The cytoplasmic domain of alphavirus E2 glycoprotein contains a short linear recognition signal required for viral budding

Monoclonal antibody targeting chikungunya virus envelope 1 protein inhibits virus release

Chikungunya virus (CHIKV) causes an acute clinical illness characterized by sudden high fever, intense joint pain, and skin rash. Recent outbreaks of chikungunya disease in Africa and Asia are a major public health concern; however, there is currently no effective licensed vaccine or specific treatment. This study reported the development of a mouse monoclonal antibody (MAb), CK47, which recognizes domain III within the viral envelope 1 protein and inhibited the viral release process, thereby preventing the production of progeny virus. The MAb had no effect on virus entry and replication processes. Thus, CK47 may be a useful tool for studying the mechanisms underlying CHIKV release and may show potential as a therapeutic agent.

Crystal structure of aura virus capsid protease and its complex with dioxane: new insights into capsid-glycoprotein molecular contacts

The nucleocapsid core interaction with endodomains of glycoproteins plays a critical role in the alphavirus life cycle that is essential to virus budding. Recent cryo-electron microscopy (cryo-EM) studies provide structural insights into key interactions between capsid protein (CP) and trans-membrane glycoproteins E1 and E2. CP possesses a chymotrypsin-like fold with a hydrophobic pocket at the surface responsible for interaction with glycoproteins. In the present study, crystal structures of the protease domain of CP from Aura virus and its complex with dioxane were determined at 1.81 and 1.98 Å resolution respectively. Due to the absence of crystal structures, homology models of E1 and E2 from Aura virus were generated. The crystal structure of CP and structural models of E1 and E2 were fitted into the cryo-EM density map of Venezuelan equine encephalitis virus (VEEV) for detailed analysis of CP-glycoprotein interactions. Structural analysis revealed that the E2 endodomain consists of a helix-loop-helix motif where the loop region fits into the hydrophobic pocket of CP. Our studies suggest that Cys397, Cys418 and Tyr401 residues of E2 are involved in stabilizing the structure of E2 endodomain. Density map fitting analysis revealed that Pro405, a conserved E2 residue is present in the loop region of the E2 endodomain helix-loop-helix structure and makes intermolecular hydrophobic contacts with the capsid. In the Aura virus capsid protease (AVCP)-dioxane complex structure, dioxane occupies the hydrophobic pocket on CP and structurally mimics the hydrophobic pyrollidine ring of Pro405 in the loop region of E2.

Probing the early temporal and spatial interaction of the Sindbis virus capsid and E2 proteins with reverse genetics

A 7-Å cryoelectron microscopy-based reconstruction of Sindbis virus (SINV) was recently generated. Fitting the crystal structure of the SINV capsid protein (Cp) into the density map revealed that the F2-G2 loop of the Cp was shifted away from cytoplasmic domain of E2 (cdE2) in the 7-Å reconstruction relative to its position in the Cp crystal structure. Furthermore, the reconstruction demonstrated that residue E395 in region I of the cytoplasmic domain of the E2 envelope protein (cdE2-RI) and K252 of Cp, part of the Cp F2-G2 loop, formed a putative salt bridge in the virion. We generated amino acid substitutions at residues K250 and K252 of the SINV Cp and explored the resulting phenotypes. In the context of cells infected with wild-type or mutant virus, reversing the charge of these two residues resulted in the appearance of Cp aggregates around cytopathic vacuole type I (CPV-I) structures, the absence of nucleocapsid (NC) formation, and a lack of virus particle release in the infected mammalian cell. However, expressing the same Cp mutants in the cell without the envelope proteins or expressing and purifying the mutants from an Escherichia coli expression system and assembling in vitro yielded NC assembly in all cases. In addition, second-site mutations within cdE2 restored NC assembly but not release of infectious particles. Our data suggest an early temporal and spatial interaction between cdE2-RI and the Cp F2-G2 loop that, when ablated, leads to the absence of NC assembly. This interaction also appears to be important for budding of virus particles.

Interactions of the cytoplasmic domain of Sindbis virus E2 with nucleocapsid cores promote alphavirus budding

Alphavirus budding from the plasma membrane occurs through the specific interaction of the nucleocapsid core with the cytoplasmic domain of the E2 glycoprotein (cdE2). Structural studies of the Sindbis virus capsid protein (CP) have suggested that these critical interactions are mediated by the binding of cdE2 into a hydrophobic pocket in the CP. Several molecular genetic studies have implicated amino acids Y400 and L402 in cdE2 as important for the budding of alphaviruses. In this study, we characterized the role of cdE2 residues in structural polyprotein processing, glycoprotein transport, and capsid interactions. Along with hydrophobic residues, charged residues in the N terminus of cdE2 were critical for the effective interaction of cores with cdE2, a process required for virus budding. Mutations in the C-terminal signal sequence region of cdE2 affected E2 protein transport to the plasma membrane, while nonbudding mutants that were defective in cdE2-CP interaction accumulated E2 on the plasma membrane. The interaction of cdE2 with cytoplasmic cores purified from infected cells and in vitro-assembled core-like particles suggests that cdE2 interacts with assembled cores to mediate budding. We hypothesize that these cdE2 interactions induce a change in the organization of the nucleocapsid core upon binding leading to particle budding and priming of the nucleocapsid cores for disassembly that is required for virus infection.

Crystallization, high-resolution data collection and preliminary crystallographic analysis of Aura virus capsid protease and its complex with dioxane

The C-terminal protease domain of capsid protein from Aura virus expressed in a bacterial expression system has been purified to homogeneity and crystallized. Crystals suitable for X-ray diffraction analysis were obtained by the vapour-diffusion method using 0.1 M bis-tris and polyethylene glycol monomethyl ether 2000. Crystals of the C-terminal protease domain of capsid protein in complex with dioxane were also produced and crystal data were obtained. Both crystals belonged to space group C2, with unit-cell parameters a = 79.6, b = 35.2, c = 49.5 Å. High-resolution data sets were collected to a resolution of 1.81 Å for the native protein and 1.98 Å for the complex. Preliminary crystallographic studies suggested the presence of a single molecule in the crystallographic asymmetric unit, with a solvent content of 38.5%.

A conserved region in the prM protein is a critical determinant in the assembly of flavivirus particles

Flaviviruses are assembled to bud into the lumen of the endoplasmic reticulum (ER) and are secreted through the vesicle transport pathway, but the details of the molecular mechanism of virion assembly remain largely unknown. In this study, a highly conserved region in the prM protein was identified among flaviviruses. In the subviral particle (SP) system of tick-borne encephalitis virus (TBEV) and Japanese encephalitis virus, secretion of SPs was impaired by a mutation in the conserved region in the prM protein. Viral proteins were sparse in the Golgi complex and accumulated in the ER. Ultrastructural analysis revealed that long filamentous structures, rather than spherical SPs, were observed in the lumen of the ER as a result of the mutation. The production of infectious virions derived from infectious cDNA of TBEV was also reduced by mutations in the conserved region. Molecular modelling analysis suggested that the conserved region is important for the association of prM-envelope protein heterodimers in the formation of a spike of immature virion. These results are the first demonstration that the conserved region in the prM protein is a molecular determinant for the flavivirus assembly process.

Heterogeneous pathways of maternal-fetal transmission of human viruses (review)

Several viruses can pass the maternal-fetal barrier, and cause diseases of the fetus or the newborn. Recently, however, it became obvious, that viruses may invade fetal cells and organs through different routes without acute consequences. Spermatozoa, seminal fluid and lymphocytes in the sperm may transfer viruses into the human zygotes. Viruses were shown to be integrated into human chromosomes and transferred into fetal tissues. The regular maternal-fetal transport of maternal cells has also been discovered. This transport might implicate that lymphotropic viruses can be released into the fetal organs following cellular invasion. It has been shown that many viruses may replicate in human trophoblasts and syncytiotrophoblast cells thus passing the barrier of the maternal-fetal interface. The transport of viral immunocomplexes had also been suggested, and the possibility has been put forward that even anti-idiotypes mimicking viral epitopes might be transferred by natural mechanisms into the fetal plasma, in spite of the selective mechanisms of apical to basolateral transcytosis in syncytiotrophoblast and basolateral to apical transcytosis in fetal capillary endothelium. The mechanisms of maternal-fetal transcytosis seem to be different of those observed in differentiated cells and tissue cultures. Membrane fusion and lipid rafts of high cholesterol content are probably the main requirements of fetal transcytosis. The long term presence of viruses in fetal tissues and their interactions with the fetal immune system might result in post partum consequences as far as increased risk of the development of malignancies and chronic pathologic conditions are discussed.

A structural and functional perspective of alphavirus replication and assembly

Alphaviruses are small, spherical, enveloped, positive-sense ssRNA viruses responsible for a considerable number of human and animal diseases. Alphavirus members include Chikungunya virus, Sindbis virus, Semliki Forest virus, the western, eastern and Venezuelan equine encephalitis viruses, and the Ross River virus. Alphaviruses can cause arthritic diseases and encephalitis in humans and animals and continue to be a worldwide threat. The viruses are transmitted by blood-sucking arthropods, and replicate in both arthropod and vertebrate hosts. Alphaviruses form spherical particles (65-70 nm in diameter) with icosahedral symmetry and a triangulation number of four. The icosahedral structures of alphaviruses have been defined to very high resolutions by cryo-electron microscopy and crystallographic studies. In this review, we summarize the major events in alphavirus infection: entry, replication, assembly and budding. We focus on data acquired from structural and functional studies of the alphaviruses. These structural and functional data provide a broader perspective of the virus lifecycle and structure, and allow additional insight into these important viruses.

Oligomerization of hepatitis C virus core protein is crucial for interaction with the cytoplasmic domain of E1 envelope protein

Hepatitis C virus (HCV) contains two membrane-associated envelope glycoproteins, E1 and E2, which assemble as a heterodimer in the endoplasmic reticulum (ER). In this study, predictive algorithms and genetic analyses of deletion mutants and glycosylation site variants of the E1 glycoprotein were used to suggest that the glycoprotein can adopt two topologies in the ER membrane: the conventional type I membrane topology and a polytopic topology in which the protein spans the ER membrane twice with an intervening cytoplasmic loop (amino acid residues 288 to 360). We also demonstrate that the E1 glycoprotein is able to associate with the HCV core protein, but only upon oligomerization of the core protein in the presence of tRNA to form capsid-like structures. Yeast two-hybrid and immunoprecipitation analyses reveal that oligomerization of the core protein is promoted by amino acid residues 72 to 91 in the core. Furthermore, the association between the E1 glycoprotein and the assembled core can be recapitulated using a fusion protein containing the putative cytoplasmic loop of the E1 glycoprotein. This fusion protein is also able to compete with the intact E1 glycoprotein for binding to the core. Mutagenesis of the cytoplasmic loop of E1 was used to define a region of four amino acids (residues 312 to 315) that is important for interaction with the assembled HCV core. Taken together, our studies suggest that interaction between the self-oligomerized HCV core and the E1 glycoprotein is mediated through the cytoplasmic loop present in a polytopic form of the E1 glycoprotein.

Association of sindbis virus capsid protein with phospholipid membranes and the E2 glycoprotein: implications for alphavirus assembly

A late stage in assembly of alphaviruses within infected cells is thought to be directed by interactions between the nucleocapsid and the cytoplasmic domain of the E2 protein, a component of the viral E1/E2 glycoprotein complex that is embedded in the plasma membrane. Recognition between the nucleocapsid protein and the E2 protein was explored in solution using NMR spectroscopy, as well as in binding assays using a model phospholipid membrane system that incorporated a variety of Sindbis virus E2 cytoplasmic domain (cdE2) and capsid protein constructs. In these binding assays, synthetic cdE2 peptides were reconstituted into phospholipid vesicles to simulate the presentation of cdE2 on the inner leaflet of the plasma membrane. Results from these binding assays showed a direct interaction between a peptide containing the C-terminal 16 amino acids of the cdE2 sequence and a Sindbis virus capsid protein construct containing amino acids 19-264. Additional experiments that probed the sequence specificity of this cdE2-capsid interaction are also described. Further binding assays demonstrated an interaction between the 19-264 capsid protein and artificial vesicles containing neutral or negatively charged phospholipids, while capsid protein constructs with N-terminal truncations displayed either little or no affinity for such vesicles. The membrane-binding property of the capsid protein suggests that the membrane may play an active role in alphavirus assembly. The results are consistent with an assembly process involving an initial membrane association, whereby an association with E2 glycoprotein further enhances capsid binding to facilitate membrane envelopment of the nucleocapsid for budding. Collectively, these experiments elucidate certain requirements for the binding of Sindbis virus capsid protein to the cytoplasmic domain of the E2 glycoprotein, a critical event in the alphavirus maturation pathway.

Rhabdovirus assembly and budding

Rhabdoviruses are a diverse, widely-distributed group of enveloped viruses that assemble and bud from the plasma membrane of host cells. Recent advances in the identification of domains on both the envelope glycoprotein and the matrix protein of rhabdoviruses that contribute to virus assembly and release have allowed us to refine current models of rhabdovirus budding and to describe in better detail the interplay between both viral and cellular components involved in the budding process. In this review we discuss the steps involved in rhabdovirus assembly beginning with genome encapsidation and the association of nucleocapsid-matrix protein pre-assembly complexes with the inner leaflet of the plasma membrane, how condensation of these complexes may occur, how microdomains containing the envelope glycoprotein facilitate bud site formation, and how multiple forms of the matrix protein may participate in virion extrusion and release.

Single point mutation in tick-borne encephalitis virus prM protein induces a reduction of virus particle secretion

Flaviviruses are assembled to bud into the lumen of the endoplasmic reticulum (ER) and are secreted through the vesicle transport pathway. Virus envelope proteins play important roles in this process. In this study, the effect of mutations in the envelope proteins of tick-borne encephalitis (TBE) virus on secretion of virus-like particles (VLPs), using a recombinant plasmid expression system was analysed. It was found that a single point mutation at position 63 in prM induces a reduction in secretion of VLPs. The mutation in prM did not affect the folding of the envelope proteins, and chaperone-like activity of prM was maintained. As observed by immunofluorescence microscopy, viral envelope proteins with the mutation in prM were scarce in the Golgi complex, and accumulated in the ER. Electron microscopic analysis of cells expressing the mutated prM revealed that many tubular structures were present in the lumen. The insertion of the prM mutation at aa 63 into the viral genome reduced the production of infectious virus particles. This data suggest that prM plays a crucial role in the virus budding process.

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.

Venezuelan equine encephalomyelitis virus structure and its divergence from old world alphaviruses

Although alphaviruses have been extensively studied as model systems for the structural organization of enveloped viruses, no structures exist for the phylogenetically distinct eastern equine encephalomyelitis (EEE)-Venezuelan equine encephalomyelitis (VEE) lineage of New World alphaviruses. Here we report the 25-A structure of VEE virus, obtained from electron cryomicroscopy and image reconstruction. The envelope spike glycoproteins of VEE virus have a T=4 icosahedral arrangement, similar to that observed in Old World Sindbis, Semliki Forest, and Ross River alphaviruses. However, VEE virus has pronounced differences in its nucleocapsid structure relative to nucleocapsid structures repeatedly observed in Old World alphaviruses.

Identification of the glycoprotein 41(TM) cytoplasmic tail domains of human immunodeficiency virus type 1 that interact with Pr55Gag particles

We investigated the protein/protein interactions that occur during human immunodeficiency virus (HIV-1) budding. We evaluated the binding to Pr55Gag particles of peptides mapping to the cytoplasmic tail of gp41TM and of host-cell proteins, in a cell-free, in vitro assay. Host-cell proteins and irrelevant viral envelope peptides did not bind. Peptides corresponding to a large central domain of the gp41TM cytoplasmic tail (93 residues) bound to Pr55Gag particles. This demonstrates that a Gag/Env interaction is responsible for the specific incorporation of the Env glycoprotein into nascent HIV-1 virions, and defines more accurately the gp41TM domain involved in this interaction.

The membrane-proximal stem region of vesicular stomatitis virus G protein confers efficient virus assembly

In this report, we show that the glycoprotein of vesicular stomatitis virus (VSV G) contains within its extracellular membrane-proximal stem (GS) a domain that is required for efficient VSV budding. To determine a minimal sequence in GS that provides for high-level virus assembly, we have generated a series of recombinant DeltaG-VSVs which express chimeric glycoproteins having truncated stem sequences. The recombinant viruses having chimeras with 12 or more membrane-proximal residues of the G stem, and including the G protein transmembrane-cytoplasmic tail domains, produced near-wild-type levels of particles. In contrast, viruses encoding chimeras with shorter or no G-stem sequences produced approximately 10- to 20-fold less. This budding domain when present in chimeric glycoproteins also promoted their incorporation into the VSV envelope. We suggest that the G-stem budding domain promotes virus release by inducing membrane curvature at sites where virus budding occurs or by recruiting condensed nucleocapsids to sites on the plasma membrane which are competent for efficient virus budding.

Role of rubella virus glycoprotein domains in assembly of virus-like particles

Rubella virus is a small enveloped positive-strand RNA virus that assembles on intracellular membranes in a variety of cell types. The virus structural proteins contain all of the information necessary to mediate the assembly of virus-like particles in the Golgi complex. We have recently identified intracellular retention signals within the two viral envelope glycoproteins. E2 contains a Golgi retention signal in its transmembrane domain, whereas a signal for retention in the endoplasmic reticulum has been localized to the transmembrane and cytoplasmic domains of E1 (T. C. Hobman, L. Woodward, and M. G. Farquhar, Mol. Biol. Cell 6:7-20, 1995; T. C. Hobman, H. F. Lemon, and K. Jewell, J. Virol. 71:7670-7680, 1997). In the present study, we have analyzed the role of these retention signals in the assembly of rubella virus-like particles. Deletion or replacement of these domains with analogous regions from other type I membrane glycoproteins resulted in failure of rubella virus-like particles to be secreted from transfected cells. The E1 transmembrane and cytoplasmic domains were not required for targeting of the structural proteins to the Golgi complex and, surprisingly, assembly and budding of virus particles into the lumen of this organelle; however, the resultant particles were not secreted. In contrast, replacement or alteration of the E2 transmembrane or cytoplasmic domain, respectively, abrogated the targeting of the structural proteins to the budding site, and consequently, no virion formation was observed. These results indicate that the transmembrane and cytoplasmic domains of E2 and E1 are required for early and late steps respectively in the viral assembly pathway and that rubella virus morphogenesis is very different from that of the structurally similar alphaviruses.

Cytoplasmic domain of Sendai virus HN protein contains a specific sequence required for its incorporation into virions

In the assembly of paramyxoviruses, interactions between viral proteins are presumed to be specific. The focus of this study is to elucidate the protein-protein interactions during the final stage of viral assembly that result in the incorporation of the viral envelope proteins into virions. To this end, we examined the specificity of HN incorporation into progeny virions by transiently transfecting HN cDNA genes into Sendai virus (SV)-infected cells. SV HN expressed from cDNA was efficiently incorporated into progeny Sendai virions, whereas Newcastle disease virus (NDV) HN was not. This observation supports the theory of a selective mechanism for HN incorporation. To identify the region on HN responsible for the selective incorporation, we constructed chimeric SV and NDV HN cDNAs and evaluated the incorporation of expressed proteins into progeny virions. Chimera HN that contained the SV cytoplasmic domain fused to the transmembrane and external domains of the NDV HN was incorporated to SV particles, indicating that amino acids in the cytoplasmic domain are responsible for the observed specificity. Additional experiments using the chimeric HNs showed that 14 N-terminal amino acids are sufficient for the specificity. Further analysis identified five consecutive amino acids (residues 10 to 14) that were required for the specific incorporation of HN into SV. These residues are conserved among all strains of SV as well as those of its counterpart, human parainfluenza virus type 1. These results suggest that this region near the N terminus of HN interacts with another viral protein(s) to lead to the specific incorporation of HN into progeny virions.

Mutations in the Sindbis virus capsid gene can partially suppress mutations in the cytoplasmic domain of the virus E2 glycoprotein spike

Assembly and budding of alphaviruses are postulated to occur by protein-protein interactions between sites on the cytoplasmic domain of the transmembranal envelope E2 glycoprotein and on the surface of the nucleocapsid protein subunits. Genetic data to support this model have been obtained by isolating revertants of two slow-growth mutants of Sindbis virus and analyzing the sequences of the genes encoding their structural proteins. The slow-growth phenotypes of the mutants were previously shown to result from site-directed mutations of 2 amino acids in the sequence corresponding to the 33 amino acids at the carboxyl terminus of E2, which are localized to the cytoplasmic face of the plasma membrane. Putative revertants of these two mutants with faster growth rates were isolated by sequential passaging of virus grown on insect cells or chicken embryo fibroblasts. Sequence analysis of plaque-purified viruses that grew significantly better than the original mutant revealed that the original E2 mutation was present and that there were additional amino acid changes in the virus capsid. Two of the latter were introduced separately into the wild-type virus cDNA and into the genomes of the original mutants. The new strains of virus that contained both capsid and E2 mutations produced many more extracellular particles than those with the E2 mutations alone, indicating substantial suppression of the original E2 mutation. Both capsid mutations appear to be localized near a hydrophobic pocket of the capsid, which is postulated to be the site for docking of hydrophobic amino acids of the E2 cytoplasmic domain. This genetic study provides strong support for the current models of alphavirus assembly.

Pleiotropic Effects of Small Peptides Corresponding in Sequence to the Cytoplasmic Domain of the Influenza Virus Haemagglutinin on Influenza, Vesicular Stomatitis and Sindbis Viruses

Studies on the antiviral effects of short peptides of six to 10 amino acids that correspond in sequence to the cytoplasmic domains of enveloped virus transmembrane glycoproteins have been extended to include additional kinds of assay in order to determine a site for inhibition of virus replication. Based on these experiments, the antiviral activity previously described for a decapeptide with the influenza virus haemagglutinin HA2C-terminal sequence was not specific for influenza virus and the integrity of newly released, extracellular vesicular stomatitis virus particles was affected by the peptide. A shortened, six amino acid form of this peptide inactivated cell-free preparations of influenza, vesicular stomatitis and Sindbis viruses and also bound effectively to virus-encoded structural proteins. For this virus-protein interaction, the peptide sequence was highly specific with respect to its hydrophobicity and net ionic charge.

Alphavirus budding is dependent on the interaction between the nucleocapsid and hydrophobic amino acids on the cytoplasmic domain of the E2 envelope glycoprotein

The interaction between the nucleocapsid core and the glycoprotein spikes is a critical component in the budding process of alphaviruses. A molecular model was previously proposed which suggested that this interaction was mediated by the binding of the cytoplasmic domain of glycoprotein E2 into a hydrophobic pocket found on the surface of the nucleocapsid protein [S. Lee, K. E. Owen, H.-K. Choi, H. Lee, G. Lu, G. Wengler, D. T. Brown, M. G. Rossmann, and R. J. Kuhn (1996) Structure 4, 531-541; U. Skoging, M. Vihinen, L. Nilsson, and P. Liljeström (1996) Structure 4, 519-529]. Two hydrophobic amino acids in the cytoplasmic domain of E2 were predicted to be important in the contact between the proteins. One of the residues, Y400 (Sindbis virus numbering), had previously been shown by mutational studies to be important in the budding of Semliki Forest virus [H. Zhao, B. Lindqvist, H. Garoff, C. H. von Bonsdorf, and P. Liljeström (1994) EMBO J. 13, 4204-4211]. The role of the second residue, L402, had not been examined. By creating a panel of amino acid substitutions at this residue, followed by phenotypic analysis of rescued mutant viruses, we now show that L402 is critical for the production of Sindbis virus. Substitutions at this amino acid inhibit budding, and the data suggest the L402 plays an important role in the interaction, between the glycoprotein and the nucleocapsid core. These data support the model and suggest that the proposed molecular interactions are important for the budding of alphaviruses from the cell.

Identification of a protein binding site on the surface of the alphavirus nucleocapsid and its implication in virus assembly

BACKGROUND

Many enveloped viruses exit cells by budding from the plasma membrane. The driving force for budding is the interaction of an inner protein nucleocapsid core with transmembrane glycoprotein spikes. The molecular details of this process are ill defined. Alphaviruses, such as Sindbis virus (SINV) and Semliki Forest virus (SFV), represent some of the simplest enveloped viruses and have been well characterized by structural, genetic and biochemical techniques. Although a high-resolution structure of an alphavirus has not yet been attained, cryo-electron microscopy (cryo-EM) has been used to show the multilayer organization at 25 A resolution. In addition, atomic resolution studies are available of the C-terminal domain of the nucleocapsid protein and this has been modeled into the cryo-EM density.

RESULTS

A recombinant form of Sindbis virus core protein (SCP) was crystallized and found to diffract much better than protein extracted from the virus (2.0 A versus 3.0 A resolution). The new structure showed that amino acids 108 to 111 bind to a specific hydrophobic pocket in neighboring molecules. Re-examination of the structures derived from virus-extracted protein also showed this 'N-terminal arm' binding to the same hydrophobic pocked in adjacent molecules. It is proposed that the binding of these capsid residues into the hydrophobic pocket of SCP mimics the binding of E2 (one of two glycoproteins that penetrate the lipid bilayer of the viral envelope) C-terminal residues in the pocket. Mutational studies of capsid residues 108 and 110 confirm their role in capsid assembly.

CONCLUSIONS

Structural and mutational analyses of residues within the hydrophobic pocket suggest that budding results in a switch between two conformations of the capsid hydrophobic pocket. This is the first description of a viral budding mechanism in molecular detail.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

Semliki Forest virus core protein fragmentation: its possible role in nucleocapsid disassembly

Semliki Forest virus (SFV) envelope proteins function as proton pores under mildly acidic conditions and translocate protons across the viral membrane [Schlegel, A., Omar, A., Jentsch, P., Morell, A. and Kemp, F. C. (1991) Biosci. Rep. 11, 243-255]. As a consequence, during uptake of SFV by cells via receptor-mediated endocytosis the nucleocapsid is supposed to be exposed to protons. In this paper the effects of mildly acidic pH on SFV nucleocapsids were examined. A partial proteolytic fragmentation of core proteins was observed when nucleocapsids were exposed to mildly acidic pH. A similar proteolytic event was detected when intact SFV virions were exposed to identical conditions. Protease protection assays with exogenous bromelain provided evidence that the capsid protein degradation was due to an endogenous proteolytic activity and not to a proteolytic contamination. Detergent solubilization of virus particles containing degraded nucleocapsids followed by sucrose gradient centrifugation led to a separation of capsid protein fragments and remaining nucleocapsids. These data are discussed in terms of a putative biological significance, namely that the core protein fragmentation may play a role in nucleocapsid disassembly.

Analysis of a human immunodeficiency virus type 1 isolate carrying a truncated transmembrane glycoprotein

We have recently reported the isolation of a human immunodeficiency virus type 1 (HIV-1), KB-1gp32 carrying a shorter size (32 kDa) of transmembrane glycoprotein (TMP) from TALL-1 cells persistently infected with KB-1gp41 virus strain (Shimizu et al., 1990a). Endoglycosidase treatments showed that the different size of the TMP between the two strains was due to a truncation of 9 kDa of polypeptide in the KB-1gp32 TMP coding region. Sequence analysis revealed the substitution of a CAG codon to a TAG stop codon just downstream of the putative membrane-spanning domain of the TMP of KB-1gp32. This resulted in a truncation of some 133 amino acids of the cytoplasmic domain of TMP. The data indicate that a premature stop codon in KB-1gp32 has been introduced during adaption of the parental virus to TALL-1 cells. We have constructed two chimeric clones between the env region of a clone pKB-1, derived from KB-1gp32, and an infectious molecular clone pNL-432. We have also constructed a site-directed mutant of pNL-432 carrying a premature stop codon at the same position as the env stop codon of pKB-1. Among the three clones carrying a premature stop codon in env, only one chimeric clone was infectious to TALL-1 but not MT-2 cells. This clone contained the entire tat, rev, vpu, and env genes of pKB-1. The pNL-432 mutant was not infectious. The results suggest that some sequences of pKB-1 might compensate for the truncation of the TMP during replication in TALL-1 cells.