in their ability to mediate synergism when infected with PVX. In contrast, two of three transgenic lines with mutations in the HC-Pro coding region were unable to induce the synergistic increases in either symptom severity or PVX accumulation. Loss of synergistic function was associated with mutations within the region encoding the central domain of HC-Pro, while the ability to induce synergism was retained in a transgenic line expressing HC-Pro with an alteration in the amino-terminal "zinc-finger domain." In coinoculation experiments, a TEV mutant lacking the sequence encoding the zinc-linger domain of HC-Pro induced a typical synergistic response in interaction with PVX. The results indicate that the zinc-finger domain comprising the first 66 amino acid residues of HC-Pro is dispensable for induction of synergistic disease and transactivation of PVX multiplication, while regions within the central domain of HC-Pro are essential for both of these responses.

Sequence of the 3'-terminal region of a Zimbabwe isolate of cowpea aphid-borne mosaic virus (CABMV)

The 3'-terminal 1221 nucleotides of a Zimbabwe isolate of cowpea aphid-borne mosaic potyvirus (CABMV) genome have been sequenced. The sequence comprises an open reading frame (ORF) of 990 nucleotides and a 3' non-coding-region of 231 nucleotides followed by a poly-A. The ORF has high similarity to NIb and coat proteins (CP) of potyviruses. A potential CP Q/S cleavage site was identified, yielding a CP of 30.5 kDa containing 275 amino acids. The CABMV sequence is closely related to that of South African passiflora virus (SAPV) which should therefore be regarded as a strain of CABMV.

Construction and analysis of infectious transcripts from a resistance-breaking strain of tobacco vein mottling potyvirus

The Burley tobacco (Nicotiana tabacum) cultivar TN 86 is "resistant" to most strains of tobacco vein mottling potyvirus (TVMV), the virus being restricted to epidermal cells of inoculated leaves. One strain, designated TVMV-S, overcomes this resistance and infects cv TN 86 systemically. To begin our investigation of the molecular basis for the resistance-breaking phenomenon, we have completed the cloning and sequencing of the TVMV-S RNA genome. The complete cDNA clone, under the control of a T7 RNA polymerase promoter, was used to produce infectious transcripts which were tested for their ability to reproduce the characteristics of TVMV-S RNA on three types of tobacco (N. tabacum cv TN 86, N. tabacum cv KY 14, and N. benthamiana). Timing of symptom appearance, symptom type, and titer of virus were identical to those of plants inoculated with TVMV-S RNA. As a step toward mapping the responsible genetic region(s) that contribute(s) to resistance-breaking by TVMV-S, the nucleotide and deduced amino acid sequences were compared to those of wild-type TVMV, a strain that does not overcome cv TN 86 resistance. Variant TVMV-S transcripts containing changes within the VPg cistron exhibited an altered pattern of infectivity on cv TN 86.

Evidence that the potyvirus P1 proteinase functions in trans as an accessory factor for genome amplification

The tobacco etch potyvirus (TEV) polyprotein is proteolytically processed by three viral proteinases (NIa, HC-Pro, and P1). While the NIa and HC-Pro proteinases each provide multiple functions essential for viral infectivity, the role of the P1 proteinase beyond its autoproteolytic activity is understood poorly. To determine if P1 is necessary for genome amplification and/or virus movement from cell to cell, a mutant lacking the entire P1 coding region (delta P1 mutant) was produced with a modified TEV strain (TEV-GUS) expressing beta-glucuronidase (GUS) as a reporter, and its replication and movement phenotypes were assayed in tobacco protoplasts and plants. The delta P1 mutant accumulated in protoplasts to approximately 2 to 3% the level of parental TEV-GUS, indicating that the P1 protein may contribute to but is not strictly required for viral RNA amplification. The delta P1 mutant was capable of cell-to-cell and systemic (leaf-to-leaf) movement in plants but at reduced rates compared with parental virus. This is in contrast to the S256A mutant, which encodes a processing-defective P1 proteinase and which was nonviable in plants. Both delta P1 and S256A mutants were complemented by P1 proteinase expressed in a transgenic host. In transgenic protoplasts, genome amplification of the delta P1 mutant relative to parental virus was stimulated five- to sixfold. In transgenic plants, the level of accumulation of the delta P1 mutant was stimulated, although the rate of cell-to-cell movement was the same as in nontransgenic plants. Also, the S256A mutant was capable of replication and systemic infection in P1-expressing transgenic plants. These data suggest that, in addition to providing essential processing activity, the P1 proteinase functions in trans to stimulate genome amplification.

Long-distance movement factor: a transport function of the potyvirus helper component proteinase

Transport of viruses from cell to cell in plants typically involves one or more viral proteins that supply dedicated movement functions. Transport from leaf to leaf through phloem, or long-distance transport, is a poorly understood process with requirements differing from those of cell-to-cell movement. Through genetic analysis of tobacco etch virus (TEV; potyvirus group), a novel long-distance movement factor was identified that facilitates vascular-associated movement in tobacco. A mutation in the central region of the helper component proteinase (HC-Pro), a TEV-encoded protein with previously described activities in aphid-mediated transmission and polyprotein processing, inactivated long-distance movement. This mutant virus exhibited only minor defects in genome amplification and cell-to-cell movement functions. In situ histochemical analysis revealed that the mutant was capable of infecting mesophyll, bundle sheath, and phloem cells within inoculated leaves, suggesting that the long-distance movement block was associated with entry into or exit from sieve elements. The long-distance movement defect was specifically complemented by HC-Pro supplied in trans by a transgenic host. The data indicate that HC-Pro functions in one or more steps unique to long-distance transport.

5' proximal potyviral sequences mediate potato virus X/potyviral synergistic disease in transgenic tobacco

The interaction of potato virus X (PVX) and potato virus Y (PVY) in tobacco causes a synergistic disease characterized by a dramatic increase in symptom severity, a change in the regulation of PVX RNA replication, and an increase in accumulation of PVX. In this study we demonstrate that PVX also interacts synergistically with three other members of the potyvirus group of plant viruses, tobacco vein mottling virus (TVMV), tobacco etch virus (TEV), and pepper mottle virus. These synergisms resemble the classic PVX/PVY synergism with respect to both the increase in host response and the change in PVX replication. To determine if the induction of PVX/potyviral synergism requires potyviral genome replication per se or if the response is mediated by expression of one or more potyviral genes, we used tobacco plants stably transformed with various subsets of the TVMV genome. PVX infections of transgenic plants expressing the 5'-proximal region of the TVMV genome, including the protease-1, helper component protease, and protein-3 genes, result in symptoms resembling those of PVX/potyviral synergism. A similar synergistic-like response occurs when transgenic tobacco plants expressing the analogous but smaller region from the 5'-proximal region of the TEV genome were infected with PVX. Replication of PVX RNA is altered in transgenic plants expressing 5'-proximal sequences of either TVMV or TEV, and in a manner similar to that observed in double infections. These results indicate that replication of the potyviral genome is not required for PVX/potyviral synergism and that the response is mediated by expression of potyviral sequences which have been localized to the 5'-proximal third of the genomic RNAs of both TVMV and TEV.

A mutation in Arabidopsis that leads to constitutive expression of systemic acquired resistance

Systemic acquired resistance (SAR) is a nonspecific defense response in plants that is associated with an increase in the endogenous level of salicylic acid (SA) and elevated expression of pathogenesis-related (PR) genes. To identify mutants involved in the regulation of PR genes and the onset of SAR, we transformed Arabidopsis with a reporter gene containing the promoter of a beta-1,3-glucanase-encoding PR gene (BGL2) and the coding region of beta-glucuronidase (GUS). The resulting transgenic line (BGL2-GUS) was mutagenized, and the M2 progeny were scored for constitutive GUS activity. We report the characterization of one mutant, cpr1 (constitutive expressor of PR genes), that was identified in this screen and shown by RNA gel blot analysis also to have elevated expression of the endogenous PR genes BGL2, PR-1, and PR-5. Genetic analyses indicated that the phenotype conferred by cpr1 is caused by a single, recessive nuclear mutation and is suppressed in plants producing a bacterial salicylate hydroxylase, which inactivates SA. Furthermore, biochemical analysis showed that the endogenous level of SA is elevated in the mutant. Finally, the cpr1 plants were found to be resistant to the fungal pathogen Peronospora parasitica NOCO2 and the bacterial pathogen Pseudomonas syringae pv maculicola ES4326, which are virulent in wild-type BGL2-GUS plants. Because the cpr1 mutation is recessive and associated with an elevated endogenous level of SA, we propose that the CPR1 gene product acts upstream of SA as a negative regulator of SAR.

A mutation in Arabidopsis that leads to constitutive expression of systemic acquired resistance

Systemic acquired resistance (SAR) is a nonspecific defense response in plants that is associated with an increase in the endogenous level of salicylic acid (SA) and elevated expression of pathogenesis-related (PR) genes. To identify mutants involved in the regulation of PR genes and the onset of SAR, we transformed Arabidopsis with a reporter gene containing the promoter of a beta-1,3-glucanase-encoding PR gene (BGL2) and the coding region of beta-glucuronidase (GUS). The resulting transgenic line (BGL2-GUS) was mutagenized, and the M2 progeny were scored for constitutive GUS activity. We report the characterization of one mutant, cpr1 (constitutive expressor of PR genes), that was identified in this screen and shown by RNA gel blot analysis also to have elevated expression of the endogenous PR genes BGL2, PR-1, and PR-5. Genetic analyses indicated that the phenotype conferred by cpr1 is caused by a single, recessive nuclear mutation and is suppressed in plants producing a bacterial salicylate hydroxylase, which inactivates SA. Furthermore, biochemical analysis showed that the endogenous level of SA is elevated in the mutant. Finally, the cpr1 plants were found to be resistant to the fungal pathogen Peronospora parasitica NOCO2 and the bacterial pathogen Pseudomonas syringae pv maculicola ES4326, which are virulent in wild-type BGL2-GUS plants. Because the cpr1 mutation is recessive and associated with an elevated endogenous level of SA, we propose that the CPR1 gene product acts upstream of SA as a negative regulator of SAR.

In vitro characterization of a cassette to accumulate multiple proteins through synthesis of a self-processing polypeptide

The strategy for processing the polyprotein encoded by plant potyviruses has been mimicked by constructing an expression cassette based on the nuclear inclusion (Nla) proteinase from tobacco etch virus (TEV). This cassette (pPR01), includes the TEV Nla coding region flanked on each side by its heptapeptide cleavage sequence and cloning sites for the in frame insertion of two different open reading frames. pPR01 allows the synthesis, under the control of a single transcriptional promoter, of two proteins in equimolar amounts as part of a polyprotein which is cleaved into individual mature products by the TEV protease. In in vitro reactions the cassette functioned as expected when several different protein-coding sequences were used. The potential uses of pPR01 are discussed.

Detection of a 45 kD protein derived from the N terminus of the pea seedborne mosaic potyvirus polyprotein in vivo and in vitro

A 45 kD protein (Pro1) derived from the N terminus of the pea seedborne mosaic potyvirus (PSbMV) polyprotein has been detected in extracts of infected pea plants and among in vitro translation products of PSbMV genomic RNA. The genomic region coding for the first 231 amino acids of the PSbMV polyprotein was cloned and expressed in Escherichia coli as a fusion protein with beta-galactosidase. A rabbit antiserum raised against the fusion protein recognized an approximately 45 kD protein in immunoblots of extracts of PSbMV-infected pea leaves that was not present in extracts of healthy leaves. The highest concentration of the 45 kD protein was found in extracts of young leaves, suggesting the protein may be rapidly degraded in vivo. After in vitro translation of PSbMV genomic RNA in a wheat germ extract, the antiserum immunoprecipitated a 45 kD polypeptide as well as some lower molecular weight translation products. On the other hand, an approximately 90 kD polypeptide was immunoprecipitated from in vitro translation products of genomic RNA in a rabbit reticulocyte lysate, corresponding to the combined molecular weights of Pro1 and the helper component predicted from genomic sequence data.

Expression of a cysteine proteinase inhibitor (oryzacystatin-I) in transgenic tobacco plants

Expression of cysteine proteinase inhibitors (cystatins) in tobacco or other plants has the potential for improving resistance against pathogens and insects that possess cysteine proteinases. A chimeric gene containing a cDNA clone of rice cystatin (oryzacystatin-I; OC-I), the cauliflower mosaic virus 35S promoter, and the nopaline synthase 3' region was introduced into tobacco plants by Agrobacterium tumefaciens. The presence of the chimeric gene in transgenic plants was detected by a polymerase chain reaction-amplified assay, and transcriptional activity was shown by RNA blot analysis. Heated extracts from transgenic tobacco plants, as well as from progeny which were obtained by selfing a primary transformant, contained protein bands that corresponded in molecular mass to OC-I and reacted with antibodies raised against rOC, a recombinant OC-I protein produced by Escherichia coli. Similar bands were absent in extracts from untransformed control plants. OC-I levels reached 0.5% and 0.6% of the total soluble proteins in leaves and roots, respectively, of some progeny. On a fresh weight basis, the OC-I content was higher in leaves (50 micrograms/g) than in roots (30 micrograms/g). OC-I was partially purified from protein extracts of rice seeds and from transgenic tobacco leaves by affinity to anti-rOC antibodies. OC-I from both sources was active against papain.

Resistance to cymbidium ringspot tombusvirus infection in transgenic Nicotiana benthamiana plants expressing the virus coat protein gene

Transgenic Nicotiana benthamiana plants expressing the coat protein gene of cymbidium ringspot virus (CyRSV) were tested for resistance against infection with CyRSV. Transgenic plants showed resistance to infection only when the purified virions concentration in the inoculum was as low as 0.05 micrograms/ml. No protection was observed in transgenic plants inoculated with virion concentrations of 0.5 and 5.0 micrograms/ml or when the inoculum was in vitro synthesized genomic RNA.

Immunodetection of the plum pox virus helper component in infected plants and expression of its gene in transgenic plants

Tobacco plants (Nicotiana tabacum cv. Xanthi) have been transformed via Agrobacterium tumefaciens vectors, with cDNAs corresponding to the plum pox virus (PPV) cistron 2 encoding helper component (HC-Pro) and with the first two and half cistrons of the PPV genome. Presence of the HC-Pro in PPV-infected plants and transgenic plants transformed with the gene coding for this protein was investigated using specific polyclonal antibodies produced against the PPV HC-Pro. The results suggest that two proteases are involved in the processing of the PPV N-terminal polyprotein to yield a protein of 48 k (HC-Pro). HC-Pro autolytically cleaves at its carboxyl-terminus and a proteolytic activity, probably associated with the protein (P1) encoded by the cistron 1, is required for the cleavage in planta between the proteins derived from cistrons 1 and 2.

Mutational analysis of the tobacco etch potyviral 35-kDa proteinase: identification of essential residues and requirements for autoproteolysis

The tobacco etch potyvirus (TEV) polyprotein is processed by three virus-encoded proteinases, termed Nla, HC-Pro, and the 35-kDa proteinase. The 35-kDa proteinase is derived from the amino-terminal region of the polyprotein. Analysis of polyproteins containing beta-glucuronidase fused to the expected carboxy terminus of the 35-kDa proteinase confirmed the previously identified Tyr304-Ser305 dipeptide as the cleavage site between the 35-kDa proteinase and HC-Pro. The 35-kDa proteinase of TEV was unable to catalyze proteolysis when synthetic substrate polyproteins were supplied in a bimolecular or trans reaction, suggesting that processing occurs by an autolytic mechanism. The results of a mutational analysis within the 35-kDa proteolytic domain indicated that His214, Asp223, Ser256, and Asp288 were required for optimal autoproteolytic activity. Replacement of Ser256 with either Thr or Cys resulted in low but detectable proteinase activity, as did substitution of Asp223 and Asp288 with Glu. These results are consistent with the hypothesis that the 35-kDa proteinase resembles cellular serine-type proteinases, with Ser256 functioning as the nucleophilic residue within the active site. Cleavage mediated by the 35-kDa proteinase has been shown previously to occur after polyprotein synthesis in wheat germ extracts and transgenic plants, but not in rabbit reticulocyte lysate. We were able to demonstrate that processing in vitro may require a heat-labile factor present in wheat germ extracts.

Characterization of the potyviral HC-pro autoproteolytic cleavage site

The helper component-proteinase (HC-Pro) encoded by potyviruses functions to cleave the viral polyprotein by an autoproteolytic mechanism at the HC-Pro C-terminus. This protein belongs to a group of viral cysteine-type proteinases and has been shown previously to catalyze proteolysis between a Gly-Gly dipeptide. The amino acid sequence requirements surrounding the HC-Pro C-terminal cleavage site of the tobacco etch virus polyprotein have been investigated using site-directed mutagenesis and in vitro expression systems. A total of 51 polyprotein derivatives, each differing by the substitution of a single amino acid between the P5 and P2' positions, were tested for autoproteolytic activity. Substitutions of Tyr (P4), Val (P2), Gly (P1), and Gly (P1') were found to eliminate or nearly eliminate proteolysis. Substitutions of Thr (P5), Asn (P3), and Met (P2'), on the other hand, were permissive for proteolysis, although the apparent processing rates of some polyproteins containing these alterations were reduced. These results suggest that auto-recognition by HC-Pro involves the interaction of the enzymatic binding site with four amino acids surrounding the cleavage site. Comparison of the homologous sequences of five potyviral polyproteins revealed that the residues essential for processing are strictly conserved, whereas the nonessential residues are divergent. The relationship between HC-Pro and other viral and cellular cysteine-type proteinases is discussed.

Serological relationships involving potyviral nonstructural proteins

This report represents a compilation of many of the publications on antigenic properties of potyviral-specified nonstructural proteins. Polyclonal antisera have been prepared for use in characterization of six nonstructural proteins. These include antisera to the cylindrical inclusion proteins of at least 28 potyviruses, to small nuclear inclusion protein (protease) of four potyviruses, to large nuclear inclusion protein (putative replicase) of three viruses, helper component-protease or amorphous inclusion protein of at least four viruses, to the P1 protein (located at the N-terminus of the polyprotein) of one virus, and to the P3 protein (located between helper component protease and cylindrical inclusion protein) of one virus. Monoclonal antibodies also have been prepared to several of these nonstructural proteins. The evidence thus far indicates that cylindrical inclusions of different potyviruses have both conserved and unique epitopes. Nuclear inclusion proteins and amorphous inclusion proteins also may have conserved and unique epitopes. Antigenic relationships of potyviral nonstructural proteins have potential for the identification and classification of potyviruses.

In vitro cleavage at or near the N-terminus of the helper component protein in the tobacco vein mottling virus polyprotein

Translation of tobacco vein mottling virus (TVMV) RNA in a wheat germ system resulted in two products that were not observed in a rabbit reticulocyte system. One of these was the N-terminal protein, based on its being the most abundant product and its migration on SDS-PAGE at about 34 kDa. The second product was similar or identical to helper component (HC) isolated from TVMV-infected plants, based on co-migration with HC on SDS-PAGE and immunoprecipitation with anti-HC antibodies. The N-terminus of this product was determined by radiochemical Edman degradation to be Ser-257 of the polyprotein. This assignment was supported by peptide mapping with a tryptophan-specific reagent. A similar cleavage was observed when tobacco etch virus was translated in a wheat germ system. Comparison with homologous regions in five other potyviruses indicated conservation of amino acid residues on both sides of the proposed cleavage site. Conversion of Phe-256 to Met, Pro, Arg, His, or Trp by site-directed mutagenesis of a TVMV RNA transcription template inhibited cleavage in the wheat germ system. These results suggest that in vitro cleavage occurs between Phe-256 and Ser-257 and that this cleavage is the same as the in vivo cleavage which liberates the N-terminus of HC.

The 35-kDa protein from the N-terminus of the potyviral polyprotein functions as a third virus-encoded proteinase

The polyprotein encoded by plant potyviruses is proteolytically processed to at least eight mature products by viral-encoded proteinases. While the proteinases that catalyze processing at most of the cleavage sites have been identified, the enzyme responsible for cleavage between the 35-kDa protein and helper component-proteinase (HC-Pro), near the N-terminus of the viral polyprotein, has not been mapped or characterized previously. Polyproteins containing the 35-kDa protein and HC-Pro were synthesized in the wheat germ system using defined RNA transcripts and were demonstrated to undergo proteolysis to generate products that resemble fully processed proteins. The C-terminal half of the 35-kDa protein was found to be required for proteolysis, whereas most of the HC-Pro sequence was dispensable. Amino acid substitutions affecting three positions, each of which are conserved in the 35-kDa protein encoded by five potyviruses, were shown to inhibit protein processing. These data suggest that the 35-kDa protein functions as a proteinase to cleave at its C-terminus. A model that accounts for all proteolytic processing events in the potyviral polyprotein is presented.