Events in the endoplasmic reticulum abrogate the temperature sensitivity of Sindbis virus mutant ts23

Structural differences observed in arboviruses of the alphavirus and flavivirus genera

Arthropod borne viruses have developed a complex life cycle adapted to alternate between insect and vertebrate hosts. These arthropod-borne viruses belong mainly to the families Togaviridae, Flaviviridae, and Bunyaviridae. This group of viruses contains many pathogens that cause febrile, hemorrhagic, and encephalitic disease or arthritic symptoms which can be persistent. It has been appreciated for many years that these viruses were evolutionarily adapted to function in the highly divergent cellular environments of both insect and mammalian phyla. These viruses are hybrid in nature, containing viral-encoded RNA and proteins which are glycosylated by the host and encapsulate viral nucleocapsids in the context of a host-derived membrane. From a structural perspective, these virus particles are macromolecular machines adapted in design to assemble into a packaging and delivery system for the virus genome and, only when associated with the conditions appropriate for a productive infection, to disassemble and deliver the RNA cargo. It was initially assumed that the structures of the virus from both hosts were equivalent. New evidence that alphaviruses and flaviviruses can exist in more than one conformation postenvelopment will be discussed in this review. The data are limited but should refocus the field of structural biology on the metastable nature of these viruses.

Helical virus particles formed from morphological subunits of a membrane containing icosahedral virus

The classic publication by Caspar and Klug in 1962 [Physical principles in the construction of regular viruses. Cold Spring Harbor Symp. Quant. Biol. 27:1-24.] has formed the basis of much research on virus assembly. Caspar and Klug predicted that a single virus morphological unit could form a two dimensional lattice composed of 6-fold arrays (primitive plane), a family of icosahedra of increasing triangulation numbers (T) and helical arrays of varying length. We have shown that icosahedral viruses of varying T numbers can be produced using Sindbis virus [Ferreira, D. F. et al. 2003. Morphological variants of Sindbis virus produced by a mutation in the capsid protein. Virology 307:54-66]. Other studies have shown that Sindbis glycoproteins can also form a 2-dimensional lattice confirming Caspar and Klug's prediction of the primitive plane as a biologically relevant structure [VonBonsdorff, C. H., and S. C. Harrison. 1978. Sindbis virus glycoproteins form a regular icosahedral surface lattice. J. Virol. 28:578]. In this study we have used mutations in the glycoproteins of membrane containing Sindbis virus to create helical-virus-like particles from the morphological subunits of a virus of icosahedral geometry. The resulting virus particles were examined for subunit organization and were determined to be constructed of only 6-fold rotational arrays of the virus glycoproteins. A model of the tubular virus particles created from the 6-fold rotational arrays of Sindbis virus confirmed the observed structure. These experiments show that a common morphological unit (the Sindbis E1-E2 heterodimer) can produce three different morphological entities of varying dimensions in a membrane-containing virus system.

A conserved region between the heptad repeats of paramyxovirus fusion proteins is critical for proper F protein folding

Paramyxoviruses are a diverse family that utilizes a fusion (F) protein to enter cells via fusion of the viral lipid bilayer with a target cell membrane. Although certain regions of the F protein are known to play critical roles in membrane fusion, the function of much of the protein remains unclear. Sequence alignment of a set of paramyxovirus F proteins and analysis utilizing Block Maker identified a region of conserved amino acid sequence in a large domain between the heptad repeats of F1, designated CBF1. We employed site-directed mutagenesis to analyze the function of completely conserved residues of CBF1 in both the simian virus 5 (SV5) and Hendra virus F proteins. The majority of CBF1 point mutants were deficient in homotrimer formation, proteolytic processing, and transport to the cell surface. For some SV5 F mutants, proteolytic cleavage and surface expression could be restored by expression at 30 degrees C, and varying levels of fusion promotion were observed at this temperature. In addition, the mutant SV5 F V402A displayed a hyperfusogenic phenotype at both 30 and 37 degrees C, indicating that this mutation allows for efficient fusion with only an extremely small amount of cleaved, active protein. The recently published prefusogenic structure of PIV5/SV5 F (Yin, H. S., et al. (2006) Nature 439, 38-44) indicates that residues within and flanking CBF1 interact with the fusion peptide domain. Together, these data suggest that CBF1-fusion peptide interactions are critical for the initial folding of paramyxovirus F proteins from this important viral family and can also modulate subsequent membrane fusion promotion.

Differential inhibition of cellular and Sindbis virus translation by brefeldin A

Brefeldin A is a macrolide compound that interferes with the secretory pathway and also affects protein synthesis in mammalian cells. As a result, this antibiotic impedes the maturation of viral glycoproteins of enveloped viruses and viral genome replication in several virus species. In the present work, we show that translation of subgenomic mRNA from Sindbis virus, which in contrast to cellular translation is resistant to brefeldin A after prolonged treatment. The phosphorylation of eIF2alpha as a result of brefeldin A treatment correlates with the inhibition of cellular translation, while late viral protein synthesis is resistant to this phosphorylation. The effect of brefeldin A on Sindbis virus replication was also examined using a Sindbis virus replicon. Although brefeldin A delayed viral RNA synthesis, translation by non-replicative viral RNAs was not affected, reinforcing the idea that brefeldin A delays viral RNA replication, but does not directly affect Sindbis virus protein synthesis.

Avian reovirus temperature-sensitive mutant tsA12 has a lesion in major core protein sigmaA and is defective in assembly

Members of our laboratory previously generated and described a set of avian reovirus (ARV) temperature-sensitive (ts) mutants and assigned 11 of them to 7 of the 10 expected recombination groups, named A through G (M. Patrick, R. Duncan, and K. M. Coombs, Virology 284:113-122, 2001). This report presents a more detailed analysis of two of these mutants (tsA12 and tsA146), which were previously assigned to recombination group A. The capacities of tsA12 and tsA146 to replicate at a variety of temperatures were determined. Morphological analyses indicated that cells infected with tsA12 at a nonpermissive temperature produced approximately 100-fold fewer particles than cells infected at a permissive temperature and accumulated core particles. Cells infected with tsA146 at a nonpermissive temperature also produced approximately 100-fold fewer particles, a larger proportion of which were intact virions. We crossed tsA12 with ARV strain 176 to generate reassortant clones and used them to map the temperature-sensitive lesion in tsA12 to the S2 gene. S2 encodes the major core protein sigmaA. Sequence analysis of the tsA12 S2 gene showed a single alteration, a cytosine-to-uracil transition, at nucleotide position 488. This alteration leads to a predicted amino acid change from proline to leucine at amino acid position 158 in the sigmaA protein. An analysis of the core crystal structure of the closely related mammalian reovirus suggested that the Leu(158) substitution in ARV sigmaA lies directly under the outer face of the sigmaA protein. This may cause a perturbation in sigmaA such that outer capsid proteins are incapable of condensing onto nascent cores. Thus, the ARV tsA12 mutant represents a novel assembly-defective orthoreovirus clone that may prove useful for delineating virus assembly.

Generation and genetic characterization of avian reovirus temperature-sensitive mutants

There currently is little known about the genetic and biological functions of avian reovirus (ARV), an atypical member of the family Reoviridae and the prototype of all nonenveloped viruses that induce syncytia formation. In this study, we created ARV temperature-sensitive (ts) mutants by chemical mutagenesis of ARV strain 138. We developed a novel efficiency of lysis (EOL) screening technique and used it and the classical efficiency of plating (EOP) assay to identify 17 ARV ts mutants. Pairwise mixed infections of these mutants and evaluation of recombinant progeny ts status led to their organization into seven recombination groups. This indicates that these new groups of mutants represent the majority of the ARV genome. To phenotypically characterize the ts mutants, progeny double-stranded RNA (dsRNA) produced at permissive and nonpermissive temperature was measured. Some mutants were capable of dsRNA synthesis at the restrictive temperature (RNA(+)), which indicates the effects of their ts lesions occur after RNA replication. Most mutants were RNA(-), which suggests their mutations affect stages in viral replication that precede progeny genome synthesis.

A single deletion in the membrane-proximal region of the Sindbis virus glycoprotein E2 endodomain blocks virus assembly

The envelopment of the Sindbis virus nucleocapsid in the modified cell plasma membrane involves a highly specific interaction between the capsid (C) protein and the endodomain of the E2 glycoprotein. We have previously identified a domain of the Sindbis virus C protein involved in binding to the E2 endodomain (H. Lee and D. T. Brown, Virology 202:390-400, 1994). The C-E2 binding domain resides in a hydrophobic cleft with C Y180 and W247 on opposing sides of the cleft. Structural modeling studies indicate that the E2 domain, which is proposed to bind the C protein (E2 398T, 399P, and 400Y), is located at a sufficient distance from the membrane to occupy the C protein binding cleft (S. Lee, K. E. Owen, H. K. Choi, H. Lee, G. Lu, G. Wengler, D. T. Brown, M. G. Rossmann, and R. J. Kuhn, Structure 4:531-541, 1996). To measure the critical spanning length of the E2 endodomain which positions the TPY domain into the putative C binding cleft, we have constructed a deletion mutant, DeltaK391, in which a nonconserved lysine (E2 K391) at the membrane-cytoplasm junction of the E2 tail has been deleted. This mutant was found to produce very low levels of virus from BHK-21 cells due to a defect in an unidentified step in nucleocapsid binding to the E2 endodomain. In contrast, DeltaK391 produced wild-type levels of virus from tissue-cultured mosquito cells. We propose that the phenotypic differences displayed by this mutant in the two diverse host cells arise from fundamental differences in the lipid composition of the insect cell membranes which affect the physical and structural properties of membranes and thereby virus assembly. The data suggest that these viruses have evolved properties adapted specifically for assembly in the diverse hosts in which they grow.

Oligomerization-dependent folding of the membrane fusion protein of Semliki Forest virus

The spikes of alphaviruses are composed of three copies of an E2-E1 heterodimer. The E1 protein possesses membrane fusion activity, and the E2 protein, or its precursor form, p62 (sometimes called PE2), controls this function. Both proteins are, together with the viral capsid protein, translated from a common C-p62-E1 coding unit. In an earlier study, we showed that the p62 protein of Semliki Forest virus (SFV) dimerizes rapidly and efficiently in the endoplasmic reticulum (ER) with the E1 protein originating from the same translation product (so-called heterodimerization in cis) (B.-U. Barth, J. M. Wahlberg, and H. Garoff, J. Cell Biol. 128:283-291, 1995). In the present work, we analyzed the ER translocation and folding efficiencies of the p62 and E1 proteins of SFV expressed from separate coding units versus a common one. We found that the separately expressed p62 protein translocated and folded almost as efficiently as when it was expressed from a common coding unit, whereas the independently expressed E1 protein was inefficient in both processes. In particular, we found that the majority of the translocated E1 chains were engaged in disulfide-linked aggregates. This result suggests that the E1 protein needs to form a complex with p62 to avoid aggregation. Further analyses of the E1 aggregation showed that it occurred very rapidly after E1 synthesis and could not be avoided significantly by the coexpression of an excess of p62 from a separate coding unit. These latter results suggest that the p62-E1 heterodimerization has to occur very soon after E1 synthesis and that this is possible only in a cis-directed reaction which follows the synthesis of p62 and E1 from a common coding unit. We propose that the p62 protein, whose synthesis precedes that of the E1 protein, remains in the translocon of the ER and awaits the completion of E1. This strategy enables the p62 protein to complex with the E1 protein immediately after the latter has been made and thereby to control (suppress) its fusion activity.

The formation of intramolecular disulfide bridges is required for induction of the Sindbis virus mutant ts23 phenotype

The Sindbis virus envelope protein spike is a hetero-oligomeric complex composed of a trimer of glycoprotein E1-E2 heterodimers. Spike assembly is a multistep process which occurs in the endoplasmic reticulum (ER) and is required for the export of E1 from the ER. PE2 (precursor to E2), however, can transit through the secretory pathway and be expressed at the cell surface in the absence of E1. Although oligomer formation does not appear to be required for the export of PE2, there is evidence that defects in E1 folding can affect PE2 transit from the ER. Temperature-sensitive mutant ts23 of Sindbis virus contains two amino acid substitutions in E1, while PE2 and capsid protein have the wild-type sequence; however, at the nonpermissive temperature, both E1 and PE2 are retained within the ER and can be isolated in protein aggregates with the molecular chaperone GRP78-BiP. We previously demonstrated that the temperature sensitivity for ts23 was lost as oligomer formation took place at the permissive temperature, suggesting that temperature sensitivity is initiated early in the process of viral spike assembly (M. Carleton and D. T. Brown, J. Virol. 70:952-959, 1996). Experiments described herein investigated the defects in envelope protein maturation that occur in ts23-infected cells and which result in retention of both envelope proteins in the ER. The data demonstrate that in ts23-infected cells incubated at the nonpermissive temperature, E1 folding is disrupted early after synthesis, resulting in the rapid incorporation of both E1 and PE2 into disulfide-stabilized aggregates. Furthermore, the aberrant E1 conformation which is responsible for induction of the ts phenotype requires the formation of intramolecular disulfide bridges formed prior to E1 association with PE2 and the completion of E1 folding.

Characterization of two temperature-sensitive mutants of coronavirus mouse hepatitis virus strain A59 with maturation defects in the spike protein

Two temperature-sensitive (ts) mutants of mouse hepatitis virus strain A59, ts43 and ts379, have been described previously to be ts in infectivity but unaffected in RNA synthesis (M. J. M. Koolen, A. D. M. E. Osterhaus, G. van Steenis, M. C. Horzinek, and B. A. M. van der Zeijst, Virology 125:393-402, 1983). We present a detailed analysis of the protein synthesis of the mutant viruses at the permissive (31 degrees C) and nonpermissive (39.5 degrees C) temperatures. It was found that synthesis of the nucleocapsid protein N and the membrane protein M of both viruses was insensitive to temperature. However, the surface protein S of both viruses was retained in the endoplasmic reticulum at the nonpermissive temperature. This was shown first by analysis of endoglycosidase H-treated and immunoprecipitated labeled S proteins. The mature Golgi form of S was not present at the nonpermissive temperature for the ts viruses, in contrast to wild-type (wt) virus. Second, gradient purification of immunoprecipitated S after pulse-chase labeling showed that only wt virus S was oligomerized. We conclude that the lack of oligomerization causes the retention of the ts S proteins in the endoplasmic reticulum. As a result, ts virus particles that were devoid of S were produced at the nonpermissive temperature. This result could be confirmed by biochemical analysis of purified virus particles and by electron microscopy.

Role of glycoprotein PE2 in formation and maturation of the Sindbis virus spike

Sindbis virus envelope assembly is a multistep process resulting in the maturation of a rigid, highly ordered T=4 icosahedral protein lattice containing 80 spikes composed of trimers of E1-E2 heterodimers. Intramolecular disulfide bonds within E1 stabilize E1-E1 associations required for envelope formation and maintenance of the envelope's structural integrity. The structural integrity of the envelope protein lattice is resistant to reduction by dithiothreitol (DTT), indicating that E1 disulfides which stabilize structural domains become inaccessible to DTT at some point during virus maturation. The development of E1 resistance to DTT occurs prior to the completion of E1 folding and is temporally correlated with spike assembly in the endoplasmic reticulum. From these data we have predicted that in the final stages of spike assembly, E1 intramolecular disulfides, which stabilize the structural integrity of the envelope protein lattice, are buried within the spike and become inaccessible to the reductive activity of DTT. The spike is formed prior to the completion of E1 folding, and we have suggested that PE2 (the precursor to E2) may play a critical role in E1 folding after PE2-E1 oligomer formation has occurred. In this study we have investigated the role of PE2 in E1 folding, oligomer formation, and development of E1 resistance to both protease digestion and reduction by DTT by using a Sindbis virus replicon (SINrep/E1) which allows for the expression of E1 in the presence of truncated PE2. Through pulse-chase analysis of both Sindbis virus- and SINrep/E1-infected cells, we have determined that the folding of E1 into a trypsin-resistant conformation and into its most compact and stable form is not dependent upon association of E1 with PE2. However, E1 association with PE2 is required for oligomer formation, the export of E1 from the endoplasmic reticulum, and E1 acquisition of resistance to DTT.

Disulfide bridge-mediated folding of Sindbis virus glycoproteins

The Sindbis virus envelope is composed of 80 E1-E2 (envelope glycoprotein) heterotrimers organized into an icosahedral protein lattice with T=4 symmetry. The structural integrity of the envelope protein lattice is maintained by E1-E1 interactions which are stabilized by intramolecular disulfide bonds. Structural domains of the envelope proteins sustain the envelope's icosahedral lattice, while functional domains are responsible for virus attachment and membrane fusion. We have previously shown that within the mature Sindbis virus particle, the structural domains of the envelope proteins are significantly more resistant to the membrane-permeative, sulfhydryl-reducing agent dithiothreitol (DTT) than are the functional domains (R. P. Anthony, A. M. Paredes, and D. T. Brown, Virology 190:330-336, 1992). We have used DTT to probe the accessibility of intramolecular disulfides within PE2 (the precursor to E2) and E1, as these proteins fold and are assembled into the spike heterotrimer. We have determined through pulse-chase analysis that intramolecular disulfide bonds within PE2 are always sensitive to DTT when the glycoproteins are in the endoplasmic reticulum. The reduction of these disulfides results in the disruption of PE2-E1 associations. E1 acquires increased resistance to DTT as it folds through a series of disulfide intermediates (E1alpha, -beta, and -gamma) prior to assuming its native and most compact conformation (E1epsilon). The transition from a DTT-sensitive form into a form which exhibits increased resistance to DTT occurs after E1 has folded into its E1beta conformation and correlates temporally with the dissociation of BiP-E1 complexes and the formation of PE2-E1 heterotrimers. We propose that the disulfide bonds within E1 which stabilize the protein domains required for maintaining the structural integrity of the envelope protein lattice form early within the folding pathway of E1 and become inaccessible to DTT once the heterotrimer has formed.