Molecular studies on bromovirus capsid protein

Virus Assembly Pathways: Straying Away but Not Too Far

Non-enveloped RNA viruses pervade all domains of life. In a cell, they co-assemble from viral RNA and capsid proteins. Virus-like particles can form in vitro where virtually any non-cognate polyanionic cargo can be packaged. How only viral RNA gets selected for packaging in vivo, in presence of myriad other polyanionic species, has been a puzzle. Through a combination of charge detection mass spectrometry and cryo-electron microscopy, it is determined that co-assembling brome mosaic virus (BMV) coat proteins and nucleic acid oligomers results in capsid structures and stoichiometries that differ from the icosahedral virion. These previously unknown shell structures are strained and less stable than the native one. However, they contain large native structure fragments that can be recycled to form BMV virions, should a viral genome become available. The existence of such structures suggest the possibility of a previously unknown regulatory pathway for the packaging process inside cells.

Genome organization and interaction with capsid protein in a multipartite RNA virus

We report the asymmetric reconstruction of the single-stranded RNA (ssRNA) content in one of the three otherwise identical virions of a multipartite RNA virus, brome mosaic virus (BMV). We exploit a sample consisting exclusively of particles with the same RNA content-specifically, RNAs 3 and 4-assembled in planta by agrobacterium-mediated transient expression. We find that the interior of the particle is nearly empty, with most of the RNA genome situated at the capsid shell. However, this density is disordered in the sense that the RNA is not associated with any particular structure but rather, with an ensemble of secondary/tertiary structures that interact with the capsid protein. Our results illustrate a fundamental difference between the ssRNA organization in the multipartite BMV viral capsid and the monopartite bacteriophages MS2 and Qβ for which a dominant RNA conformation is found inside the assembled viral capsids, with RNA density conserved even at the center of the particle. This can be understood in the context of the differing demands on their respective lifecycles: BMV must package separately each of several different RNA molecules and has been shown to replicate and package them in isolated, membrane-bound, cytoplasmic complexes, whereas the bacteriophages exploit sequence-specific "packaging signals" throughout the viral RNA to package their monopartite genomes.

Disassembly Intermediates of the Brome Mosaic Virus Identified by Charge Detection Mass Spectrometry

Capsid disassembly and genome release are critical steps in the lifecycle of a virus. However, their mechanisms are poorly understood, both in vivo and in vitro. Here, we have identified two in vitro disassembly pathways of the brome mosaic virus (BMV) by charge detection mass spectrometry and transmission electron microscopy. When subjected to a pH jump to a basic environment at low ionic strength, protein-RNA interactions are disrupted. Under these conditions, BMV appears to disassemble mainly through a global cleavage event into two main fragments: a near complete capsid that has released the RNA and the released RNA complexed to a small number of the capsid proteins. Upon slow buffer exchange to remove divalent cations at neutral pH, capsid protein interactions are disrupted. The BMV virions swell but there is no measurable loss of the RNA. Some of the virions break into small fragments, leading to an increase in the abundance of species with masses less than 1 MDa. The peak attributed to the BMV virion shifts to a higher mass with time. The mass increase is attributed to additional capsid proteins associating with the disrupted capsid protein-RNA complex, where the RNA is presumably partially exposed. It is likely that this pathway is more closely related to how the capsid disassembles in vivo, as it offers the advantage of protecting the RNA with the capsid protein until translation begins.

Regulation of Positive-Strand Accumulation by Capsid Protein During Brome mosaic virus Infection In Planta

A hallmark feature of (+)-strand RNA viruses of eukaryotic cells is that progeny (+)-strands are accumulated 100-fold over (-)-strands. Previous experimental evidence suggests that, in Brome mosaic virus (BMV), a plant-infecting member of the alphavirus-like superfamily, the addition of RNA3 and, specifically, translation of the wild-type (WT) coat protein (CP) gene contributes to increased accumulation of (+)-strands. It is unclear whether this stimulation of (+)-strand accumulation by CP is due to direct regulation of viral RNA replication or RNA stabilization via encapsidation. Analysis of BMV progeny RNA in Nicotiana benthamiana plants revealed that expression of RNA3 variants that did not express WT CP led to a severe defect in BMV (+)-strand accumulation. The (+)-strand accumulation could be rescued when CP was complemented in trans. To verify whether stimulation of (+)-strand accumulation is coupled with encapsidation, two independent mutations were engineered into CP open reading frames. An N-terminal deletion that prevented CP binding to the viral RNAs resulted in a severe reduction of BMV (+)-strand accumulation but stimulated (-)-strand accumulation over the WT. On the other hand, a C-terminal mutation affecting CP dimerization caused a significant decrease in (+)-strand accumulation but had no detectable effect on (-)-strand accumulation. Nucleotide sequences in the movement protein-coding region were also found to contribute to (+)-strand accumulation, in part by providing packaging signals for efficient RNA3 encapsidation. Overall, these results show that RNA encapsidation is a significant determinant of BMV RNA intracellular accumulation.

Characterisation of Structural Proteins from Chronic Bee Paralysis Virus (CBPV) Using Mass Spectrometry

Chronic bee paralysis virus (CBPV) is the etiological agent of chronic paralysis, an infectious and contagious disease in adult honeybees. CBPV is a positive single-stranded RNA virus which contains two major viral RNA fragments. RNA 1 (3674 nt) and RNA 2 (2305 nt) encode three and four putative open reading frames (ORFs), respectively. RNA 1 is thought to encode the viral RNA-dependent RNA polymerase (RdRp) since the amino acid sequence derived from ORF 3 shares similarities with the RdRP of families Nodaviridae and Tombusviridae. The genomic organization of CBPV and in silico analyses have suggested that RNA 1 encodes non-structural proteins, while RNA 2 encodes structural proteins, which are probably encoded by ORFs 2 and 3. In this study, purified CBPV particles were used to characterize virion proteins by mass spectrometry. Several polypeptides corresponding to proteins encoded by ORF 2 and 3 on RNA 2 were detected. Their role in the formation of the viral capsid is discussed.

Viral RNAs are unusually compact

A majority of viruses are composed of long single-stranded genomic RNA molecules encapsulated by protein shells with diameters of just a few tens of nanometers. We examine the extent to which these viral RNAs have evolved to be physically compact molecules to facilitate encapsulation. Measurements of equal-length viral, non-viral, coding and non-coding RNAs show viral RNAs to have among the smallest sizes in solution, i.e., the highest gel-electrophoretic mobilities and the smallest hydrodynamic radii. Using graph-theoretical analyses we demonstrate that their sizes correlate with the compactness of branching patterns in predicted secondary structure ensembles. The density of branching is determined by the number and relative positions of 3-helix junctions, and is highly sensitive to the presence of rare higher-order junctions with 4 or more helices. Compact branching arises from a preponderance of base pairing between nucleotides close to each other in the primary sequence. The density of branching represents a degree of freedom optimized by viral RNA genomes in response to the evolutionary pressure to be packaged reliably. Several families of viruses are analyzed to delineate the effects of capsid geometry, size and charge stabilization on the selective pressure for RNA compactness. Compact branching has important implications for RNA folding and viral assembly.

N-terminal basic amino acid residues of Beet black scorch virus capsid protein play a critical role in virion assembly and systemic movement

BACKGROUND

Beet black scorch virus (BBSV) is a small single-stranded, positive-sense RNA plant virus belonging to the genus Necrovirus, family Tombusviridae. Its capsid protein (CP) contains a 13 amino acid long basic region at the N-terminus, rich in arginine and lysine residues, which is thought to interact with viral RNA to initiate virion assembly.

RESULTS

In the current study, a series of BBSV mutants containing amino acid substitutions as well as deletions within the N-terminal region were generated and examined for their effects on viral RNA replication, virion assembly, and long distance spread in protoplasts and whole host plants of BBSV. The RNA-binding activities of the mutated CPs were also evaluated in vitro. These experiments allowed us to identify two key basic amino acid residues in this region that are responsible for initiating virus assembly through RNA-binding. Proper assembly of BBSV particles is in turn needed for efficient viral systemic movement.

CONCLUSIONS

We have identified two basic amino acid residues near the N-terminus of the BBSV CP that bind viral RNA with high affinity to initiate virion assembly. We further provide evidence showing that systemic spread of BBSV in infected plants requires intact virions. This study represents the first in-depth investigation of the role of basic amino acid residues within the N-terminus of a necroviral CP.

Subcellular localization and rearrangement of endoplasmic reticulum by Brome mosaic virus capsid protein

Genome packaging in the plant-infecting Brome mosaic virus (BMV), a member of the alphavirus-like superfamily, as well as in other positive-strand RNA viruses pathogenic to humans (e.g., poliovirus) and animals (e.g., Flock House virus), is functionally coupled to replication. Although the subcellular localization site of BMV replication has been identified, that of the capsid protein (CP) has remained elusive. In this study, the application of immunofluorescence confocal microscopy to Nicotiana benthamiana leaves expressing replication-derived BMV CP as a green fluorescent protein (GFP) fusion, in conjunction with antibodies to the CP and double-stranded RNA, a presumed marker of RNA replication, revealed that the subcellular localization sites of replication and CP overlap. Our temporal analysis by transmission electron microscopy of ultrastructural modifications induced in BMV-infected N. benthamiana leaves revealed a reticulovesicular network of modified endoplasmic reticulum (ER) incorporating large assemblies of vesicles derived from ER accumulated in the cytoplasm during BMV infection. Additionally, for the first time, we have found by ectopic expression experiments that BMV CP itself has the intrinsic property of modifying ER to induce vesicles similar to those present in BMV infections. The significance of CP-induced vesicles in relation to CP-organized viral functions that are linked to replication-coupled packaging is discussed.

Effects of amino-acid substitutions in the Brome mosaic virus capsid protein on RNA encapsidation

Brome mosaic virus (BMV) packages its genomic RNAs (RNA1, RNA2, and RNA3) and subgenomic RNA4 into three different particles. However, since the RNAs in the virions have distinct lengths and electrostatic charges, we hypothesize that subsets of the virions should have distinct properties. A glutamine to cysteine substitution at position 120 of the capsid protein (CP) was found to result in a mutant virus named QC that exhibited a dramatically altered ratio of the RNAs in virions. RNA2 was far more abundant than the other RNAs, although the ratios could be affected by the host plant species. RNAs with the QC mutation were competent for replication early in the infection, suggesting that they were either selectively packaged or degraded after packaging. In support of the latter idea, low concentrations of truncated RNA1 that co-migrated with RNA2 were found in the QC virions. Spectroscopic analysis and peptide fingerprinting experiments showed that the QC virus capsid interacted with the encapsidated RNAs differently than did the wild type. Furthermore, wild-type BMV RNA1 was found to be more susceptible to nuclease digestion relative to RNA2 as a function of the buffer pH. Other BMV capsid mutants also had altered ratios of packaged RNAs.

Functional analysis of brome mosaic virus coat protein RNA-interacting domains

The coat proteins (CP) of cowpea chlorotic mottle (CCMV) and brome mosaic virus (BMV), two members of the genus Bromovirus, share 70% identity at the amino acid (aa) level and contain four highly conserved regions, identified as putative RNA-interacting domains (RIDs). To assess the contribution of the conserved aa sequence within each RID and the structural features contained therein toward virion assembly and RNA packaging, we engineered a set of fourteen independent mutations (deletions and substitutions) encompassing all four RIDs. The effect of each mutation on viral biology, pathogenesis, and RNA packaging was analyzed in whole-plant infection assays. Among the four RIDs, two mutations engineered into the N-proximal domain (RID I) and two of the four mutations engineered into the C-proximal domain (RID IV) proved to be more debilitating (compared to wild-type) while only selected regions in the central domains (RID II or III) showed a detectable effect. Neutral effects were observed when aa residues that are predicted to affect calcium binding were mutated. To further analyze the importance of N and C terminal interactions leading to virus assembly and RNA packaging, four CP hybrids were constructed by precisely exchanging either the N-terminal 77 or the C-terminal 113/112aa between BMV and CCMV. Despite the fact that the CP composition of the hybrid viruses is distinct from either of the parents, the symptom phenotype in Chenopodium quinoa, migration pattern of CP in Western blots and virion mobility in agarose gels was indistinguishable from the respective parent providing the genetic background. Collectively, the data provide insight for assessing the relative importance of each RID during genome packaging and in molecular processes regulating the overall architecture of the assembled virions.

In vivo particle polymorphism results from deletion of a N-terminal peptide molecular switch in brome mosaic virus capsid protein

The interaction between brome mosaic virus (BMV) coat protein (CP) and viral RNA is a carefully orchestrated process resulting in the formation of homogeneous population of infectious virions with T=3 symmetry. Expression in vivo of either wild type or mutant BMV CP through homologous replication never results in the assembly of aberrant particles. In this study, we report that deletion of amino acid residues 41-47 from the N-proximal region of BMV CP resulted in the assembly of polymorphic virions in vivo. Purified virions from symptomatic leaves remain non-infectious and Northern blot analysis of virion RNA displayed packaging defects. Biochemical characterization of variant CP by circular dichroism and MALDI-TOF, respectively, revealed that the engineered deletion affected the protein structure and capsid dynamics. Most significantly, CP subunits dissociated from polymorphic virions are incompetent for in vitro reassembly. Based on these observations, we propose a chaperon-mediated mechanism for the assembly of variant CP in vivo and also hypothesize that (41)KAIKAIA(47) N-proximal peptide functions as a molecular switch in regulating T=3 virion symmetry.

In vivo packaging of brome mosaic virus RNA3, but not RNAs 1 and 2, is dependent on a cis-acting 3' tRNA-like structure

The four encapsidated RNAs of brome mosaic virus (BMV; B1, B2, B3, and B4) contain a highly conserved 3' 200-nucleotide (nt) region encompassing the tRNA-like structure (TLS) which is required for packaging in vitro (Y. G. Choi, T. W. Dreher, and A. L. N. Rao, Proc. Natl. Acad. Sci. USA 99:655-660, 2002). To validate these observations in vivo, we performed packaging assays using Agrobacterium-mediated transient expression of RNAs and coat protein (CP) (P. Annamalai and A. L. N. Rao, Virology 338:96-111, 2005). Coexpression of TLS-less constructs of B1 or B2 or B3 and CP mRNAs in Nicotiana benthamiana leaves resulted in packaging of TLS-less B1 and B2 but not B3, suggesting that packaging of B3 requires the TLS in cis. This conjecture was confirmed by the efficient packaging of a B3 chimera in which the viral TLS was replaced with a cellular tRNA(Tyr). When N. benthamiana leaves were infiltrated with a mixture of transformants containing wild-type B1 (wtB1) plus wtB2 plus a TLS-less B3 (wtB1+wtB2+TLS-lessB3), the 3' end of progeny B3 was restored by heterologous recombination with that of either B1 or B2. This intrinsic cis-requirement of TLS in promoting B3 packaging was further confirmed when a mixture containing agrotransformants of TLS-less B1+B2+B3 was supplemented with either wtB4 or a 3' 200-nt or 3' 336-nt untranslated region (UTR) of B3. Northern blot analysis followed by sequencing of B3 progeny revealed that replication of TLS-less B3, but not TLS-less B1 or B2, was fully restored due to recombination with TLS from transiently expressed wtB4 or the B3 3' UTR. Collectively, these observations suggested that the requirement of a cis-acting TLS is distinct for B3 compared with B1 or B2.

Packaging of brome mosaic virus subgenomic RNA is functionally coupled to replication-dependent transcription and translation of coat protein

In Brome mosaic virus (BMV), genomic RNA1 (gB1) and RNA2 (gB2), encoding the replication factors, are packaged into two separate virions, whereas genomic RNA3 (gB3) and its subgenomic coat protein (CP) mRNA (sgB4) are copackaged into a third virion. In vitro assembly assays performed between a series of deletion variants of sgB4 and wild-type (wt) CP subunits demonstrated that packaging of sgB4 is independent of sequences encoding the CP open reading frame. To confirm these observations in vivo and to unravel the mechanism of sgB4 copackaging, an Agrobacterium-mediated transient in vivo expression system (P. Annamalai and A. L. N. Rao, Virology 338:96-111, 2005) that effectively uncouples replication from packaging was used. Cultures of agrotransformants, engineered to express sgB4 and CP subunits either transiently (sgB4(Trans) and CP(Trans)) or in replication-dependent transcription and translation when complemented with gB1 and gB2 (sgB4(Rep) and CP(Rep)), were mixed in all four pair-wise combinations and infiltrated to Nicotiana benthamiana leaves to systematically evaluate requirements regulating sgB4 packaging. The data revealed that (i) in the absence of replication, packaging was nonspecific, since transiently expressed CP subunits efficiently packaged ubiquitous cellular RNA as well as transiently expressed sgB4 and its deletion variants; (ii) induction of viral replication increased specificity of RNA packaging; and most importantly, (iii) efficient packaging of sgB4, reminiscent of the wt scenario, is functionally coupled not only to its transcription via replication but also to translation of CP from replication-derived mRNA, a mechanism that appears to be conserved among positive-strand RNA viruses of plants (this study), animals (flock house virus), and humans (poliovirus).

Genome packaging by spherical plant RNA viruses

The majority of positive-strand RNA viruses of plants replicate and selectively encapsidate their progeny genomes into stable virions in cytoplasmic compartments of the cell where the opportunity to copackage cellular RNA also exists. Remarkably, highly purified infectious virions contain almost exclusively viral RNA, suggesting that mechanisms exist to regulate preferential packaging of viral genomes. The general principle that governs RNA packaging is an interaction between the structural CP and a specific RNA signal. Mechanisms that enhance selective packaging of viral genomes and formation of infectious virions may involve factors other than CP and nucleic acid sequences. The possible involvement of replicase proteins is an example. Our knowledge concerning genome packaging among spherical plant RNA viruses is still maturing. The main focus of this review is to discuss factors that have limited progress and to evaluate recent technical breakthroughs likely to help unravel the mechanism of RNA packaging among viruses of agronomic importance. A key breakthrough is the development of in vivo systems and comparisons with results obtained in vitro.

The Role of Arginine-rich Motif and β-Annulus in the Assembly and Stability of Sesbania Mosaic Virus Capsids

Sesbania mosaic virus (SeMV) capsids are stabilized by protein-protein, protein-RNA and calcium-mediated protein-protein interactions. The N-terminal random domain of SeMV coat protein (CP) controls RNA encapsidation and size of the capsids and has two important motifs, the arginine-rich motif (ARM) and the beta-annulus structure. Here, mutational analysis of the arginine residues present in the ARM to glutamic acid was carried out. Mutation of all the arginine residues in the ARM almost completely abolished RNA encapsidation, although the assembly of T=3 capsids was not affected. A minimum of three arginine residues was found to be essential for RNA encapsidation. The mutant capsids devoid of RNA were less stable to thermal denaturation when compared to wild-type capsids. The results suggest that capsid assembly is entirely mediated by CP-dependent protein-protein inter-subunit interactions and encapsidation of genomic RNA enhances the stability of the capsids. Because of the unique structural ordering of beta-annulus segment at the icosahedral 3-folds, it has been suggested as the switch that determines the pentameric and hexameric clustering of CP subunits essential for T=3 capsid assembly. Surprisingly, mutation of a conserved proline within the segment that forms the beta-annulus to alanine, or deletion of residues 48-53 involved in hydrogen bonding interactions with residues 54-58 of the 3-fold related subunit or deletion of all the residues (48-59) involved in the formation of beta-annulus did not affect capsid assembly. These results suggest that the switch for assembly into T=3 capsids is not the beta-annulus. The ordered beta-annulus observed in the structures of many viruses could be a consequence of assembly to optimize intersubunit interactions.

Atomic force microscopy investigation of a chlorella virus, PBCV-1

A virus PBCV-1, which infects certain fresh water algae and has been shown by transmission and cryo-electron microscopy to exist as a triskaidecahedron, was imaged using atomic force microscopy (AFM). From AFM the particles have diameters of about 190nm and the overall structure is in all important respects consistent with existing models. The surface lattice of the virion is composed of trimeric capsid proteins distributed according to p3 symmetry to create a honeycomb arrangement of raised edges forming quasi-hexagonal cells. At the pentagonal vertices are five copies of a different protein forming an exact pentagon, and this has yet another unique protein in its center. The apical protein exhibits some unusual mechanical properties in that it can be made to retract into the virion interior when subjected to AFM tip pressure. When PBCV-1 virions degrade, they give rise to small, uniform, spherical, and virus like particles (VLP) consistent with T=1 or 3 icosahedral products. Also observed upon disintegration are strands of linear dsDNA. Fibers of unknown function are also occasionally seen associated with some virions.

Replication-independent expression of genome components and capsid protein of brome mosaic virus in planta: a functional role for viral replicase in RNA packaging

To begin elucidation of the relationship between Brome mosaic virus (BMV) replication and encapsidation, we used a T-DNA-based Agrobacterium-mediated transient expression (agroinfiltration) system in Nicotiana benthamiana leaves to express either individual or desired pairs of the three genomic RNAs. The packaging competence of these RNAs into virions formed by the transiently expressed coat protein (CP) was analyzed. We found that in the absence of a functional replicase, assembled virions contained non-replicating viral RNAs (RNA1 or RNA2 or RNA3 or RNA1 + RNA3 or RNA2 + RNA3) as well as cellular RNAs. By contrast, virions assembled in the presence of a functional replicase contained only viral RNAs. To further elucidate the specificity exhibited by the functional viral replicase in RNA packaging, replication-defective RNA1 and RNA2 were constructed by deleting the 3' tRNA-like structure (3' TLS). Co-expression of TLS-less RNA1 and RNA2 with wt RNA3 resulted in efficient synthesis of subgenomic RNA4. Virions recovered from leaves co-expressing TLS-less RNA1 and RNA2 and either CP mRNA or wt RNA3 exclusively contained viral RNAs. These results demonstrated that packaging of BMV genomic RNAs is not replication dependent whereas expression of a functional viral replicase plays an active role in increasing specificity of RNA packaging.

Deletion of highly conserved arginine-rich RNA binding motif in cowpea chlorotic mottle virus capsid protein results in virion structural alterations and RNA packaging constraints

The N-proximal region of cowpea chlorotic mottle virus (CCMV) capsid protein (CP) contains an arginine-rich RNA binding motif (ARM) that is also found in the CPs of other members of Bromoviridae and in other RNA binding proteins such as the Tat and Rev proteins of human immunodeficiency virus. To assess the critical role played by this motif during encapsidation, a variant of CCMV RNA3 (C3) precisely lacking the ARM region (C3/Delta919) of its CP gene was constructed. The biology and the competence of the matured CP derived in vivo from C3/Delta919 to assemble and package progeny RNA was examined in whole plants. Image analysis and computer-assisted three-dimensional reconstruction of wild-type and mutant virions revealed that the CP subunits bearing the engineered deletion assembled into polymorphic virions with altered surface topology. Northern blot analysis of virion RNA from mutant progeny demonstrated that the engineered mutation down-regulated packaging of all four viral RNAs; however, the packaging effect was more pronounced on genomic RNA1 and RNA2 than genomic RNA3 and its CP mRNA. In vitro assembly assays with mutant CP subunits and RNA transcripts demonstrated that the mutant CP is inherently not defective in packaging genomic RNA1 (53%) and RNA2 (54%), but their incorporation into virions was competitively inhibited by the presence of other viral RNAs. Northern blot analysis of RNA encapsidation in vivo of two distinct bromovirus RNA3 chimeras, constructed by exchanging CPs having the Delta919 deletion, demonstrated that the role of the conserved N-terminal ARM in recognizing and packaging specific RNA is distinct for each virus.

Crystallographic Structure of the T=1 Particle of Brome Mosaic Virus

T=1 icosahedral particles of amino terminally truncated brome mosaic virus (BMV) protein were created by treatment of the wild-type T=3 virus with 1M CaCl2 and crystallized from sodium malonate. Diffraction data were collected from frozen crystals to beyond 2.9 A resolution and the structure determined by molecular replacement and phase extension. The particles are composed of pentameric capsomeres from the wild-type virions which have reoriented with respect to the original particle pentameric axes by rotations of 37 degrees , and formed tenuous interactions with one another, principally through conformationally altered C-terminal polypeptides. Otherwise, the pentamers are virtually superimposable upon those of the original T=3 BMV particles. The T=1 particles, in the crystals, are not perfect icosahedra, but deviate slightly from exact symmetry, possibly due to packing interactions. This suggests that the T=1 particles are deformable, which is consistent with the loose arrangement of pentamers and latticework of holes that penetrate the surface. Atomic force microscopy showed that the T=3 to T=1 transition could occur by shedding of hexameric capsomeres and restructuring of remaining pentamers accompanied by direct condensation. Knowledge of the structures of the BMV wild-type and T=1 particles now permit us to propose a tentative model for that process. A comparison of the BMV T=1 particles was made with the reassembled T=1 particles produced from the coat protein of trypsin treated alfalfa mosaic virus (AlMV), another bromovirus. There is little resemblance between the two particles. The BMV particle, with a maximum diameter of 195 A, is made from distinctive pentameric capsomeres with large holes along the 3-fold axis, while the AlMV particle, of approximate maximum diameter 220 A, has subunits closely packed around the 3-fold axis, large holes along the 5-fold axis, and few contacts within pentamers. In both particles crucial linkages are made about icosahedral dyads.

Packaging, replication and recombination of the segmented genome of bacteriophage Phi6 and its relatives

The genomes of bacteriophage Phi6 and its relatives are packaged through a mechanism that involves the recognition and translocation of the three different plus strand transcripts of the segmented dsRNA genomes into preformed polyhedral structures called procapsids or inner cores. The packaging requires hydrolysis of NTPs and takes place in the order S:M:L. Minus strand synthesis begins after the completion of the plus strand packaging. The packaging and replication reactions can be studied in vitro with purified components. A model has been presented that proposes that the program of serially dependent packaging is determined by the conformational changes at the surface of the procapsid due to the amount of RNA packaged at each step. The in vitro packaging and replication system has facilitated the application of reverse genetics and the study of recombination in the family of Cystoviridae.

Interaction with capsid protein alters RNA structure and the pathway for in vitro assembly of cowpea chlorotic mottle virus

Viruses use sophisticated mechanisms to allow the specific packaging of their genome over that of host nucleic acids. We examined the in vitro assembly of the Cowpea chlorotic mottle virus (CCMV) and observed that assembly with viral RNA follows two different mechanisms. Initially, CCMV capsid protein (CP) dimers bind RNA with low cooperativity and form virus-like particles of 90 CP dimers and one copy of RNA. Longer incubation reveals a different assembly path. At a stoichiometry of about ten CP dimers per RNA, the CP slowly folds the RNA into a compact structure that can be bound with high cooperativity by additional CP dimers. This folding process is exclusively a function of CP quaternary structure and is independent of RNA sequence. CP-induced folding is distinct from RNA folding that depends on base-pairing to stabilize tertiary structure. We hypothesize that specific encapsidation of viral RNA is a three-step process: specific binding by a few copies of CP, RNA folding, and then cooperative binding of CP to the "labeled" nucleoprotein complex. This mechanism, observed in a plant virus, may be applicable to other viruses that do not halt synthesis of host nucleic acid, including HIV.

cis-acting elements required for efficient packaging of brome mosaic virus RNA3 in barley protoplasts

Brome mosaic virus (BMV) is a positive-sense RNA plant virus, the tripartite genomic RNAs of which are separately packaged into virions. RNA3 is copackaged with subgenomic RNA4. In barley protoplasts coinoculated with RNA1 and RNA2, an RNA3 mutant with a 69-nucleotide (nt) deletion in the 3'-proximal region of the 3a open reading frame (ORF) was very poorly packaged compared with other RNA3 mutants and wild-type RNA3, despite their comparable accumulation in the absence of coat protein. Computer analysis of RNA secondary structure predicted two stem-loop (SL) structures (i.e., SL-I and SL-II) in the 69-nt region. Disruption of SL-II, but not of SL-I, significantly reduced RNA3 packaging. A chimeric BMV RNA3 (B3Cmp), with the BMV 3a ORF replacing that of cucumber mosaic virus (CMV), was packaged negligibly, whereas RNA4 was packaged efficiently. Replacement of the 3'-proximal region of the CMV 3a ORF in B3Cmp with the 3'-proximal region of the BMV 3a ORF significantly improved packaging efficiency, and the disruption of SL-II in the substituted BMV 3a ORF region greatly reduced packaging efficiency. These results suggest that the 3'-proximal region of the BMV 3a ORF, especially SL-II predicted between nt 904 and 933, plays an important role in the packaging of BMV RNA3 in vivo. Furthermore, the efficient packaging of RNA4 without RNA3 in B3Cmp-infected cells implies the presence of an element in the 3a ORF of BMV RNA3 that regulates the copackaging of RNA3 and RNA4.

Basic residues of the helix six domain of influenza virus M1 involved in nuclear translocation of M1 can be replaced by PTAP and YPDL late assembly domain motifs

Influenza type A virus matrix (M1) protein possesses multiple functional motifs in the helix 6 (H6) domain (amino acids 91 to 105), including nuclear localization signal (NLS) (101-RKLKR-105) involved in translocating M1 from the cytoplasm into the nucleus. To determine the role of the NLS motif in the influenza virus life cycle, we mutated these and the neighboring sequences by site-directed mutagenesis, and influenza virus mutants were generated by reverse genetics. Our results show that infectious viruses were rescued by reverse genetics from all single alanine mutations of amino acids in the H6 domain and the neighboring region except in three positions (K104A and R105A within the NLS motif and E106A in loop 6 outside the NLS motif). Among the rescued mutant viruses, R101A and R105K exhibited reduced growth and small-plaque morphology, and all other mutant viruses showed the wild-type phenotype. On the other hand, three single mutations (K104A, K105A, and E106A) and three double mutations (R101A/K102A, K104A/K105A, and K102A/R105A) failed to generate infectious virus. Deletion (Delta YRKL) or mutation (4A) of YRKL also abolished generation of infectious virus. However, replacement of the YRKL motif with PTAP or YPDL as well as insertion of PTAP after 4A mutation yielded infectious viruses with the wild-type phenotype. Furthermore, mutant M1 proteins (R101A/K102A, Delta YRKL, 4A, PTAP, 4A+PTAP, and YPDL) when expressed alone from cloned cDNAs were only cytoplasmic, whereas the wild-type M1 expressed alone was both nuclear and cytoplasmic as expected. These results show that the nuclear translocation function provided by the positively charged residues within the NLS motif does not play a critical role in influenza virus replication. Furthermore, these sequences of H6 domain can be replaced by late (L) domain motifs and therefore may provide a function similar to that of the L domains of other negative-strand RNA and retroviruses.

Positional effect of deletions on viability, especially on encapsidation, of Brome mosaic virus D-RNA in barley protoplasts

Brome mosaic virus (BMV), a tripartite RNA plant virus, accumulates RNA3-derived defective RNAs (D-RNAs) in which 477-500 nucleotides (nt) are deleted in the central region of the 3a protein open reading frame (ORF), after prolonged infection in barley. In the present study, six artificial D-RNAs (AD-RNAs), having deletions of the same size as the naturally occurring D-RNA but at different positions in the 3a ORF, were constructed and tested for their amplification and encapsidation in barley protoplasts by coinoculation with BMV RNA1 and 2, or RNA1, 2, and 3. Northern blot analysis of RNA accumulation in total and virion fractions showed that deletions of 492 nt in the 3'-proximal and the 5'-proximal regions of the 3a ORF decreased encapsidation efficiency of the AD-RNAs compared with that of RNA3, whereas deletions in the central region enhanced encapsidation efficiency. The present results also show that deletion positions affect competition with RNA3 in the amplification and encapsidation of AD-RNAs.

Crystallization of Brome mosaic virus and T = 1 Brome mosaic virus particles following a structural transition

Brome mosaic virus (BMV), a T = 3 icosahedral plant virus, can be dissociated into coat protein subunits and subunit oligomers at pH 7.5 in the presence of concentrated salts. We have found that during the course of this treatment the coat protein subunits are cleaved, presumably by plant cell proteases still present in the preparation, between amino acids 35 and 36. The truncated protein subunits will then reorganize into T = 1 icosahedral particles and can be crystallized from sodium malonate. Quasi elastic light scattering and atomic force microscopy results suggest that the transition from T = 3 to T = 1 particles can occur by separate pathways, dissociation into coat protein subunits and oligomers and reassembly into T = 1 particles, or direct condensation of the T = 3 virions to T = 1 particles with the shedding of hexameric capsomeres. The latter process has been directly visualized using atomic force microscopy. Native T = 3 virions have been crystallized in several different crystal forms, but neither a rhombohedral form nor either of two orthorhombic forms diffract beyond about 3.4 A. Tetragonal crystals of the T = 1 particles, however, diffract to at least 2.5 A resolution. Evidence suggests that the T = 1 particles are more structurally uniform and ordered than are native T = 3 virions. A variety of anomalous virus particles having diverse sizes have been visualized in preparations of BMV used for crystallization. In some cases these aberrant particles are incorporated into growing crystals where they are frequently responsible for defect formation.

In vitro analysis of an RNA binding site within the N-terminal 30 amino acids of the southern cowpea mosaic virus coat protein

Southern cowpea mosaic virus (SCPMV) is a positive-sense RNA virus with T = 3 icosahedral symmetry. The coat protein (CP) has two domains, the random (R) domain and the shell (S) domain. The R domain is formed by the N-terminal 64 amino acids (aa) and is localized to the interior of the particle where it is expected to interact with the viral RNA. The R domain (aa 1--57) was expressed in Escherichia coli as a recombinant protein (rWTR) containing a nonviral C-terminal extension with two histidine tags. The RNA binding site of the R domain was identified by Northwestern blotting and electrophoretic mobility shift assay (EMSA) using recombinant wild-type and mutant R domain proteins. Deletions within the R domain revealed that the RNA binding site is localized to its N-terminal 30 aa. RNA binding by this element was found to be nonspecific with regard to RNA sequence and was sensitive to high salt concentrations, suggesting that electrostatic interactions are important for RNA binding by the R domain. The RNA binding site includes 11 basic residues, eight of which are located in the arginine-rich region between aa 22 and 30. It was demonstrated using alanine substitution mutants that the basic residues of the arginine-rich region but not those present at positions 3, 4, and 7 are necessary for RNA binding. None of the basic residues within the arginine-rich region are specifically required for RNA binding, but the overall charge of the N-terminal 30 aa is important. Proline substitution mutations within the N-terminal 30 aa, and alanine substitutions for prolines at positions 18, 20, and 21, did not affect the RNA binding activity of the R domain. However, it was demonstrated by circular dichroism (CD) that the conformation of the N-terminal 30 aa of the R domain changes from a random coil to an alpha-helix in the presence of 50% trifluoroethanol (TFE). The possible role for this structural change in RNA binding by the R domain is discussed.

The C terminus of brome mosaic virus coat protein controls viral cell-to-cell and long-distance movement

To investigate the functional domains of the coat protein (CP; 189 amino acids) of Brome mosaic virus, a plant RNA virus, 19 alanine-scanning mutants were constructed and tested for their infectivity in barley and Nicotiana benthamiana. Despite its apparent normal replicative competence and CP production, the C-terminal mutant F184A produced no virions. Furthermore, virion-forming C-terminal mutants P178A and D182A failed to move from cell to cell in both plant species, and mutants D181A and V187A showed host-specific movement. These results indicate that the C-terminal region of CP plays some important roles in virus movement and encapsidation. The specificity of certain mutations for viral movement in two different plant species is evidence for the involvement of host-specific factors.

Packaging of tobacco mosaic virus subgenomic RNAs by Brome mosaic virus coat protein exhibits RNA controlled polymorphism

The coat protein (CP) of icosahedral Brome mosaic virus (BMV) was expressed from a genetically engineered rod-shape Tobacco mosaic virus. Molecular characterization of the progeny recovered from symptomatic plants revealed that BMV CP selectively packaged the three subgenomic RNAs of the hybrid virus into two differently sized icosahedral virus-like particles (VLPs). The smaller VLPs packaged only the two smaller subgenomic RNAs. Additional in vitro reassembly assays with BMV CP subunits and transcripts of hybrid subgenomic RNAs further demonstrated that the ability of BMV capsids to display polymorphism is not dependent on the RNA size alone and appears to be controlled by some other feature(s) of the genetically engineered RNA.

Molecular studies on bromovirus capsid protein. VII. Selective packaging on BMV RNA4 by specific N-terminal arginine residuals

An arginine-rich RNA-binding motif (ARM) found at the N-proximal region of Brome mosaic virus (BMV) coat protein (CP) adopts alpha-helical conformation and shares homology with CPs of plant and insect RNA viruses, HIV-Rev and Tat proteins, bacterial antiterminators, and ribosomal splicing factors. The ARM of BMV CP, consisting of amino acids 9 through 21 with six arginine residues, is essential for RNA binding and subsequent packaging. In this study analysis of the alpha-helical contents of wild-type and mutant peptides by circular dichroism spectra identified protein determinants required for such conformation. Electrophoretic mobility-shift assays between viral RNA and BMV CP peptides with either proline or alanine substitutions revealed that the interaction is nonspecific. Expression in vivo of mature full-length BMV CP subunits, having the same substitutions for each arginine within the ARM, derived from biologically active clones was found to be competent to assemble into infectious virions and cause visible symptom phenotypes in whole plants. However, analysis of virion progeny RNA profiles of CP variants and subsequent in vitro reassembly assays between mutant CP and four BMV RNAs unveiled the ability of arginine residues at positions 10, 13, or 14 of the ARM to confer selective packaging of BMV RNA4. Thus, BMV CP contains determinants that specifically interact with RNA4 to ensure selective packaging.