Proteolytic processing of the Sindbis virus membrane protein precursor PE2 is nonessential for growth in vertebrate cells but is required for efficient growth in invertebrate cells

Chikungunya virus host range E2 transmembrane deletion mutants induce protective immunity against challenge in C57BL/6J mice

A vaccine against Chikungunya virus (ChikV), a reemerging pathogenic arbovirus, has been made by attenuating wild-type (WT) virus via truncation of the transmembrane domain (TMD) of E2 and selecting for host range (HR) mutants. Mice are a standard model system for ChikV disease and display the same symptoms of the disease seen in humans. Groups of mice were inoculated with one of three ChikV HR mutants to determine the ability of each mutant strain to elicit neutralizing antibody and protective immunity upon virus challenge. One mutant, ChikV TM17-2, fulfilled the criteria for a good vaccine candidate. It displayed no reactogenicity at the site of injection, no tissue disease in the foot/ankle and quadriceps, and no evidence of viral persistence in foot/ankle tissues 21 days after infection. Upon challenge with a highly pathogenic strain of ChikV, the mutant blocked viral replication in all tissues tested. This study identified a ChikV HR mutant that grows to high levels in insect cells but was restricted in the ability to assemble virus in mammalian cells in vitro. The study demonstrates that these HR strains are attenuated in the mammalian host and warrant further development as live-attenuated vaccine strains.

Virus maturation

We examine virus maturation of selected nonenveloped and enveloped single-stranded RNA viruses, retroviruses, bacteriophages, and herpesviruses. Processes associated with maturation in the RNA viruses range from subtle (nodaviruses and picornaviruses) to dramatic (tetraviruses and togaviruses). The elaborate assembly and maturation pathway of HIV is discussed in contrast to the less sophisticated but highly efficient processes associated with togaviruses. Bacteriophage assembly and maturation are discussed in general terms, with specific examples chosen for emphasis. Finally the herpesviruses are compared with bacteriophages. The data support divergent evolution of nodaviruses, picornaviruses, and tetraviruses from a common ancestor and divergent evolution of alphaviruses and flaviviruses from a common ancestor. Likewise, bacteriophages and herpesviruses almost certainly share a common ancestor in their evolution. Comparing all the viruses, we conclude that maturation is a convergent process that is required to solve conflicting requirements in biological dynamics and function.

An arthropod enzyme, Dfurin1, and a vertebrate furin homolog display distinct cleavage site sequence preferences for a shared viral proprotein substrate

Alphaviruses replicate in vertebrate and arthropod cells and utilize a cellular enzyme called furin to process the PE2 glycoprotein precursor during virus replication in both cell types. Furin cleaves PE2 at a site immediately following a highly conserved four residue cleavage signal. Prior studies demonstrated that the amino acid immediately adjacent to the cleavage site influenced PE2 cleavage differently in vertebrate and mosquito cells (HW Heidner et al. 1996 . Journal of Virology 70: 2069-2073.). This finding was tentatively attributed to potential differences in the substrate specificities of the vertebrate and arthropod furin enzymes or to differences in the carbohydrate processing phenotypes of arthropod and vertebrate cells. To further address this issue, we evaluated Sindbis virus replication and PE2 cleavage in the Chinese hamster, Cricetulus griseus Milne-Edwards (Rodentia: Cricetidae) ovary cells (CHO-K1) and in a CHO-K1-derived furin-negative cell line (RPE.40) engineered to stably express the Dfurin1 enzyme of Drosophila melanogaster Meigen (Diptera: Drosophilidae). Expression of Dfurin1 enhanced Sindbis virus titers in RPE.40 cells by a factor of 10(2)-10(3), and this increase correlated with efficient cleavage of PE2. The PE2-cleavage phenotypes of viruses containing different amino acid substitutions adjacent to the furin cleavage site were compared in mosquito (C6/36), CHO-K1, and Dfurin1-expressing RPE.40 cells. This analysis confirmed that the substrate specificities of Dfurin1 and the putative mosquito furin homolog present in C6/36 cells are similar and suggested that the alternative PE2 cleavage phenotypes observed in vertebrate and arthropod cells were due to differences in substrate specificity between the arthropod and vertebrate furin enzymes and not to differences in host cell glycoprotein processing pathways.

Sindbis virus-associated pathology in Aedes albopictus (Diptera: Culicidae)

Virus dissemination and associated pathology were examined in Aedes albopictus after intrathoracic inoculation of Sindbis virus (SIN), the prototypic Alphavirus. At 10 days postinfection, virus RNA was detected in all three-body segments of the insect. Colocalization of virus antigen with structural pathology was observed in mosquito salivary glands and midgut-associated visceral muscles, representing yet another example of arbovirus-associated pathology in a mosquito host. SIN antigen and gross pathology were detected in lateral lobes, but not the medial lobe of salivary glands, whereas virus antigen, vacuolated cytoplasm, and myofilament misalignment were detected in the visceral muscles at the midgut exterior surface. Early in the midgut infection, virus antigen was localized in small foci on the organ surface that progressed to a grate work-like banding pattern that eventually cleared. Both the salivary glands and the midgut are essential to insect survival and reproduction. Additionally, these organs provide a pathway for virus transmission in nature. Although SIN infection may not shorten the mosquito life span, persistent coexistence could permit survival of both host and microbe as well as contribute to alterations in insect behavior.

PE2 cleavage mutants of Sindbis virus: correlation between viral infectivity and pH-dependent membrane fusion activation of the spike heterodimer

The spike glycoprotein E2 of Sindbis virus (SIN) is synthesized in the infected cell as a PE2 precursor protein, which matures through cleavage by a cellular furin-like protease. Previous work has shown that SIN mutants impaired in PE2 cleavage are noninfectious on BHK-21 cells, the block in infection being localized at a step after virus-receptor interaction but prior to RNA replication. Here, we studied the membrane fusion properties of SIN PE2 cleavage mutants and observed that these viruses are impaired in their ability to form an E1 homotrimer and to fuse with liposomes at a mildly acidic pH. The block in spike rearrangement and fusion could be overridden by exposure of the mutant viruses to very low pH (<4.5). Cleavage mutants with second-site resuscitating mutations in PE2 were highly infectious for BHK-21 cells. The ability of these viruses to form E1 homotrimers and to fuse at a mildly acidic pH was completely restored despite a sustained lack of PE2 cleavage.

Linkage of an alphavirus host-range restriction to the carbohydrate-processing phenotypes of the host cell

The Sindbis virus mutant NE2G216 retains PE2 in place of E2 in its virion structure. NE2G216 is a host-range mutant that replicates with near-normal kinetics in vertebrate cells, but displays severely restricted growth in cultured mosquito cells (C6/36) due to defects in the virus maturation process. In this study we tested the hypothesis that the host-range phenotype of NE2G216 was linked to the differences in carbohydrate-processing phenotypes between vertebrate and arthropod cells. Arthropod cell-derived glycoproteins are distinguishable from those synthesized in vertebrate cells by the absence of complex- and hybrid-type N-linked oligosaccharides. To test our hypothesis we compared the growth of the wild-type virus, TRSB, NE2G216 and three PE2-containing, C6/36 cell-adapted variants, in vertebrate cells treated with 1-deoxymannojirimycin (1-dMM). 1-dMM inhibits the Golgi alpha-mannosidase I enzyme and limits oligosaccharide processing to high-mannose forms (Man(8-9)GlcNAc(2)). The growth of TRSB was not restricted by the action of 1-dMM; however, NE2G216 was restricted in a dose-dependent manner. In contrast, the growth of each PE2-containing, C6/36 cell-adapted mutant was enhanced by low concentrations of 1-dMM (up to 1500%) and was only slightly affected by the higher concentrations. These results demonstrate that virion maturation functions of NE2G216 are sensitive to the structure of cis-linked oligosaccharides, and indicate that the carbohydrate-processing phenotypes of the host cell can influence viral host-range and function as a selective pressure in alphavirus evolution.

Endoproteolytic processing of the ebola virus envelope glycoprotein: cleavage is not required for function

Proteolytic processing is required for the activation of numerous viral glycoproteins. Here we show that the envelope glycoprotein from the Zaire strain of Ebola virus (Ebo-GP) is proteolytically processed into two subunits, GP1 and GP2, that are likely covalently associated through a disulfide linkage. Murine leukemia virions pseudotyped with Ebo-GP contain almost exclusively processed glycoprotein, indicating that this is the mature form of Ebo-GP. Mutational analysis identified a dibasic motif, reminiscent of furin-like protease processing sites, as the Ebo-GP cleavage site. However, analysis of Ebo-GP processing in LoVo cells that lack the proprotein convertase furin demonstrated that furin is not required for processing of Ebo-GP. In sharp contrast to other viral systems, we found that an uncleaved mutant of Ebo-GP was able to mediate infection of various cell lines as efficiently as the wild-type, proteolytically cleaved glycoprotein, indicating that cleavage is not required for the activation of Ebo-GP despite the conservation of a dibasic cleavage site in all filoviral envelope glycoproteins.

Suppressors of cleavage-site mutations in the p62 envelope protein of Semliki Forest virus reveal dynamics in spike structure and function

The E2 spike glycoprotein of Semliki Forest virus is produced as a p62 precursor protein, which is cleaved by host proteases to its mature form, E2. Cleavage is not necessary for particle formation or release but is necessary for infectivity. Previous results had shown that phenotypic revertants of cleavage-deficient p62 mutants are generated, and here we show that these may contain second-site suppressor mutations in the vicinity of the cleavage site. These hot-spot sites were mutated to abolish the generation of such suppressor mutations; however, secondary mutations in another distant domain of the E2 protein appeared instead, all of which still caused cleavage-deficient mutations. Such mutants grew very poorly and were inefficient in virus entry and release. The mutated sites define domains of the spike protein which probably interact to regulate its structure and function. Because of their highly attenuated phenotype and the lower probability of reversion, the new mutations close to the cleavage site were used to make new helper vectors for packaging of recombinant RNA into infectious particles, thus increasing further the biosafety of the vector system based on the Semliki Forest virus replicon.

Differential processing of sindbis virus glycoprotein PE2 in cultured vertebrate and arthropod cells

A step in the maturation of Sindbis virus glycoproteins is the cleavage of the precursor glycoprotein PE2 into E3 and E2 by furin or a furin-like host cell protease. The results presented here suggest that PE2 cleavage is an obligatory event for Sindbis virus maturation in C6/36 cells and demonstrate that certain mutants display a cell-specific PE2 cleavage phenotype. We previously have described Sindbis virus variants which fail to cleave PE2 because of incorporation of a signal for N-linked glycosylation immediately adjacent to the PE2 cleavage site but are viable in BHK-21 cells by virtue of an additional mutation at E2 216 or E2 191 (TRSB-NE2G216 and TRSB-NE2T191, respectively) (H. W. Heidner, K. L. McKnight, N. L. Davis, and R. E. Johnston, J. Virol. 68:2683-2692, 1994). Other viable PE2 cleavage-defective mutants were constructed by substituting the parental residue at E2 position 1 (Arg), with Leu or Val (TRSB-E2L1 and TRSB-E2V1, respectively) (H.W. Heidner and R. E. Johnston, J. Virol. 68:8064-8070, 1994). When grown in BHK-21 cells, all four of these viruses replicated normally and incorporated PE2 in place of E2 in released virions. However, growth of TRSB-NE2G216 and TRSB-NE2T191 was severely restricted in cultured arthropod cells (C6/36 cells). Analysis of infected C6/36 cells by flow cytometry demonstrated that the restricted growth of TRSB-NE2G216 and TRSB-NE2T191 was not due to an impaired ability to initiate infection. In addition, TRSB-NE2G216 and TRSB-NE2T191 remained growth restricted in C6/36 cells following introduction of in vitro transcriptions by electroporation. In contrast, the PE2 cleavage defect of TRSB-E2L1 and TRSB-E2V1 was cell type specific. In C6/36 cells, the majority of PE2 was converted to E2, and these viruses replicated normally in C6/36 cells. These results demonstrated a consistent link between expression of a PE2 cleavage defect and restricted growth in C6/36 cells and suggest that cleavage of PE2 is required for maturation of Sindbis virus late in infection of C6/36 cells.

Host-dependent evolution of the Sindbis virus promoter for subgenomic mRNA synthesis

Alphaviruses are alternately transmitted between arthropod and vertebrate hosts. In each host, the virus transcribes a subgenomic mRNA that encodes the viral structural proteins which encapsidate the genome to form progeny virions. Transcription initiates at an internal site called the promoter. To determine if promoter utilization varies in mammalian versus mosquito cells, we used these cells as hosts to select for active promoters among a library of different mutant promoters. Compared with that in BHK-21 cells, selection was more rapid in mosquito (C7-10) cells, with much less diversity of promoters remaining after fewer passages. Thus, promoter selection is host dependent. With further passaging, both BHK-21 and C7-10 cells selected for similar sequences that closely resemble the wild-type promoter sequence. The difference in the rates of selection is not because BHK-21-derived promoters cannot function in mosquito cells. Instead, part of the host dependence is probably due to posttranscriptional differences between BHK-21 and C7-10 cells that may require more active promoters in mosquito cells. Part of the host dependence may also be attributed to the decreased rate of transcription versus that of replication in mosquito cells. This change in regulation of subgenomic to genomic RNA synthesis appears to correlate with the extent of cleavage or pausing of the genomic RNA synthesis at or close to the promoter.

The amino-terminal residue of Sindbis virus glycoprotein E2 influences virus maturation, specific infectivity for BHK cells, and virulence in mice

The E2 glycoprotein of Sindbis virus is synthesized as a precursor, PE2, which is cleaved by furin or a furin-like host cell protease at a late stage of maturation. The four-residue PE2 cleavage signal conforms to the basic amino acid-X-basic-basic motif which is present in many other viral and cellular glycoproteins which are processed by the cellular enzyme(s). In this report, we present evidence that the amino acid which immediately follows the signal, the N-terminal residue of E2, can influence protease recognition, binding, and/or cleavage of PE2. Constructs encoding nine different amino acids at E2 position 1 (E2 1) were produced by site-directed mutagenesis of the full-length cDNA clone of our laboratory strain of Sindbis virus AR339 (pTRSB). Viruses derived from clones encoding Arg (TRSB), Asp, Ser, Phe, His, and Asn in a nonglycosylated form at E2 1 contained predominantly E2. Viruses encoding Ile, Leu, or Val at E2 1 contained the uncleaved form of PE2. The specific infectivity of TRSB (E2 Arg-1) for baby hamster kidney (BHK-21) cells was from 5- to greater than 100-fold higher than those of isogenic constructs with other residues at E2 1, suggesting that E2 Arg-1 represents a BHK-21 cell adaptive mutation in our laboratory strain. In newborn CD-1 mice, TRSB was more virulent than the PE2-containing viruses but less virulent than other PE2-cleaving viruses with alternative amino acids at E2 1. These results indicate that in TRSB, E2 Arg-1 increased the efficiency of virus-cell interactions in cultured BHK-21 cells but simultaneously decreased the ability of virus to mediate in vivo virus-cell interactions critical for the induction of disease. This suggests that the N terminus of E2 may participate in or be associated with virion domains which mediate these viral functions.

Visualization of fusion activation in the Semliki Forest virus spike

BACKGROUND

Viral spike proteins such as those of Semliki Forest virus (SFV) undergo a conformational change triggered by low pH which results in the fusion of the viral envelope with cellular membranes. The viral spike precursor of SFV is insensitive to low pH, and hence is fusion incompetent, until it is proteolytically cleaved to give the fusion competent mature form.

RESULTS

Three-dimensional image reconstructions from cryo-electron micrographs were used to compare the virion structure of wild-type SFV with that of a mutant SFV in which cleavage of the spike precursor had been blocked. Upon maturation to the fusion competent form, the spike undergoes a conformational change in which copies of the polypeptide containing the fusion sequence (E1) move from peripheral to lateral positions bringing them closer together.

CONCLUSIONS

This first visualization of the maturation of a viral spike protein complex suggests a mechanism for the conformational change which controls the fusion process.

Lethality of PE2 incorporation into Sindbis virus can be suppressed by second-site mutations in E3 and E2

Sindbis virions contain two glycoproteins, E1 and E2. E2 is produced initially as a precursor, PE2, from which the amino-terminal 64 amino acids are cleaved by a cellular protease at a late stage in virion maturation. A mutation at E2 position 1 (Arg to Asn) was placed into Sindbis virus AR339 by site-directed mutagenesis of a full-length AR339 cDNA clone, pTRSB, to produce pTRSB-N. The mutation created a signal for N-linked glycosylation immediately adjacent to the PE2 cleavage signal. Virions derived from pTRSB-N were glycosylated at E2 position 1, and they quantitatively incorporated PE2 in place of E2. When pTRSB-N transcripts were electroporated into BHK-21 cells, TRSB-N particles were released with nearly normal efficiency; however, the specific infectivity of TRSB-N particles was very low. Analysis of seven infectious revertants of TRSB-N revealed that reversion was linked to (i) mutations that eliminated the signal for N-linked glycosylation and thus restored the PE2 cleavage phenotype or (ii) conservation of the PE2 cleavage defect combined with incorporation of suppressor mutations in E3 or E2. The genotype of each revertant was reconstructed in the genetic background of TRSB-N, and each reverting mutation also was replaced individually into the genetic background of wild-type virus (TRSB). Each PE2-containing revertant was attenuated in newborn CD-1 mice and replicated poorly in cultured mosquito cells (C6/36). Reverting mutations in the genetic background of TRSB did not reduce virulence in mice or growth in mosquito cells, suggesting that the phenotypes of attenuation in mice and reduced growth in mosquito cells were linked to failure of PE2 cleavage and not to the reverting mutations themselves.

Temperature-sensitive steps in the transport of Semliki Forest virus envelope proteins in mosquito C6/36 cells

We have analysed the temperature dependence of the transport of Semliki Forest virus (SFV) envelope proteins in mosquito cells, the natural host cells of alphaviruses. These cells are cultivated at a lower temperature (28 degrees C) and have a different lipid composition as compared to mammalian cells. When the incubation temperature was reduced at early times after infection, the onset of virus shedding was delayed and the maximal titers decreased correspondingly to the temperature. No virus was shed at 12 degrees C. No evidence was observed for a block of virus release due to a shift of the sites of virus maturation. When the incubation temperature was reduced at later times after infection a critical temperature of 12 degrees C was again observed. At this temperature no transport of viral proteins took place, p62 remained uncleaved, the glycan processing of E1 did not occur and the envelope proteins accumulated in a pre-Golgi compartment. We suggest a mathematical formula which allows the extrapolation of transport data to the temperature at which intracellular protein transport becomes blocked.

Immunogens of encephalitis viruses

The equine encephalitis viruses are members of the genus Alphavirus, in the family Togaviridae. Three main virus serogroups represented by western (WEE), eastern (EEE) and Venezuelan equine encephalitis (VEE) viruses cause epizootic and enzootic infection of horses throughout the western hemisphere. All equine encephalitis viruses are transmitted through the bite of an infected mosquito. The first equine encephalitis virus vaccines were produced by virus inactivation. Problems with inadequate inactivation, which may have caused a major epidemic/epizootic of VEE in central America and Texas in the 1970s, led to the development of a live attenuated VEE virus vaccine (TC-83) derived by cell culture passage. Inactivated vaccines are still used to prevent equine infections with WEE and EEE viruses. Alphaviruses are small single stranded, positive sense RNA viruses. The 12000 nucleotide genome is enclosed in an icosahedral nucleocapsid composed of multiple copies of the capsid (C) protein. The virion is enveloped. The membrane is modified by the insertion of heterodimers of two glycoproteins: E1 and E2. Monoclonal antibody analysis of the surface glycoproteins have provided a detailed understanding of important protective antigens. Recent studies comparing gene sequences from virulent and avirulent VEE viruses have begun to delineate mechanisms of alphavirus attenuation.

Palmitoylation of Semliki Forest virus glycoproteins in insect cells (C6/36) occurs in an early compartment and is coupled to the cleavage of the precursor p62

The acylation of the envelope proteins of Semliki Forest virus by palmitic acid in infected mosquito (C6/36) cells was investigated. It is shown that in these cells palmitic acid was incorporated post-translationally via hydroxylamine-labile linkages onto cysteines in the inner domains of the viral envelope proteins. The kinetics of incorporation, however, differed considerably as compared to higher eukaryotic cells. (i) The precursor of the envelope proteins E2 and E3, p62, was weakly and incompletely palmitoylated irrespective of the duration of labeling. (ii) Under all conditions tested complete acylation of E2 was delayed as compared to E1. (iii) Heavy protein complexes were formed consisting of unacylated p62 and partially unacylated E1. From this data, we conclude that during the maturation of SFV glycoproteins in mosquito cells differently acylated intermediates of p62/E2 exist. Furthermore, acylation of p62/E2 and cleavage of p62 are coupled events, occurring in an early compartment and allowing the release of the envelope oligomers for transport.

The Murray Valley encephalitis virus prM protein confers acid resistance to virus particles and alters the expression of epitopes within the R2 domain of E glycoprotein

To study the role of the precursor to the membrane protein (prM) in flavivirus maturation, we inhibited the proteolytic processing of the Murray Valley encephalitis (MVE) virus prM to membrane protein in infected cells by adding the acidotropic agent ammonium chloride late in the virus replication cycle. Viruses purified from supernatants of ammonium chloride-treated cells contained prM protein and were unable to fuse C6/36 mosquito cells from without. When ammonium chloride was removed from the cells, both the processing of prM and the fusion activity of the purified viruses were partially restored. By using monoclonal antibodies (MAbs) specific for the envelope (E) glycoprotein of MVE virus, we found that at least three epitopes were less accessible to their corresponding antibodies in the prM-containing MVE virus particles. Amino-terminal sequencing of proteolytic fragments of the E protein which were reactive with sequence-specific peptide antisera or MAb enabled us to estimate the site of the E protein interacting with the prM to be within amino acids 200 to 327. Since prM-containing viruses were up to 400-fold more resistant to a low pH environment, we conclude that the E-prM interaction might be necessary to protect the E protein from irreversible conformational changes caused by maturation into the acidic vesicles of the exocytic pathway.

Two mutations in the envelope glycoprotein E2 of Semliki Forest virus affecting the maturation and entry patterns of the virus alter pathogenicity for mice

The prototype strain of Semliki Forest virus (SFV) of known sequence and virus produced by the cDNA clone derived from it were lethal following intranasal (i.n.) infection of 40-day-old and intraperitoneal (i.p.) infection of pregnant BALB/c mice; this lethality was related to neuronal necrosis in the central nervous system (CNS). We conclude that the virulence of the prototype strain, and virus from the cDNA clone derived from it, is similar to that of L10 (the original SFV isolate). The effects of two mutations in the p62 envelope protein region of the clone were determined. Substitution of Glu for Lys at position 162 (mut64) extended the mean time of death following i.n. inoculation of 40-day-old mice. Pregnant mice infected with this virus survived but lethal infection of some fetuses did occur. Substitution of Leu for Arg at position 66 (mL), the cleavage site of the E2 and E3 proteins, results in the production of particles containing uncleaved p62. These particles were less virulent than the prototype strain when inoculated i.n. and induced immunity to virulent SFV challenge. The virus also induced the formation of multifocal glial nodules in the CNS of surviving mice. The differences in pathogenicity between the two mutants and the virulent parental virus are probably related to differences in the efficiency of virus multiplication in infected mice. The mut64 mutation attenuated the virus and allowed survival of pregnant mice infected i.p. so that the effects of fetal infection could be detected. The mL mutation allowed survival of i.n.-infected mice so that the later effects of virus multiplication in the CNS could be assessed. In the former case, this is probably a result of reduced virus release, whereas in the latter case it is due to inefficient entry of host cells. The results are consistent with our previous suggestion that lethality for virulent SFV infection results from a lethal threshold of damage to neurons in the CNS and that attenuating mutations may reduce neuronal damage below this threshold level.