Viral coat protein peptides with limited sequence homology bind similar domains of alfalfa mosaic virus and tobacco streak virus RNAs

The coat protein of the ilarvirus prunus necrotic ringspot virus mediates long-distance movement

The coat protein (CP) of plant viruses generally has multiple functions involving infection, replication, movement and pathogenicity. Functions of the CP of prunus necrotic ringspot virus (PNRSV), the causal agent of several threatening diseases of Prunus fruit trees, are poorly studied. Previously, we identified a novel virus in apple, apple necrotic mosaic virus (ApNMV), which is phylogenetically related to PNRSV and probably associated with apple mosaic disease in China. Full-length cDNA clones of PNRSV and ApNMV were constructed, and both are infectious in cucumber (Cucumis sativus L.), an experimental host. PNRSV exhibited higher systemic infection efficiency with more severe symptoms than ApNMV. Reassortment analysis of genomic RNA segments 1-3 found that RNA3 of PNRSV could enhance the long-distance movement of an ApNMV chimaera in cucumber, indicating the association of RNA3 of PNRSV with viral long-distance movement. Deletion mutagenesis of the PNRSV CP showed that the basic motif from amino acids 38 to 47 was crucial for the CP to maintain the systemic movement of PNRSV. Moreover, we found that arginine residues 41, 43 and 47 codetermine viral long-distance movement. The findings demonstrate that the CP of PNRSV is required for long-distance movement in cucumber, which expands the functions of ilarvirus CPs in systemic infection. For the first time, we identified involvement of Ilarvirus CP protein during long-distance movement.

The N-terminal region containing the zinc finger domain of tobacco streak virus coat protein is essential for the formation of virus-like particles

Tobacco streak virus (TSV), a member of the genus Ilarvirus (family Bromoviridae), has a tripartite genome and forms quasi-isometric virions. All three viral capsids, encapsidating RNA 1, RNA 2 or RNA 3 and subgenomic RNA 4, are constituted of a single species of coat protein (CP). Formation of virus-like particles (VLPs) could be observed when the TSV CP gene was cloned and the recombinant CP (rCP) was expressed in E. coli. TSV VLPs were found to be stabilized by Zn(2+) ions and could be disassembled in the presence of 500 mM CaCl2. Mutational analysis corroborated previous studies that showed that an N-terminal arginine-rich motif was crucial for RNA binding; however, the results presented here demonstrate that the presence of RNA is not a prerequisite for assembly of TSV VLPs. Instead, the N-terminal region containing the zinc finger domain preceding the arginine-rich motif is essential for assembly of these VLPs.

The molecular biology of ilarviruses

Ilarviruses were among the first 16 groups of plant viruses approved by ICTV. Like Alfalfa mosaic virus (AMV), bromoviruses, and cucumoviruses they are isometric viruses and possess a single-stranded, tripartite RNA genome. However, unlike these other three groups, ilarviruses were recognized as being recalcitrant subjects for research (their ready lability is reflected in the sigla used to create the group name) and were renowned as unpromising subjects for the production of antisera. However, it was recognized that they shared properties with AMV when the phenomenon of genome activation, in which the coat protein (CP) of the virus is required to be present to initiate infection, was demonstrated to cross group boundaries. The CP of AMV could activate the genome of an ilarvirus and vice versa. Development of the molecular information for ilarviruses lagged behind the knowledge available for the more extensively studied AMV, bromoviruses, and cucumoviruses. In the past 20 years, genomic data for most known ilarviruses have been developed facilitating their detection and allowing the factors involved in the molecular biology of the genus to be investigated. Much information has been obtained using Prunus necrotic ringspot virus and the more extensively studied AMV. A relationship between some ilarviruses and the cucumoviruses has been defined with the recognition that members of both genera encode a 2b protein involved in RNA silencing and long distance viral movement. Here, we present a review of the current knowledge of both the taxonomy and the molecular biology of this genus of agronomically and horticulturally important viruses.

Rubella virus-like replicon particles: analysis of encapsidation determinants and non-structural roles of capsid protein in early post-entry replication

Rubella virus (RUBV) contains a plus-strand RNA genome with two ORFs, one encoding the non-structural replicase proteins (NS-ORF) and the second encoding the virion structural proteins (SP-ORF). This study describes development and use of a trans-encapsidation system for the assembly of infectious RUBV-like replicon particles (VRPs) containing RUBV replicons (self replicating genomes with the SP-ORF replaced with a reporter gene). First, this system was used to map signals within the RUBV genome that mediate packaging of viral RNA. Mutations within a proposed packaging signal did not significantly affect relative packaging efficiency. The insertion of various fragments derived from the RUBV genome into Sindbis virus replicons revealed that there are several regions within the RUBV genome capable of enhancing encapsidation of heterologous replicon RNAs. Secondly, the trans-encapsidation system was used to analyse the effect of alterations within the capsid protein (CP) on release of VRPs and subsequent initiation of replication in newly infected cells. Deletion of the N-terminal eight amino acids of the CP reduced VRP titre significantly, which could be partially complemented by native CP provided in trans, indicating that this mutation affected an entry or post-entry event in the replication cycle. To test this hypothesis, the trans-encapsidation system was used to demonstrate the rescue of a lethal deletion within P150, one of the virus replicase proteins, by CP contained within the virus particle. This novel finding substantiated the functional role of CP in early post-entry replication.

Biochemical approaches for characterizing RNA-protein complexes in preparation for high resolution structure analysis

RNA-protein interactions control viral RNA replication, transcription, translation, and particle assembly. Progress toward understanding the functional significance of RNA-protein complexes in the viral life cycle is hindered by the lack of high resolution structural information. Challenges to acquiring structural data include RNA's inherent instability and conformational plasticity, coupled with the comparatively high cost of generating large quantities of RNA for biophysical experiments. The potential for successful structure determination is increased by conducting biochemical experiments that outline interacting domains and identify key residues. These approaches are aimed at defining and characterizing RNA and protein substrates that are suitable for high resolution structural analysis.

Arabidopsis thaliana is an asymptomatic host of Alfalfa mosaic virus

The susceptibility of Arabidopsis thaliana ecotypes to infection by Alfalfa mosaic virus (AMV) was evaluated. Thirty-nine ecotypes supported both local and systemic infection, 26 ecotypes supported only local infection, and three ecotypes could not be infected. No obvious symptoms characteristic of virus infection developed on the susceptible ecotypes under standard conditions of culture. Parameters of AMV infection were characterized in ecotype Col-0, which supported systemic infection and accumulated higher levels of AMV than the symptomatic host Nicotiana tabacum. The formation of infectious AMV particles in infected Col-0 was confirmed by infectivity assays on a hypersensitive host and by electron microscopy of purified virions. Replication and transcription of AMV was confirmed by de novo synthesis of AMV subgenomic RNA in Col-0 protoplasts transfected with AMV RNA or plasmids harboring AMV cDNAs.

Coat protein activation of alfalfa mosaic virus replication is concentration dependent

Alfalfa mosaic virus (AMV) and ilarvirus RNAs are infectious only in the presence of the viral coat protein; therefore, an understanding of coat protein's function is important for defining viral replication mechanisms. Based on in vitro replication experiments, the conformational switch model states that AMV coat protein blocks minus-strand RNA synthesis (R. C. Olsthoorn, S. Mertens, F. T. Brederode, and J. F. Bol, EMBO J. 18:4856-4864, 1999), while another report states that coat protein present in an inoculum is required to permit minus-strand synthesis (L. Neeleman and J. F. Bol, Virology 254:324-333, 1999). Here, we report on experiments that address these contrasting results with a goal of defining coat protein's function in the earliest stages of AMV replication. To detect coat-protein-activated AMV RNA replication, we designed and characterized a subgenomic luciferase reporter construct. We demonstrate that activation of viral RNA replication by coat protein is concentration dependent; that is, replication was strongly stimulated at low coat protein concentrations but decreased progressively at higher concentrations. Genomic RNA3 mutations preventing coat protein mRNA translation or disrupting coat protein's RNA binding domain diminished replication. The data indicate that RNA binding and an ongoing supply of coat protein are required to initiate replication on progeny genomic RNA transcripts. The data do not support the conformational switch model's claim that coat protein inhibits the initial stages of viral RNA replication. Replication activation may correlate with low local coat protein concentrations and low coat protein occupancy on the multiple binding sites present in the 3' untranslated regions of the viral RNAs.

Replication of alfamo- and ilarviruses: role of the coat protein

In the family Bromoviridae, a mixture of the three genomic RNAs of bromo-, cucumo-, and oleaviruses is infectious as such, whereas the RNAs of alfamo- and ilarviruses require binding of a few molecules of coat protein (CP) to the 3' end to initiate infection. Most studies on the early function of CP have been done on the alfamovirus Alfalfa mosaic virus (AMV). The 3' 112 nucleotides of AMV RNAs can adopt two different conformations. One conformer consists of a tRNA-like structure that, together with an upstream hairpin, is required for minus-strand promoter activity. The other conformer consists of four hairpins interspersed by AUGC-sequences and represents a strong binding site for CP. Binding of CP to this conformer enhances the translational efficiency of viral RNAs in vivo 40-fold and blocks viral minus-strand RNA synthesis in vitro. AMV CP is proposed to initiate infection by mimicking the function of the poly(A)-binding protein.

The coat protein of prunus necrotic ringspot virus specifically binds to and regulates the conformation of its genomic RNA

Binding of coat protein (CP) to the 3' nontranslated region (3'-NTR) of viral RNAs is a crucial requirement to establish the infection of Alfamo- and Ilarviruses. In vitro binding properties of the Prunus necrotic ringspot ilarvirus (PNRSV) CP to the 3'-NTR of its genomic RNA using purified E. coli- expressed CP and different synthetic peptides corresponding to a 26-residue sequence near the N-terminus were investigated by electrophoretic mobility shift assays. PNRSV CP bound to, at least, three different sites existing on the 3'-NTR. Moreover, the N-terminal region between amino acid residues 25 to 50 of the protein could function as an independent RNA-binding domain. Single exchange of some arginine residues by alanine eliminated the RNA-interaction capacity of the synthetic peptides, consistent with a crucial role for Arg residues common to many RNA-binding proteins possessing Arg-rich domains. Circular dichroism spectroscopy revealed that the RNA conformation is altered when amino-terminal CP peptides bind to the viral RNA. Finally, mutational analysis of the 3'-NTR suggested the presence of a pseudoknotted structure at this region on the PNRSV RNA that, when stabilized by the presence of Mg(2+), lost its capability to bind the coat protein. The existence of two mutually exclusive conformations for the 3'-NTR of PNRSV strongly suggests a similar regulatory mechanism at the 3'-NTR level in Alfamo- and Ilarvirus genera.

Translation Inhibition of Capped and Uncapped Viral RNAs Mediated by Ribosome-Inactivating Proteins

ABSTRACT Ribosome-inactivating proteins (RIPs) are N-glycosidases that remove specific purine residues from the sarcin/ricin (S/R) loop of the large rRNA and arrest protein synthesis at the translocation step. In addition to their enzymatic activity, RIPs have been reputed to be potent antiviral agents against many plant, animal, and human viruses. We recently showed that pokeweed antiviral protein (PAP), an RIP from pokeweed, inhibits translation in cell extracts by binding to the cap structure of eukaryotic mRNA and viral RNAs and depurinating these RNAs at multiple sites downstream of the cap structure. In this study, we examined the activity of three different RIPs against capped and uncapped viral RNAs. PAP, Mirabilis expansa RIP (ME1), and the Saponaria officinalis RIP (saporin) depurinated the capped Tobacco mosaic virus and Brome mosaic virus RNAs, but did not depurinate the uncapped luciferase RNA, indicating that other type I RIPs besides PAP can distinguish between capped and uncapped RNAs. We did not detect depurination of Alfalfa mosaic virus (AMV) RNAs at multiple sites by PAP or ME1. Because AMV RNAs are capped, these results indicate that recognition of the cap structure alone is not sufficient for depurination of the RNA at multiple sites throughout its sequence. Furthermore, PAP did not cause detectable depurination of uncapped RNAs from Tomato bushy stunt virus (TBSV), Satellite panicum mosaic virus (SPMV), and uncapped RNA containing poliovirus internal ribosome entry site (IRES). However, in vitro translation experiments showed that PAP inhibited translation of AMV, TBSV, SPMV RNAs, and poliovirus IRES dependent translation. These results demonstrate that PAP does not depurinate every capped RNA and that PAP can inhibit translation of uncapped viral RNAs in vitro without causing detectable depurination at multiple sites. Thus, the cap structure is not the only determinant for inhibition of translation by PAP.

Selection of RRE RNA binding peptides using a kanamycin antitermination assay

The arginine-rich domains of several RNA-binding proteins have been shown to bind their cognate RNAs with high affinities and specificities as isolated peptides, adopting different conformations within different complexes. The sequence simplicity and structural diversity of the arginine-rich motif has made it a good framework for constructing combinatorial libraries and identifying novel RNA-binding peptides, including those targeted to the HIV Rev response element (RRE). Here we describe a modified transcription antitermination reporter assay engineered with kanamycin resistance that enables larger in vivo screens (approximately 10(9) sequences) than previously possible. We show that the assay detects only specific RNA-protein complexes, and that binders are enriched at least 300-fold per round of selection. We screened a large peptide library in which amino acids with charged, polar, and small side chains were randomly distributed within a polyarginine framework and identified a set of high affinity RRE-binding peptides. Most contain glutamine at one particular peptide position, and the best peptides display significantly higher antitermination activities than Rev or other previously described high-affinity RRE-binding peptides. The kanamycin antitermination (KAN) assay should be useful for screening relatively large libraries and thereby facilitate identification of novel RNA binders.

The importance of alfalfa mosaic virus coat protein dimers in the initiation of replication

Deletion and substitution mutations affecting the oligomerization of alfalfa mosaic virus (AMV) coat protein (CP) were studied in protoplasts to determine their effect on genome activation, an early step in AMV replication. The CP mutants that formed dimers, CPDeltaC9 and CPC-A(R)F, were highly active in initiating replication with 63-84% of wild-type (wt) CP activity. However, all mutants that did not form dimers, CPDeltaC18, CPDeltaC19, CPC-WFP, and CPC-W, were much less active with 19-33% of wt CP activity. The accumulation and solubility of mutant CPs expressed from a virus-based vector in Nicotiana benthamiana were similar to that of wt CP. Analysis of CP-RNA interactions indicated that CP dimers and CP monomers interacted very differently with AMV RNA 3' ends. These results suggest that CP dimers are more efficient for replication than CP monomers because of differences in RNA binding rather than differences in expression and accumulation of the mutant CPs in infected cells.

Alphavirus minus-strand RNA synthesis: identification of a role for Arg183 of the nsP4 polymerase

A partially conserved region spanning amino acids 142 to 191 of the Sindbis virus (SIN) nsP4 core polymerase is implicated in host restriction, elongation, and promoter recognition. We extended the analysis of this region by substituting Ser, Ala, or Lys for a highly conserved Arg183 residue immediately preceding its absolutely conserved Ser184-Ala-Val-Pro-Ser188 sequence. In chicken cells, the nsP4 Arg183 mutants had a nonconditionally lethal, temperature-sensitive (ts) growth phenotype caused by a ts defect in minus-strand synthesis whose extent varied with the particular amino acid substituted (Ser>Ala>Lys). Plus-strand synthesis by nsP4 Arg183 mutant polymerases was unaffected when corrected for minus-strand numbers, although 26S mRNA synthesis was enhanced at the elevated temperature compared to wild type. The ts defect was not due to a failure to form or accumulate nsP4 at 40 degrees C. In contrast to their growth in chicken cells, the nsP4 Arg183 mutants replicated equally poorly, if at all, in mosquito cells. We conclude that Arg183 within the Pro180-Asn-Ile-Arg-Ser184 sequence of the SIN nsP4 polymerase contributes to the efficient initiation of minus strands or the formation of its replicase and that a host factor(s) participates in this event.

Genetic dissection of the multiple functions of alfalfa mosaic virus coat protein in viral RNA replication, encapsidation, and movement

Coat protein (CP) of alfalfa mosaic virus (AMV) binds as a dimer to the 3' termini of the three genomic RNAs and is required for initiation of infection, asymmetric plus-strand RNA accumulation, virion formation, and spread of the virus in plants. A mutational analysis of the multiple functions of AMV CP was made. Mutations that interfered with CP dimer formation in the two-hybrid system had little effect on the initiation of infection or plus-strand RNA accumulation but interfered with virion formation and reduced or abolished cell-to-cell movement of the virus in plants. Six of the 7 basic amino acids in the N-terminal arm of CP (positions 5, 6, 10, 13, 16, and 25) could be deleted or mutated into alanine without affecting any step of the replication cycle except systemic movement in plants. Mutation of Arg-17 interfered with initiation of infection (as previously shown by others) and cell-to-cell movement of the virus but not with plus-strand RNA accumulation or virion formation. The results indicate that in addition to the RNA-binding domain, different domains of AMV CP are involved in initiation of infection, plus-strand RNA accumulation, virion formation, cell-to-cell movement, and systemic spread of the virus.

The complete nucleotide sequence of apple mosaic virus (ApMV) RNA 1 and RNA 2: ApMV is more closely related to alfalfa mosaic virus than to other ilarviruses

The complete nucleotide sequences of apple mosaic virus RNA 1 and 2 have been characterized. Apple mosaic virus RNA 1 is 3476 nucleotides in length and encodes a single large open reading frame (ORF), whereas apple mosaic virus RNA 2 is 2979 nucleotides in length and also encodes a single ORF. The amino acid sequences encoded by RNA 1 and 2 show similarity to all of the other ilarviruses for which sequence data are available, but both are more closely related to alfalfa mosaic virus (AMV) than to other ilarviruses. Points of similarity include the absence of ORF 2b, present on the RNA 2 of all previously characterized ilarviruses. The close relationship to AMV also occurs in the movement protein, encoded by RNA 3, but not with the coat protein. These data suggest that the present taxonomy should be revised, and that AMV should be considered an aphid-transmissible ilarvirus.

RNA determinants of a specific RNA-coat protein peptide interaction in alfalfa mosaic virus: conservation of homologous features in ilarvirus RNAs

Alfalfa mosaic virus (AMV) coat protein and tobacco streak virus (TSV) coat protein bind specifically to the 3' untranslated regions of the viral RNAs and are required with the genomic RNAs to initiate virus replication. A combination of nucleotide substitutions, hydroxyl radical footprinting, and ethylation and chemical modification interference analysis has been used to define the RNA determinants important for the specific binding of the 3'-terminal 39 nucleotides of AMV RNA 3/4 (AMV843-881) to an amino-terminal coat protein peptide (CP26). The results demonstrate that potential phosphate and base-specific contacts as well as ribose moieties protected upon peptide binding cluster in lower hairpin stems and flanking AUGC sequences of the viral RNA, without direct involvement of loop nucleotides. Nucleotides identified in the modification-interference analyses as important for RNA-protein interactions are highly conserved among AMV and the ilarvirus RNAs. This RNA sequence homology, coupled with the recent identification of an RNA binding consensus sequence for AMV and ilarvirus coat proteins, provides a framework for understanding the functional equivalence of AMV and TSV coat proteins in binding RNA and activating virus replication and may explain why heterologous AMV and ilarvirus coat protein-RNA mixtures are infectious.