Amino-terminal region of the human immunodeficiency virus type 1 nucleocapsid is required for human APOBEC3G packaging

APOBEC3G exerts its antiviral activity by targeting to retroviral particles and inducing viral DNA hypermutations in the absence of Vif. However, the mechanism by which APOBEC3G is packaged into virions remains unclear. We now report that viral genomic RNA enhances but is not essential for human APOBEC3G packaging into human immunodeficiency virus type 1 (HIV-1) virions. Packaging of APOBEC3G was also detected in HIV-1 Gag virus-like particles (VLP) that lacked all the viral genomic RNA packaging signals. Human APOBEC3G could be packaged efficiently into a divergent subtype HIV-1, as well as simian immunodeficiency virus, strain mac, and murine leukemia virus Gag VLP. Cosedimentation of human APOBEC3G and intracellular Gag complexes was detected by equilibrium density and velocity sucrose gradient analysis. Interaction between human APOBEC3G and HIV-1 Gag was also detected by coimmunoprecipitation experiments. This interaction did not require p6, p1, or the C-terminal region of NCp7. However, the N-terminal region, especially the first 11 amino acids, of HIV-1 NCp7 was critical for HIV-1 Gag and APOBEC3G interaction and virion packaging. The linker region flanked by the two active sites of human APOBEC3G was also important for efficient packaging into HIV-1 Gag VLP. Association of human APOBEC3G with RNA-containing intracellular complexes was observed. These results suggest that the N-terminal region of HIV-1 NC, which is critical for binding to RNA and mediating Gag-Gag oligomerization, plays an important role in APOBEC3G binding and virion packaging.

Structural basis for packaging the dimeric genome of Moloney murine leukaemia virus

All retroviruses specifically package two copies of their genomes during virus assembly, a requirement for strand-transfer-mediated recombination during reverse transcription. Genomic RNA exists in virions as dimers, and the overlap of RNA elements that promote dimerization and encapsidation suggests that these processes may be coupled. Both processes are mediated by the nucleocapsid domain (NC) of the retroviral Gag polyprotein. Here we show that dimerization-induced register shifts in base pairing within the Psi-RNA packaging signal of Moloney murine leukaemia virus (MoMuLV) expose conserved UCUG elements that bind NC with high affinity (dissociation constant = 75 +/- 12 nM). These elements are base-paired and do not bind NC in the monomeric RNA. The structure of the NC complex with a 101-nucleotide 'core encapsidation' segment of the MoMuLV Psi site reveals a network of interactions that promote sequence- and structure-specific binding by NC's single CCHC zinc knuckle. Our findings support a structural RNA switch mechanism for genome encapsidation, in which protein binding sites are sequestered by base pairing in the monomeric RNA and become exposed upon dimerization to promote packaging of a diploid genome.

Mapping the phosphoprotein binding site on Sendai virus NP protein assembled into nucleocapsids

To catalyze RNA synthesis, the Sendai virus P-L RNA polymerase complex first binds the viral nucleocapsid (NC) template through an interaction of the P subunit with NP assembled with the genome RNA. For replication, the polymerase utilizes an NP(0)-P complex as the substrate for the encapsidation of newly synthesized RNA which involves both NP-RNA and NP-NP interactions. Previous studies showed that the C-terminal 124 amino acids of NP (aa 401-524) contain the P-NC binding site. To further delineate the amino acids important for this interaction, C-terminal truncations and site-directed mutations in NP were characterized for their replication activity and protein-protein interactions. This C-terminal region was found in fact to be necessary for several different protein interactions. The C-terminal 492-524 aa were nonessential for the complete activity of the protein. Deletion of amino acids 472-491, however, abolished replication activity due to a specific defect in the formation of the NP(0)-P complex. Binding of the P protein of the polymerase complex to NC required aa 462-471 of NP, while self-assembly of NP into NC required aa 440-461. Site-directed mutations from aa 435 to 491 showed, however, that the charged amino acids in this region were not essential for these defects.

Epitope analysis of monoclonal antibody E5/G6, which binds to a linear epitope in the nucleocapsid protein of hantaviruses

Monoclonal antibody E5/G6 recognized a linear epitope common to hantavirus nucleocapsid proteins. Using synthetic peptides, we identified epitope E5/G6 as the 9 mer YEDVNGIRK (NP 165-173), in which D167, G170, I171, and R172 are indispensable. Furthermore, all the peptides synthesized using various hantavirus sequences bound MAb E5/G6 consistently, despite the existence of several amino acid variations in this region. These results indicate that MAb E5/G6 is a useful tool for detecting hantavirus antigen in rodent or patient tissues using Western blotting or other immunohistochemical assays.

Basic residues of the retroviral nucleocapsid play different roles in gag-gag and Gag-Psi RNA interactions

The Orthoretrovirus Gag interaction (I) domain maps to the nucleocapsid (NC) domain in the Gag polyprotein. We used the yeast two-hybrid system to analyze the role of Alpharetrovirus NC in Gag-Gag interactions and also examined the efficiency of viral assembly and release in vivo. We could delete either or both of the two Cys-His (CH) boxes without abrogating Gag-Gag interactions. We found that as few as eight clustered basic residues, attached to the C terminus of the spacer peptide separating the capsid (CA) and NC domains in the absence of NC, was sufficient for Gag-Gag interactions. Our results support the idea that a sufficient number of basic residues, rather than the CH boxes, play the important role in Gag multimerization. We also examined the requirement for basic residues in Gag for packaging of specific packaging signal (Psi)-containing RNA. Using a yeast three-hybrid RNA-protein interaction assay, second-site suppressors of a packaging-defective Gag mutant were isolated, which restored Psi RNA binding. These suppressors mapped to the p10 or CA domains in Gag and resulted in either introduction of a positively charged residue or elimination of a negatively charged one. These results imply that the structural interactions of NC with other domains of Gag are necessary for Psi RNA binding. Taken together, our results show that while Gag assembly only requires a certain number of positively charged amino acids, Gag binding to genomic RNA for packaging requires more complex interactions inherent in the protein tertiary structure.

Conformational flexibility in recombinant measles virus nucleocapsids visualised by cryo-negative stain electron microscopy and real-space helical reconstruction

Measles virus is a highly contagious virus that, despite the existence of an effective vaccine, is a major cause of illness and mortality worldwide. The virus has a negative-sense, single-stranded RNA genome that is encapsidated by the nucleocapsid protein (N) to form a helical ribonucleoprotein complex known as the nucleocapsid. This structure serves as the template for both transcription and replication. Paramyxovirus nucleocapsids are flexible structures, a trait that has hitherto hampered structural analysis even at low resolution. We have investigated the extent of this structural plasticity, using real-space methods to calculate three-dimensional reconstructions of recombinant nucleocapsids from cryo-negative stain transmission electron micrographs. Images of short sections of helix were sorted according to both pitch (the axial rise per turn) and twist (the number of subunits per turn). Our analysis indicates that there is extensive conformational flexibility within these structures, ranging in pitch from 50 Angstrom to 66 Angstrom, while twist varies from at least 13.04 to 13.44 with a greater number of helices comprising around 13.1 subunits per turn. We have also investigated the influence of the C terminus of N on helix conformation, analysing nucleocapsids after having removed this domain by trypsin digestion. We have found that this causes a marked change in both pitch and twist, such that the pitch becomes shorter, ranging from 46 Angstrom to 52 Angstrom, while more helices have a twist of approximately 13.3 subunits per turn. Our findings lead us to propose a mechanism whereby changes in conformation, influenced by interactions between viral or host proteins and the C terminus of N, might have a role in regulating the balance of transcription and replication during virus infection.

The N-terminal half of the core protein of hepatitis C virus is sufficient for nucleocapsid formation

The core (C) protein of hepatitis C virus (HCV) appears to be a multifunctional protein that is involved in many viral and cellular processes. Although its effects on host cells have been extensively discussed in the literature, little is known about its main function, the assembly and packaging of the viral genome. We have studied the in vitro assembly of several deleted versions of recombinant HCV C protein expressed in E. coli. We demonstrated that the 75 N-terminal residues of the C protein were sufficient to assemble and generate nucleocapsid-like particles (NLPs) in vitro. However, homogeneous particles of regular size and shape were observed only when NLPs were produced from at least the first 79 N-terminal amino acids of the C protein. This small protein unit fused to the endoplasmic reticulum-anchoring domain also generated NLPs in yeast cells. These data suggest that the N-terminal half of the C protein is important for formation of NLPs. Similarities between the HCV C protein and C proteins of other members of the Flaviviridae are discussed.

Role of the trans-activation response element in dimerization of HIV-1 RNA

The HIV-1 genome consists of two identical RNA strands that are linked together through non-covalent interactions. A major determinant for efficient dimerization of the two RNA strands is the interaction between palindromic sequences in the dimerization initiation site. Here we use an interplay of bioinformatics, biochemistry, and atomic force microscopy to describe another conserved palindrome in the trans-activation response element (TAR) that functions as a strong dimerization site when transiently exposed to the viral nucleocapsid protein. In conjunction with the DIS interaction, the TAR dimerization induces the formation of a 65-nm higher-order circular structure in the dimeric HIV-1 RNA. Our results provide a molecular model for the role of TAR in packaging and reverse transcription of the viral genome. The unique structure of the TAR-TAR dimer renders it an intriguing therapeutic target for the treatment of HIV-1 infection.

The nucleocapsid of the hepatitis B virus: a remarkable immunogenic structure

The hepatitis B virus nucleocapsid or core antigen is extremely immunogenic during infection and after immunization. This review summarizes several features of the nucleocapsid which explain this exceptionally high immunogenicity: a unique three-dimensional folding, the presence of a region that interacts with immunoglobulins outside the classical antibody-binding site, the presence of many CD4+ T cell epitopes, and the presence of encapsidated nucleic acids. Because of these features, nucleocapsids efficiently interact and activate antigen presenting cells, especially nai;ve B cells. This leads to the generation of a dominant Th1 immunity phenotype and the secretion of high levels of IgM and IgG anti-nucleocapsid antibodies.

Important role for the CA-NC spacer region in the assembly of bovine immunodeficiency virus Gag protein

Lentiviral Gag proteins contain a short spacer sequence that separates the capsid (CA) from the downstream nucleocapsid (NC) domain. This short spacer has been shown to play an important role in the assembly of human immunodeficiency virus type 1 (HIV-1). We have now extended this finding to the CA-NC spacer motif within the Gag protein of bovine immunodeficiency virus (BIV). Mutation of this latter spacer sequence led to dramatic reductions in virus production, which was mainly attributed to the severely disrupted association of the mutated Gag with the plasma membrane, as shown by the results of membrane flotation assays and confocal microscopy. Detailed mutagenesis analysis of the BIV CA-NC spacer region for virus assembly determinants led to the identification of two key residues, L368 and M372, which are separated by three amino acids, 369-VAA-371. Incidentally, the same two residues are present within the HIV-1 CA-NC spacer region at positions 364 and 368 and have also been shown to be crucial for HIV-1 assembly. Regardless of this conservation between these two viruses, the BIV CA-NC spacer could not be replaced by its HIV-1 counterpart without decreasing virus production, as opposed to its successful replacement by the CA-NC spacer sequences from the nonprimate lentiviruses such as feline immunodeficiency virus (FIV), equine infectious anemia virus and visna virus, with the sequence from FIV showing the highest effectiveness in this regard. Taken together, these data suggest a pivotal role for the CA-NC spacer region in the assembly of BIV Gag; however, the mechanism involved therein may differ from that for the HIV-1 CA-NC spacer.

Sindbis virus nucleocapsid assembly: RNA folding promotes capsid protein dimerization

In Sindbis virus, initiation of nucleocapsid core assembly begins with recognition of the encapsidation signal of the viral RNA genome by capsid protein. This nucleation event drives the recruitment of additional capsid proteins to fully encapsidate the genome, generating an icosahedral nucleocapsid core. The encapsidation signal of the Sindbis virus genomic RNA has previously been localized to a 132-nucleotide region of the genome within the coding region of the nsP1 protein, and the RNA-binding activity of the capsid was previously mapped to a central region of the capsid protein. It is unknown how capsid protein binding to encapsidation signal leads to ordered oligomerization of capsid protein and nucleocapsid core assembly. To address this question, we have developed a mobility shift assay to study this interaction. We have characterized a 32 amino acid peptide capable of recognizing the Sindbis virus encapsidation signal RNA. Using this peptide, we were able to observe a conformational change in the RNA induced by capsid protein binding. Binding is tight (K(d)(app) = 12 nM), and results in dimerization of the capsid peptide. Mutational analysis reveals that although almost every predicted secondary structure within the encapsidation signal is required for efficient protein binding, the identities of the bases within the helices and hairpin turns of the RNA do not need to be maintained. In contrast, two purine-rich loops are essential for binding. From these data, we have developed a model in which the encapsidation signal RNA adopts a highly folded structure and this folding process directs early events in nucleocapsid assembly.

Structure of the nucleocapsid protein of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is an enveloped RNA virus of the Arteriviridae family, genomically related to the coronaviruses. PRRSV is the causative agent of both severe and persistent respiratory disease and reproductive failure in pigs worldwide. The PRRSV virion contains a core made of the 123 amino acid nucleocapsid (N) protein, a product of the ORF7 gene. We have determined the crystal structure of the capsid-forming domain of N. The structure was solved to 2.6 A resolution by SAD methods using the anomalous signal from sulfur. The N protein exists in the crystal as a tight dimer forming a four-stranded beta sheet floor superposed by two long alpha helices and flanked by two N- and two C-terminal alpha helices. The structure of N represents a new class of viral capsid-forming domains, distinctly different from those of other known enveloped viruses, but reminiscent of the coat protein of bacteriophage MS2.

Sequence Analysis of Rabies Virus in Humans Exhibiting Encephalitic or Paralytic Rabies

Two distinct clinical patterns, encephalitic (furious) and paralytic (dumb), have been recognized in human rabies. It has been postulated that different rabies virus variants associated with particular vectors may be responsible for these different clinical manifestations. Analysis of the glycoprotein (G), nucleoprotein (N), and phosphoprotein (P) genes of rabies viruses from 2 human cases of encephalitic rabies and from 2 human cases of paralytic rabies demonstrated only minor nucleotide differences. Deduced amino-acid patterns of the N protein were identical in both human and canine samples that came from the same geographic location, regardless of the clinical form. All differences in amino-acid patterns of the G protein were found outside the ectodomain, in either the signal peptide or the transmembrane and endodomains. None of the amino-acid differences of the P protein was within the interactive site with dynein. These findings support the concept that clinical manifestations of rabies are not explained solely by the associated rabies virus variant.

Secondary structure in the nucleic acid affects the rate of HIV-1 nucleocapsid-mediated strand annealing

We studied the effects of human immunodeficiency virus type 1 (HIV-1) nucleocapsid (NC) protein on the kinetics of annealing of nucleic acids using model substrates derived from the 3' end of the HIV-1 minus-strand strong-stop DNA (-sssDNA). We used HIV-1 reverse transcriptase (RT) to monitor the annealing reaction. Using several different DNA primers and acceptor oligonucleotides, we found that the rate of annealing increased with the size of the complementary region of the primer and the acceptor strands and decreased when secondary structures could be formed in either the primer or the acceptor strands. The secondary structure had a larger effect on the rate of annealing if the secondary structure extends to the 3' end of the nucleic acid(s). NC protein reduced the rate of annealing between strands with short homologies. NC had no major effect on the rate of annealing when there were at least 13 bases of complementarity between the primer and the acceptor strands and neither strand could form a stable secondary structure. NC increased the rate of annealing when the primer and/or the acceptor strand could form a secondary structure in the region of complementarity. When two strands were in competition as acceptors in an annealing reaction, the specificity of the annealing was determined by the length of the complementarity between the primer and the acceptor strands, the presence or the absence of secondary structures in the primer and/or the acceptor strand, and the presence or the absence of NC in the reaction. This suggests that NC facilitates strand transfer where the nucleic acids have considerable secondary structure (for example, the first strand transfers for viruses whose genomes have considerable secondary structure at their 3' ends). However, NC also appears to increase the fidelity of recombination by reducing strand transfers between segments that have limited complementarity.

[Genetic analysis of wild measles virus strains isolated in the european part of the Russian Federation]

Eleven wild measles virus isolated, in 1988 and in 1999-2001 in the European territory of the Russian Federation, were investigated. On the basis of an analysis of N-gene region sequences, encoding the COOH terminal end of nucleoprotein, the isolates were divided into 2 subgroups. According to the WHO classification, subgroup 1 was in line with genotype A and subgroup 2--with genotype D. Subgroup 2 was close to genotype D4 but differed from it according to its composition of nucleotides on the average by 2.8%, and according to its amino-acid composition--by 2.6%. with respect to the WHO criteria, the latter can be referred to preliminarily as an independent genotype. Finally, the measles viruses' strains of genetic groups A and D circulated in the Russian Federation in 1988, and in 1999-2001.

Specific in vitro cleavage of Mason-Pfizer monkey virus capsid protein: evidence for a potential role of retroviral protease in early stages of infection

Processing of Gag polyproteins by viral protease (PR) leads to reorganization of immature retroviral particles and formation of a ribonucleoprotein core. In some retroviruses, such as HIV and RSV, cleavage of a spacer peptide separating capsid and nucleocapsid proteins is essential for the core formation. We show here that no similar spacer peptide is present in the capsid-nucleocapsid (CA-NC) region of Mason-Pfizer monkey virus (M-PMV) and that the CA protein is cleaved in vitro by the PR within the major homology region (MHR) and the NC protein in several sites at the N-terminus. The CA cleavage product was also identified shortly after penetration of M-PMV into COS cells, suggesting that the protease-catalyzed cleavage is involved in core disintegration.

Structures of immature flavivirus particles

Structures of prM-containing dengue and yellow fever virus particles were determined to 16 and 25 A resolution, respectively, by cryoelectron microscopy and image reconstruction techniques. The closely similar structures show 60 icosahedrally organized trimeric spikes on the particle surface. Each spike consists of three prM:E heterodimers, where E is an envelope glycoprotein and prM is the precursor to the membrane protein M. The pre-peptide components of the prM proteins in each spike cover the fusion peptides at the distal ends of the E glycoproteins in a manner similar to the organization of the glycoproteins in the alphavirus spikes. Each heterodimer is associated with an E and a prM transmembrane density. These transmembrane densities represent either an EE or prMprM antiparallel coiled coil by which each protein spans the membrane twice, leaving the C-terminus of each protein on the exterior of the viral membrane, consistent with the predicted membrane-spanning domains of the unprocessed polyprotein.

Elimination of protease activity restores efficient virion production to a human immunodeficiency virus type 1 nucleocapsid deletion mutant

The nucleocapsid (NC) region of human immunodeficiency virus type 1 (HIV-1) Gag is required for specific genomic RNA packaging. To determine if NC is absolutely required for virion formation, we deleted all but seven amino acids from NC in a full-length NL4-3 proviral clone. This construct, DelNC, produced approximately four- to sixfold fewer virions than did the wild type, and these virions were noninfectious (less than 10(-6) relative to the wild type) and severely genomic RNA deficient. Immunoblot and high-pressure liquid chromatography analyses showed that all of the mature Gag proteins except NC were present in the mutant virion preparations, although there was a modest decrease in Gag processing. DelNC virions had lower densities and were more heterogeneous than wild-type particles, consistent with a defect in the interaction assembly or I domain. Electron microscopy showed that the DelNC virions displayed a variety of aberrant morphological forms. Inactivating the protease activity of DelNC by mutation or protease inhibitor treatment restored virion production to wild-type levels. DelNC-protease mutants formed immature-appearing particles that were as dense as wild-type virions without incorporating genomic RNA. Therefore, protease activity combined with the absence of NC causes the defect in DelNC virion production, suggesting that premature processing of Gag during assembly causes this effect. These results show that HIV-1 can form particles efficiently without NC.

Importance of basic residues in binding of rous sarcoma virus nucleocapsid to the RNA packaging signal

In the context of the Rous sarcoma virus Gag polyprotein, only the nucleocapsid (NC) domain is required to mediate the specificity of genomic RNA packaging. We have previously showed that the Saccharomyces cerevisiae three-hybrid system provides a rapid genetic assay to analyze the RNA and protein components of the avian retroviral RNA-Gag interactions necessary for specific encapsidation. In this study, using both site-directed mutagenesis and in vivo random screening in the yeast three-hybrid binding assay, we have examined the amino acids in NC required for genomic RNA binding. We found that we could delete either of the two Cys-His boxes without greatly abrogating either RNA binding or packaging, although the two Cys-His boxes are likely to be required for efficient viral assembly and release. In contrast, substitutions for the Zn-coordinating residues within the boxes did prevent RNA binding, suggesting changes in the overall conformation of the protein. In the basic region between the two Cys-His boxes, three positively charged residues, as well as basic residues flanking the two boxes, were necessary for both binding and packaging. Our results suggest that the stretches of positively charged residues within NC that need to be in a proper conformation appear to be responsible for selective recognition and binding to the packaging signal (Psi)-containing RNAs.

Isolation and characterisation of the rabies virus N degrees-P complex produced in insect cells

When the nucleoprotein (N) of nonsegmented negative-strand RNA viruses is expressed in insect cells, it binds to cellular RNA and forms N-RNA complexes just like viral nucleocapsids. However, in virus-infected cells, N is prevented from binding to cellular RNA because a soluble complex is formed between N and the viral phosphoprotein (P), the N degrees -P complex. N is only released from this complex for binding to newly made viral or complementary RNA. We coexpressed rabies virus N and P proteins in insect cells and purified the N degrees -P complex. Characterisation by gel filtration, polyacrylamide gel electrophoresis, analytical ultracentrifugation, native mass spectroscopy, and electron microscopy showed that the complex consists of one N protein plus two P proteins, i.e., an N degrees -P(2) complex.

Identification of a novel protein associated with envelope of occlusion-derived virus in Spodoptera litura multicapsid nucleopolyhedrovirus

Spodoptera litura multicapsid nucleopolyhedrovirus (SpltMNPV) ORF137 (Splt137) is one of 29 unique SpltMNPV ORFs. Splt137 has the potential to code for a polypeptide of 231 amino acid residues with predicted molecular weight of 27.5 kDa. Computer-assisted analysis of the predicted amino acid sequences of Splt137 protein showed 1 N-glycosylation site and 11 phosphorylation sites. For identification of Spit137, antibody was prepared by immunization of rabbits with purified Splt137 protein produced in Escherichia coli. This antibody was used to analyse Splt137 protein using Western blot. A 36-kDa protein was found both in the infected cells and envelope fractions of occlusion-derived virus (ODV) but could not be detected in the budded virus (BV). Tunicamycin treatment of SpltMNPV infected cells suggested that the 36-kDa protein had undergone N-glycosylation. Our data suggested that Splt137 protein was a novel envelope protein of ODV and might exist as a more complex form of 79-kDa protein in intact ODV. Further, transcriptional analysis with RT-PCR and 5' RACE analysis suggested that Splt137 might perform functions early and late in infection.

Functional replacement of nucleocapsid flanking regions by heterologous counterparts with divergent primary sequences: effects of chimeric nucleocapsid on the retroviral replication cycle

Nucleocapsid (NC) proteins in most retroviruses have a well-conserved Cys-His box(es) as well as more divergent flanking regions that are rich in basic residues. Mutations in the flanking regions can affect RNA packaging, virus assembly, and reverse transcription of the viral RNA. To gain a further understanding of the roles of NC flanking regions in the retroviral replication cycle, we generated and characterized chimeric gag-pol expression constructs derived from murine leukemia virus and spleen necrosis virus by replacing an NC flanking region from one virus with the counterpart from the other virus. We found that all four chimeras were able to generate virions, package viral RNA, and complete the viral replication cycle. Two chimeras had mild defects in virus assembly that correlated with a decrease in the isoelectric points of NCs, suggesting that the basic nature of NC is important in virus assembly. This finding indicates that, although the primary sequences of these flanking regions have little homology, the heterologous sequences are functional both as part of the Gag polyprotein and as processed NC protein.

Structure of isolated nucleocapsids from venezuelan equine encephalitis virus and implications for assembly and disassembly of enveloped virus

Venezuelan equine encephalitis virus (VEEV) is an important human and equine pathogen in the Americas, with widespread reoccurring epidemics extending from South America to the southern United States. Most troubling, VEEV has been made into a weapon by several countries and is currently restricted by the Centers for Disease Control and Prevention as a potential biological warfare and terrorism agent. To facilitate the development of antiviral compounds, the structure of the nucleocapsid isolated from VEEV has been determined by electron cryomicroscopy and image reconstruction and represents the first three-dimensional structure of a nucleocapsid isolated from a single-stranded enveloped RNA virus. The isolated VEEV nucleocapsid undergoes significant reorganization relative to its structure within VEEV. However, the isolated nucleocapsid clearly exhibits T=4 icosahedral symmetry, and its characteristic nucleocapsid hexons and pentons are preserved. The diameter of the isolated nucleocapsid is approximately 11.5% larger than that of the nucleocapsid within VEEV, with radial expansion being greatest near the hexons. Significantly, this is the first direct structural evidence showing that a simple enveloped virus undergoes large conformational changes during maturation, suggesting that the lipid bilayer and the transmembrane proteins of simple enveloped viruses provide the energy necessary to reorganize the nucleocapsid during maturation.