Mutational analysis of N-linked glycosylation sites of Friend murine leukemia virus envelope protein

Native structure of a retroviral envelope protein and its conformational change upon interaction with the target cell

Enveloped viruses enter their host cells by membrane fusion. The process of attachment and fusion in retroviruses is mediated by a single viral envelope glycoprotein (Env). Conformational changes of Env in the course of fusion are a focus of intense studies. Here we provide further insight into the changes occurring in retroviral Env during its initial interaction with the cell, employing murine leukemia virus (MLV) as model system. We first determined the structure of both natively membrane anchored MLV Env and MLV Env tagged with YFP in the proline rich region (PRR) by electron cryo tomography (cET) and sub-volume averaging. At a resolution of ∼20 Å, native MLV Env presents as a hollow trimer (height ∼85 Å, diameter ∼120 Å) composed of step-shaped protomers. The major difference to the YFP-tagged protein was in regions outside of the central trimer. Next, we focused on elucidating the changes in MLV Env upon interaction with a host cell. Virus interaction with the plasma membrane occurred over a large surface and Env clustering on the binding site was observed. Sub-volume averaging did yield a low-resolution structure of Env interacting with the cell, which had lost its threefold symmetry and was elongated by ∼35 Å in comparison to the unbound protein. This indicates a major rearrangement of Env upon host cell binding. At the site of virus interaction, the otherwise clearly defined bilayer structure of the host cell plasma membrane was much less evident, indicative of integral membrane protein accumulation and/or a change in membrane lipid composition.

Furin cleavage of the Moloney murine leukemia virus Env precursor reorganizes the spike structure

The trimeric Moloney murine leukemia virus Env protein matures by two proteolytic cleavages. First, furin cleaves the Env precursor into the surface (SU) and transmembrane (TM) subunits in the cell and then the viral protease cleaves the R-peptide from TM in new virus. Here we analyzed the structure of the furin precursor, by cryoelectron microscopy. We transfected 293T cells with a furin cleavage site provirus mutant, R466G/K468G, and produced the virus in the presence of amprenavir to also inhibit the R-peptide cleavage. Although Env incorporation into particles was inhibited, enough precursor could be isolated and analyzed by cryoelectron microscopy to yield a 3D structure at 22 Å resolution. This showed an open cage-like structure like that of the R-peptide precursor and the mature Env described before. However, the middle protrusion of the protomeric unit, so prominently pointing out from the side of the more mature forms of the Env, was absent. Instead, there was extra density in the top protrusion. This suggested that the C-terminal SU domain was associated alongside the receptor binding N-terminal SU domain in the furin precursor. This was supported by mapping with a SU C-terminal domain-specific antigen binding fragment. We concluded that furin cleavage not only separates the subunits and liberates the fusion peptide at the end of TM but also allows the C-terminal domain to relocate into a peripheral position. This conformational change might explain how the C-terminal domain of SU gains the potential to undergo disulfide isomerization, an event that facilitates membrane fusion.

Unique N-linked glycosylation of CasBrE Env influences its stability, processing, and viral infectivity but not its neurotoxicity

The envelope protein (Env) from the CasBrE murine leukemia virus (MLV) can cause acute spongiform neurodegeneration analogous to that induced by prions. Upon central nervous system (CNS) infection, Env is expressed as multiple isoforms owing to differential asparagine (N)-linked glycosylation. Because N-glycosylation can affect protein folding, stability, and quality control, we explored whether unique CasBrE Env glycosylation features could influence neurovirulence. CasBrE Env possesses 6/8 consensus MLV glycosylation sites (gs) but is missing gs3 and gs5 and contains a putative site (gs*). Twenty-nine mutants were generated by modifying these three sites, individually or in combination, to mimic the amino acid sequence in the nonneurovirulent Friend 57 MLV. Three basic viral phenotypes were observed: replication defective (dead; titer < 1 focus-forming unit [FFU]/ml), replication compromised (RC) (titer = 10(2) to 10(5) FFU/ml); and wild-type-like (WTL) (titer > 10(5) FFU/ml). Env protein was undetectable in dead mutants, while RC and WTL mutants showed variations in Env expression, processing, virus incorporation, virus entry, and virus spread. The newly introduced gs3 and gs5 sites were glycosylated, whereas gs* was not. Six WTL mutants tested in mice showed no clear attenuation in disease onset or severity versus controls. Furthermore, three RC viruses tested by neural stem cell (NSC)-mediated brainstem dissemination also induced acute spongiosis. Thus, while unique N-glycosylation affected structural features of Env involved in protein stability, proteolytic processing, and virus assembly and entry, these changes had minimal impact on CasBrE Env neurotoxicity. These findings suggest that the Env protein domains responsible for spongiogenesis represent highly stable elements upon which the more variable viral functional domains have evolved.

Naturally Occurring Polymorphisms of the Mouse Gammaretrovirus Receptors CAT-1 and XPR1 Alter Virus Tropism and Pathogenicity

Gammaretroviruses of several different host range subgroups have been isolated from laboratory mice. The ecotropic viruses infect mouse cells and rely on the host CAT-1 receptor. The xenotropic/polytropic viruses, and the related human-derived XMRV, can infect cells of other mammalian species and use the XPR1 receptor for entry. The coevolution of these viruses and their receptors in infected mouse populations provides a good example of how genetic conflicts can drive diversifying selection. Genetic and epigenetic variations in the virus envelope glycoproteins can result in altered host range and pathogenicity, and changes in the virus binding sites of the receptors are responsible for host restrictions that reduce virus entry or block it altogether. These battleground regions are marked by mutational changes that have produced 2 functionally distinct variants of the CAT-1 receptor and 5 variants of the XPR1 receptor in mice, as well as a diverse set of infectious viruses, and several endogenous retroviruses coopted by the host to interfere with entry.

Effects of retroviral envelope-protein cleavage upon trafficking, incorporation, and membrane fusion

Retroviral envelope glycoproteins undergo proteolytic processing by cellular subtilisin-like proprotein convertases at a polybasic amino-acid site in order to produce the two functional subunits, SU and TM. Most previous studies have indicated that envelope-protein cleavage is required for rendering the protein competent for promoting membrane fusion and for virus infectivity. We have investigated the role of proteolytic processing of the Moloney murine leukemia virus envelope-protein through site-directed mutagenesis of the residues near the SU-TM cleavage site and have established that uncleaved glycoprotein is unable either to be incorporated into virus particles efficiently or to induce membrane fusion. Additionally, the results suggest that cleavage of the envelope protein plays an important role in intracellular trafficking of protein via the cellular secretory pathway. Based on our results it was concluded that a positively charged residue located at either P2 or P4 along with the arginine at P1 is essential for cleavage.

Reconstitution of the ancestral glycoprotein of human endogenous retrovirus k and modulation of its functional activity by truncation of the cytoplasmic domain

Endogenous retroviruses present in the human genome provide a rich record of ancient infections. All presently recognized elements, including the youngest and most intact proviruses of the human endogenous retrovirus K(HML-2) [HERV-K(HML-2)] family, have suffered postinsertional mutations during their time of chromosomal residence, and genes encoding the envelope glycoprotein (Env) have not been spared these mutations. In this study, we have, for the first time, reconstituted an authentic Env of a HERV-K(HML-2) provirus by back mutation of putative postinsertional amino acid changes of the protein encoded by HERV-K113. Aided by codon-optimized expression, we demonstrate that the reconstituted Env regained its ability to be incorporated into retroviral particles and to mediate entry. The original ancient HERV-K113 Env was synthesized as a moderately glycosylated gp95 precursor protein cleaved into surface and transmembrane (TM) subunits. Of the nine N-linked oligosaccharides, four are part of the TM subunit, contributing 15 kDa to its apparent molecular mass of 41 kDa. The carbohydrates, as well as the cytoplasmic tail, are critical for efficient intracellular trafficking, processing, stability, and particle incorporation. Whereas deletions of the carboxy-terminal 6 residues completely abrogated cleavage and virion association, more extensive truncations slightly enhanced incorporation but dramatically increased the ability to mediate entry of pseudotyped lentiviruses. Although the first HERV-K(HML-2) elements infected human ancestors about 30 million years ago, our findings indicate that their glycoproteins are in most respects remarkably similar to those of classical contemporary retroviruses and can still mediate efficient entry into mammalian cells.

Removal of either N-glycan site from the envelope receptor binding domain of Moloney and Friend but not AKV mouse ecotropic gammaretroviruses alters receptor usage

Three N-linked glycosylation sites were removed from the envelope glycoproteins of Friend, Moloney, and AKV mouse ecotropic gammaretroviruses: gs1 and gs2, in the receptor binding domain; and gs8, in a region implicated in post-binding cell fusion. Mutants were tested for their ability to infect rodent cells expressing 4 CAT-1 receptor variants. Three mutants (Mo-gs1, Mo-gs2, and Fr-gs1) infect NIH 3T3 and rat XC cells, but are severely restricted in Mus dunni cells and Lec8, a Chinese hamster cell line susceptible to ecotropic virus. This restriction is reproduced in ferret cells expressing M. dunni dCAT-1, but not in cells expressing NIH 3T3 mCAT-1. Virus binding assays, pseudotype assays, and the use of glycosylation inhibitors further suggest that restriction is primarily due to receptor polymorphism and, in M. dunni cells, to glycosylation of cellular proteins. Virus envelope glycan size or type does not affect infectivity. Thus, host range variation due to N-glycan deletion is receptor variant-specific, cell-specific, virus type-specific, and glycan site-specific.

Furin cleavage potentiates the membrane fusion-controlling intersubunit disulfide bond isomerization activity of leukemia virus Env

The membrane fusion protein of murine leukemia virus is a trimer of a disulfide-linked peripheral-transmembrane (SU-TM) subunit complex. The intersubunit disulfide bond is in SU linked to a disulfide bond isomerization motif, CXXC, with which the virus controls its fusion reaction (M. Wallin, M. Ekström, and H. Garoff, EMBO J. 23:54-65, 2004). Upon receptor binding the isomerase rearranges the intersubunit disulfide bond into a disulfide bond isomer within the motif. This facilitates SU dissociation and fusion activation in the TM subunit. In the present study we have asked whether furin cleavage of the Env precursor potentiates the isomerase to be triggered. To this end we accumulated the late form of the precursor, gp90, in the cell by incubation in the presence of a furin-inhibiting peptide. The isomerization was done by NP-40 incubation or by a heat pulse under alkylation-free conditions. The cells were lysed in the presence of alkylator, and the precursor was immunoprecipitated, gel isolated, deglycosylated, and subjected to complete trypsin digestion. Disulfide-linked peptide complexes were separated by sodium dodecyl sulfate-tricine-polyacrylamide gel electrophoresis under nonreducing conditions. This assay revealed the size of the characteristic major disulfide-linked peptide complex that differentiates the two isomers of the disulfide bond between Cys336 (or Cys339) and Cys563, i.e., the bond corresponding to the intersubunit disulfide bond. The analyses showed that the isomerase was five- to eightfold more resistant to triggering in the precursor than in the mature, cleaved form. This suggests that the isomerase becomes potentiated for triggering by a structural change in Env that is induced by furin cleavage in the cell.

Analysis and function of prototype foamy virus envelope N glycosylation

The prototype foamy virus (PFV) glycoprotein, which is essential for PFV particle release, displays a highly unusual biosynthesis, resulting in posttranslational cleavage of the precursor protein into three particle-associated subunits, i.e., leader peptide (LP), surface (SU), and transmembrane (TM). Glycosidase digestion of metabolically labeled PFV particles revealed the presence of N-linked carbohydrates on all subunits. The differential sensitivity to specific glycosidases indicated that all oligosaccharides on LP and TM are of the high-mannose or hybrid type, whereas most of those attached to SU, which contribute to about 50% of its molecular weight, are of the complex type. Individual inactivation of all 15 potential N-glycosylation sites in PFV Env demonstrated that 14 are used, i.e., 1 out of 2 in LP, 10 in SU, and 3 in TM. Analysis of the individual altered glycoproteins revealed defects in intracellular processing, support of particle release, and infectivity for three mutants, having the evolutionarily conserved glycosylation sites N8 in SU or N13 and N15 in the cysteine-rich central "sheets-and-loops" region of TM inactivated. Examination of alternative mutants with mutations affecting glycosylation or surrounding sequences at these sites indicated that inhibition of glycosylation at N8 and N13 most likely is responsible for the observed replication defects, whereas for N15 surrounding sequences seem to contribute to a temperature-sensitive phenotype. Taken together these data demonstrate that PFV Env and in particular the SU subunit are heavily N glycosylated and suggest that although most carbohydrates are dispensable individually, some evolutionarily conserved sites are important for normal Env function of FV isolates from different species.

Involvement of the C-terminal disulfide-bonded loop of murine leukemia virus SU protein in a postbinding step critical for viral entry

A role for the C-terminal domain (CTD) of murine leukemia virus (MuLV) Env protein in viral fusion was indicated by the potent inhibition of MuLV-induced fusion, but not receptor binding, by two rat monoclonal antibodies (MAbs) specific for epitopes in the CTD. Although these two MAbs, 35/56 and 83A25, have very different patterns of reactivity with viral isolates, determinants of both epitopes were mapped to the last C-terminal disulfide-bonded loop of SU (loop 10), and residues in this loop responsible for the different specificities of these MAbs were identified. Both MAbs reacted with a minor fraction of a truncated SU fragment terminating four residues after loop 10, indicating that while the deleted C-terminal residues were not part of these epitopes, they promoted their formation. Neither MAb recognized the loop 10 region expressed in isolated form, suggesting that these epitopes were not completely localized within loop 10 but required additional sequences located N terminal to the loop. Direct support for a role for loop 10 in fusion was provided by the demonstration that Env mutants containing an extra serine or threonine residue between the second and third positions of the loop were highly attenuated for infectivity and defective in fusion assays, despite wild-type levels of expression, processing, and receptor binding. Other mutations at positions 1 to 3 of loop 10 inhibited processing of the gPr80 precursor protein or led to increased shedding of SU, suggesting that loop 10 also affects Env folding and the stability of the interaction between SU and TM.

Role of N glycosylation of hepatitis B virus envelope proteins in morphogenesis and infectivity of hepatitis delta virus

Hepatitis delta virus (HDV) particles are coated with the large (L), middle (M), and small (S) hepatitis B virus envelope proteins. In the present study, we constructed glycosylation-defective envelope protein mutants and evaluated their capacity to assist in the maturation of infectious HDV in vitro. We observed that the removal of N-linked carbohydrates on the S, M, and L proteins was tolerated for the assembly of subviral hepatitis B virus (HBV) particles but was partially inhibitory for the formation of HDV virions. However, when assayed on primary cultures of human hepatocytes, virions coated with S, M, and L proteins lacking N-linked glycans were infectious. Furthermore, in the absence of M, HDV particles coated with nonglycosylated S and L proteins retained infectivity. These results indicate that carbohydrates on the HBV envelope proteins are not essential for the in vitro infectivity of HDV.

Distinct mechanisms of neutralization by monoclonal antibodies specific for sites in the N-terminal or C-terminal domain of murine leukemia virus SU

The epitope specificities and functional activities of monoclonal antibodies (MAbs) specific for the murine leukemia virus (MuLV) SU envelope protein subunit were determined. Neutralizing antibodies were directed towards two distinct sites in MuLV SU: one overlapping the major receptor-binding pocket in the N-terminal domain and the other involving a region that includes the most C-terminal disulfide-bonded loop. Two other groups of MAbs, reactive with distinct sites in the N-terminal domain or in the proline-rich region (PRR), did not neutralize MuLV infectivity. Only the neutralizing MAbs specific for the receptor-binding pocket were able to block binding of purified SU and MuLV virions to cells expressing the ecotropic MuLV receptor, mCAT-1. Whereas the neutralizing MAbs specific for the C-terminal domain did not interfere with the SU-mCAT-1 interaction, they efficiently inhibited cell-to-cell fusion mediated by MuLV Env, indicating that they interfered with a postattachment event necessary for fusion. The C-terminal domain MAbs displayed the highest neutralization titers and binding activities. However, the nonneutralizing PRR-specific MAbs bound to intact virions with affinities similar to those of the neutralizing receptor-binding pocket-specific MAbs, indicating that epitope exposure, while necessary, is not sufficient for viral neutralization by MAbs. These results identify two separate neutralization domains in MuLV SU and suggest a role for the C-terminal domain in a postattachment step necessary for viral fusion.

Covalent modifications of the ebola virus glycoprotein

The role of covalent modifications of the Ebola virus glycoprotein (GP) and the significance of the sequence identity between filovirus and avian retrovirus GPs were investigated through biochemical and functional analyses of mutant GPs. The expression and processing of mutant GPs with altered N-linked glycosylation, substitutions for conserved cysteine residues, or a deletion in the region of O-linked glycosylation were analyzed, and virus entry capacities were assayed through the use of pseudotyped retroviruses. Cys-53 was the only GP(1) ( approximately 130 kDa) cysteine residue whose replacement resulted in the efficient secretion of GP(1), and it is therefore proposed that it participates in the formation of the only disulfide bond linking GP(1) to GP(2) ( approximately 24 kDa). We propose a complete cystine bridge map for the filovirus GPs based upon our analysis of mutant Ebola virus GPs. The effect of replacement of the conserved cysteines in the membrane-spanning region of GP(2) was found to depend on the nature of the substitution. Mutations in conserved N-linked glycosylation sites proved generally, with a few exceptions, innocuous. Deletion of the O-linked glycosylation region increased GP processing, incorporation into retrovirus particles, and viral transduction. Our data support a common evolutionary origin for the GPs of Ebola virus and avian retroviruses and have implications for gene transfer mediated by Ebola virus GP-pseudotyped retroviruses.

Differential glycosylation of the Cas-Br-E env protein is associated with retrovirus-induced spongiform neurodegeneration

The wild mouse ecotropic retrovirus, Cas-Br-E, induces progressive, noninflammatory spongiform neurodegenerative disease in susceptible mice. Functional genetic analysis of the Cas-Br-E genome indicates that neurovirulence maps to the env gene, which encodes the surface glycoprotein responsible for binding and fusion of virus to host cells. To understand how the envelope protein might be involved in the induction of disease, we examined the regional and temporal expression of Cas-Br-E Env protein in the central nervous systems (CNS) of mice infected with the highly neurovirulent chimeric virus FrCas(E). We observed that multiple isoforms of Cas-Br-E Env were expressed in the CNS, with different brain regions exhibiting unique patterns of processed Env glycoprotein. Specifically, the expression of gp70 correlated with regions showing microglial infection and spongiform neurodegeneration. In contrast, regions high in neuronal infection and without neurodegenerative changes (the cerebellum and olfactory bulb) were characterized by a gp65 Env protein isoform. Sedimentation analysis of brain region extracts indicated that gp65 rather than gp70 was incorporated into virions. Biochemical analysis of the Cas-Br-E Env isoforms indicated that they result from differential processing of N-linked sugars. Taken together, these results indicate that differential posttranslational modification of the Cas-Br-E Env is associated with a failure to incorporate certain Env isoforms into virions in vivo, suggesting that defective viral assembly may be associated with the induction of spongiform neurodegeneration.

Human serum-resistant retroviral vector particles from galactosyl (alpha1-3) galactosyl containing nonprimate cell lines

Retroviral vector particles (RVP) which are resistant to inactivation by human serum will be needed for many in vivo gene therapy applications. Murine-based producer cell lines generate RVP which are inactivated by human serum, reportedly due to the presence of the galactosyl (alpha1-3) galactosyl carbohydrate moiety (alphaGal) on these and other nonprimate producer cells and RVP. Consequently, human cells (which lack the alphaGal moiety) have been developed as producer cell lines for generation of human serum-resistant RVP. In this study, we report that contrary to earlier reports, the presence of the alphaGal moiety on producer cells and RVP does not necessarily correlate with cell killing or RVP inactivation by human serum. We show that the alphaGal-positive ferret brain cell line, Mpf, is an excellent basal cell line for generation of RVP which have titers and serum resistance levels equal to or greater than RVP produced in human cell lines such as HT1080. Therefore, packaging cell lines need not be limited to those of human or primate origin for production of human serum-resistant RVP.

The hypervariable domain of the murine leukemia virus surface protein tolerates large insertions and deletions, enabling development of a retroviral particle display system

The surface proteins (SU) of murine type-C retroviruses have a central hypervariable domain devoid of cysteine and rich in proline. This 41-amino-acid region of Friend ecotropic murine leukemia virus SU was shown to be highly tolerant of insertions and deletions. Viruses in which either the N-terminal 30 amino acids or the C-terminal 22 amino acids of this region were replaced by the 7-amino-acid sequence ASAVAGA were fully infectious. Insertions of this 7-amino-acid sequence at the N terminus, center, and the C terminus of the hypervariable domain had little effect on envelope protein (Env) function, while this insertion at a position 10 amino acids following the N terminus partially destabilized the association between the SU and transmembrane subunits of Env. Large, complex domains (either a 252-amino-acid single-chain antibody binding domain [scFv] or a 96-amino-acid V1/V2 domain of HIV-1 SU containing eight N-linked glycosylation sites and two disulfides) did not interfere with Env function when inserted in the center or C-terminal portions of the hypervariable domain. The scFv domain inserted into the C-terminal region of the hypervariable domain was shown to mediate binding of antigen to viral particles, demonstrating that it folded into the active conformation and was displayed on the surface of the virion. Both positive and negative enrichment of virions expressing the V1/V2 sequence were achieved by using a monoclonal antibody specific for a conformational epitope presented by the inserted sequence. These results indicated that the hypervariable domain of Friend ecotropic SU does not contain any specific sequence or structure that is essential for Env function and demonstrated that insertions into this domain can be used to extend particle display methodologies to complex protein domains that require expression in eukaryotic cells for glycosylation and proper folding.

Moloney murine leukemia virus envelope protein subunits, gp70 and Pr15E, form a stable disulfide-linked complex

The nature and stability of the interactions between the gp70 and Pr15E/p15E molecules of murine leukemia virus (MLV) have been disputed extensively. To resolve this controversy, we have performed quantitative biochemical analyses on gp70-Pr15E complexes formed after independent expression of the amphotropic and ecotropic Moloney MLV env genes in BHK-21 cells. We found that all cell-associated gp70 molecules are disulfide linked to Pr15E whereas only a small amount of free gp70 is released by the cells. The complexes were resistant to treatment with reducing agents in vivo, indicating that the presence and stability of the disulfide interaction between gp70 and Pr15E are not dependent on the cellular redox state. However, disulfide-bonded Env complexes were disrupted in lysates of nonalkylated cells in a time-, temperature-, and pH-dependent fashion. Disruption seemed not to be caused by a cellular factor but is probably due to a thiol-disulfide exchange reaction occurring within the Env complex after solubilization. The possibility that alkylating agents induce the formation of the intersubunit disulfide linkage was excluded by showing that disulfide-linked gp70-Pr15E complexes exist in freshly made lysates of nonalkylated cells and that disruption of the complexes can be prevented by lowering the pH. Together, these data establish that gp70 and Pr15E form a stable disulfide-linked complex in vivo.

Localization of the labile disulfide bond between SU and TM of the murine leukemia virus envelope protein complex to a highly conserved CWLC motif in SU that resembles the active-site sequence of thiol-disulfide exchange enzymes

Previous studies have indicated that the surface (SU) and transmembrane (TM) subunits of the envelope protein (Env) of murine leukemia viruses (MuLVs) are joined by a labile disulfide bond that can be stabilized by treatment of virions with thiol-specific reagents. In the present study this observation was extended to the Envs of additional classes of MuLV, and the cysteines of SU involved in this linkage were mapped by proteolytic fragmentation analyses to the CWLC sequence present at the beginning of the C-terminal domain of SU. This sequence is highly conserved across a broad range of distantly related retroviruses and resembles the CXXC motif present at the active site of thiol-disulfide exchange enzymes. A model is proposed in which rearrangements of the SU-TM intersubunit disulfide linkage, mediated by the CWLC sequence, play roles in the assembly and function of the Env complex.

The critical N-linked glycan of murine leukemia virus envelope protein promotes both folding of the C-terminal domains of the precursor polyprotein and stability of the postcleavage envelope complex

The infectivity of Friend ecotropic murine leukemia virus was previously shown to be highly sensitive to modification in its envelope protein (Env) at only one of the eight signals for N-linked glycan attachment, the fourth from the N terminus (gs4). In the present study, a set of six single-amino-acid substitutions in or near gs4 was used to determine the function of this region of Env and the role played by the glycan itself. One mutant that lacked the gs4 glycan was fully infectious, while one that retained this glycan was completely noninfectious, indicating that the gs4 glycan per se is not required for Env function. Infectivity correlated with the level of mature Env complex incorporated into virus particles, which was determined by the severity of defects in transport of the envelope precursor protein (gPrEnv) from the endoplasmic reticulum into the Golgi apparatus, in cleavage of gPrEnv into the two envelope subunits (the surface protein [SU] and the transmembrane protein [TM]), and in the association of SU with cellular membranes. All of the mutants induced the wild-type level of superinfection interference, indicating that the gs4 region mutations did not interfere with proper folding of the N-terminal domain of SU. These results suggest that the gs4 region mediates folding of the C-terminal domains of gPrEnv and stability of the interaction between SU and TM. Although the gs4 glycan was not essential for infectivity, processing of all mutant Envs lacking this glycan was significantly impaired, suggesting that efficient folding of gPrEnv requires a glycan at this position. The conservation of a glycosylation site homologous to gs4 across a broad range of retroviruses suggests that this sequence may play a similar role in many retroviral Envs.

Analysis of the unique hamster cell tropism of ecotropic murine leukemia virus PVC-211

PVC-211 murine leukemia virus (MuLV) is a neuropathogenic variant of Friend MuLV (F-MuLV). Previous studies from our laboratory demonstrated that unlike the parental F-MuLV, PVC-211 MuLV can infect rat brain capillary endothelial cells efficiently and that it has acquired genetic changes responsible for its expanded cellular tropism. To determine if PVC-211 MuLV also has expanded its host range, we tested its infectivity on Chinese hamster ovary-derived CHO-K1 cells, which are generally resistant to ecotropic MuLV. The results indicated that PVC-211 MuLV, but not F-MuLV, was highly infectious for CHO-K1 cells. Studies using glycosylation inhibitors and glycosylation mutants of CHO-K1 cells, as well as interference studies, suggested that PVC-211 MuLV has acquired the ability to interact with the ecotropic MuLV receptor on CHO-K1 cells that has undergone glycosylation-dependent modification. Using chimeric viruses between PVC-211 MuLV and F-MuLV, we were able to localize the viral genetic element crucial for CHO-K1 cell tropism within the env gene of PVC-211 MuLV and show that glycine at position 116 and lysine at position 129 of the envelope glycoprotein SU were important. These viral determinants also appear to confer tropism for other hamster cells resistant to ordinary ecotropic MuLVs. Further studies on the interaction between PVC-211 MuLV and the receptor on hamster cells may provide novel insights into the molecular mechanisms for receptor recognition and binding by viral envelope glycoproteins.

Characterization of chimeras between the ecotropic Moloney murine leukemia virus and the amphotropic 4070A envelope proteins

A series of 22 chimeric envelope (env) genes were generated between the ecotropic Moloney murine leukemia virus and the amphotropic 4070A isolate. The chimeric envelopes were expressed within the complete, replication-competent provirus and tested for virus viability by transient expression assays. Eleven of the 22 viruses were viable. Five of these chimeric viruses showed an ecotropic host range, and six exhibited an amphotropic host range and viral interference. The host range determinants map to the first half of the surface (SU) protein. The N-terminal 72 amino acids of 4070A (42 of processed SU) are not required for amphotropic receptor usage. Ecotropic and amphotropic viruses differ in their ability to form large, multinucleated syncytia when cocultured with the rat XC cell line. Ecotropic murine leukemia virus forms large syncytia with XC cells, whereas no syncytia are reported for amphotropic virus. All chimeras which contained the N-terminal half of the ecotropic SU protein, encoding the receptor binding domain, formed the large multinucleated syncytia with XC cells.

Identification of a subdomain in the Moloney murine leukemia virus envelope protein involved in receptor binding

We have mutated amino acids within the receptor-binding domain of Moloney murine leukemia virus envelope in order to identify residues involved in receptor binding. Analysis of mutations in the region of amino acids 81 to 88 indicates that this region is important for specific envelope-receptor interactions. None of the aspartate 84 (D-84) mutants studied bind measurably, although they are efficiently incorporated into particles. D-84 mutants have titers that correspond to the severity of the substitution. This observation suggests that D-84 may provide a direct receptor contact. Mutations in the other charged amino acids in this domain (R-83, E-86, and E-87) yield titers similar to those of wild-type envelope, but the affinity of the mutant envelope in the binding assay is decreased by nonconservative substitutions in parallel to the severity of the change. These other amino acids may either provide secondary receptor contacts or assist in maintaining a structure in the domain that favors efficient binding. We also studied other regions of high hydrophilicity. Our initial characterization indicates that amino acids 106 to 111 and 170 to 188 do not play a major role in receptor binding. Measurements of relative binding affinity and titer indicate that most mutations in the region of amino acids 120 to 131 did not significantly affect receptor binding. However, SU encoded by mutants H123V, R124L, and C131A as well as C81A could not be detected in particles and therefore did not bind measurably. Therefore, the region encompassed by amino acids 81 to 88 appears to be directly involved in receptor binding.

The HTLV-I envelope glycoproteins: structure and functions

The human T-cell lymphotropic virus type I (HTLV-I) envelope has a structural organization shared by all retroviral envelopes, which contain two mature viral glycoproteins deriving from a common precursor: an external surface protein (SU), associated with a transmembrane protein (TM) responsible for anchoring the SU-TM complex at the cell surface or in the viral envelope. Our understanding of the tertiary structure of these proteins is extremely poor. The intracellular maturation follows the normal cellular secretory pathway, resulting in expression of the mature glycoproteins at the cell surface. The five potential N-glycosylation sites are glycosylated. Most mutations artificially introduced into the glycoproteins result in loss of function, mostly due to abnormal intracellular maturation. This probably indicates a very compact structure of these proteins, where the entire structure is involved in correct conformation. Studies using neutralizing antibodies or mutagenesis have defined functional domains in the SU protein, which is responsible for receptor binding. These domains occur throughout the SU glycoprotein. Sequence analysis of the HTLV-I TM predicts a structure, and probably functions, similar to other retrovirus TMs: involvement of this glycoprotein in the different oligomerization steps leading to a fusogenic SU-TM complex and in the fusion process itself. These features remain to be proven, and it is not yet understood why the free HTLV-I viral particle is not infectious.

Influence of transmembrane domains on the fusogenic abilities of human and murine leukemia retrovirus envelopes

The envelopes of two highly divergent oncoviruses, human T-cell leukemia virus type 1 (HTLV-1) and Friend murine leukemia virus (F-MuLV), have distinct patterns of cellular receptor recognition, fusion, and syncytium formation. To analyze the influence of the transmembrane envelope subunit (TM) on fusogenic properties, we substituted either the entire TM or distinct domains from F-MuLV for the corresponding domains in the HTLV-1 envelope. Parental, chimeric, and truncated envelopes cloned into a eukaryotic expression vector were monitored for fusogenic potential in human, rat, and murine indicator cell lines by using a quantitative assay. This highly sensitive assay allowed us to assess the fusogenic properties and syncytium-forming abilities of the HTLV-1 envelope in murine NIH 3T3 cells. All chimeric envelopes containing extracellular sequences of the F-MuLV TM were blocked in their maturation process. Although deletions of the HTLV-1 cytoplasmic domain, alone and in combination with the membrane-spanning domain, did not prevent envelope cell surface expression, they impaired and suppressed fusogenic properties, respectively. In contrast, envelopes carrying substitutions of membrane-spanning and cytoplasmic domains were highly fusogenic. Our results indicate that these two domains in F-MuLV and HTLV-1 constitute structural entities with similar fusogenic properties. However, in the absence of a cytoplasmic domain, the F-MuLV membrane-spanning domain appeared to confer weaker fusogenic properties than the HTLV-1 membrane-spanning domain.

Interaction between the dominant negative mutant and the wild-type envelope proteins of Friend murine leukemia virus

Interaction between the previously obtained dominant negative mutant, referred to as fcr (T. Matano, T. Odawara, M. Ohshima, H. Yoshikura, and A. Iwamoto, J. Virol. 67:2026-2033, 1993), and the wild-type envelope proteins (Env) of Friend murine leukemia virus was examined. The wild-type Env was bound to the fcr mutant Env and trapped in the endoplasmic reticulum. The virus receptor was not involved in this interaction.

Structural elements in glycoprotein 70 from polytropic Friend mink cell focus-inducing virus and glycoprotein 71 from ecotropic Friend murine leukemia virus, as defined by disulfide-bonding pattern and limited proteolysis

The disulfide-bonding pattern of glycoprotein 70 (gp70), the surface glycoprotein (SU) encoded by the envelope gene of polytropic Friend milk cell focus-inducing virus, was elucidated and compared with that of glycoprotein 71 (gp71), the corresponding glycoprotein of the ecotropic Friend murine leukemia virus, which had previously been determined (M. Linder, D. Linder, J. Hahnen, H.-H. Schott, and Stirm, Eur. J. Biochem. 203:65-73, 1992). In the carboxy-terminal constant domain, in which these glycoproteins have about 97% sequence homology, the location of the four disulfide bonds was found to be analogous. In the amino-terminal differential domain, with about 37% sequence homology, 8 of the 12 cysteine residues of the ecotropic SU are conserved in the polytropic SU. In this domain, a similar clustering of disulfide bonds was detected, which led to the identification of three distinct disulfide-bonded regions in both glycoproteins. However, because of deletions and sequence deviations, the glycoproteins must have significantly different three-dimensional structures in these regions. Since the receptor-binding functions of both glycoproteins have been attributed to their amino-terminal domains and since each binds to a different receptor, these disulfide-bonded structures are likely candidates for receptor-binding functions. Limited proteolysis of both glycoproteins with various endoproteinases led to the identification of preferential proteolytic sites between disulfide-bonded regions, at the beginning of the hypervariable proline-rich region, and between differential and constant domains, further confirming the structural organization of the folded glycoproteins.

A domain of murine retrovirus surface protein gp70 mediates cell fusion, as shown in a novel SC-1 cell fusion system

Virus-induced cell fusion of the fusion-from-without type was observed in SC-1 cells infected with Moloney murine leukemia virus when grown in NIH 3T3 cells. Replication-competent virus mutants with altered surface protein gp70 were examined. Fusion mutations were found in the proline-rich region of gp70. They acted on a step after binding and before or during endocytosis. The fusion mutants had an altered gp70 isomer pattern, presumably caused by different glycosylation. Other mutants with deleted glycans were analyzed, and some which also showed defective fusion were found. The interrelationship of the proline-rich region, glycosylation, and fusion is discussed.

Microglial infection by a neurovirulent murine retrovirus results in defective processing of envelope protein and intracellular budding of virus particles

The observation of murine retrovirus infection of microglial cells in brain regions expressing spongiform neurodegenerative changes suggests that these cells may play an important role in pathogenesis. To evaluate this potential in vitro, murine microglial cells were infected in mixed glial cultures with the highly neurovirulent murine retrovirus, FrCasE. The microglia were then isolated from the mixed cultures on the basis of their differential adherence and shown to be approximately 98% pure. The infected microglia expressed viral envelope protein at the plasma membrane, while viral budding was primarily intracellular. Evaluation of the viral envelope protein by immunoblotting indicated that the immunoreactive species produced was exclusively a 90-kDa precursor protein. Very little of the envelope protein was associated with particles released from these cells, and viral titers in the culture supernatant were low. Interestingly, these cells were still capable of infecting permissive target cells when seeded as infectious centers. This partially defective infection of microglial cells suggests a potential cellular means by which a neurovirulent retrovirus could disrupt normal microglia and in turn central nervous system motor system functioning.

Presentation of native epitopes in the V1/V2 and V3 regions of human immunodeficiency virus type 1 gp120 by fusion glycoproteins containing isolated gp120 domains

The immune response to viral glycoproteins is often directed against conformation- and/or glycosylation-dependent structures; synthetic peptides and bacterially expressed proteins are inadequate probes for the mapping of such epitopes. This report describes a retroviral vector system that presents such native epitopes on chimeric glycoproteins in which protein fragments of interest are fused to the C terminus of the N-terminal domain of the murine leukemia virus surface protein, gp70. The system was used to express two disulfide-bonded domains from gp120, the surface protein of human immunodeficiency virus type 1 (HIV-1), that include potent neutralization epitopes. The resulting fusion glycoproteins were synthesized at high levels and were efficiently transported and secreted. A fusion protein containing the HXB2 V1/V2 domain was recognized by an HIVIIIB-infected patient serum as well as by 17 of 36 HIV-1 seropositive hemophiliac, homosexual male and intravenous drug user patient sera. Many of these HIV+ human sera reacted with V1/V2 domains from several HIV-1 clones expressed in fusion glycoproteins, indicating the presence of cross-reactive antibodies against epitopes in the V1/V2 domain. Recognition of gp(1-263):V1/V2HXB2 by the HIVIIIB-infected human patient serum was largely blocked by synthetic peptides matching V1 but not V2 sequences, while recognition of this construct by a broadly cross-reactive hemophiliac patient serum was not blocked by individual V1 or V2 peptides or by mixtures of these peptides. A construct containing the V3 domain of the IIIB strain of HIV-1, gp(1-263):V3HXB2, was recognized by sera from a human and a chimpanzee that had been infected by HIVIIIB but not by sera from hemophiliac patients who had been infected with HIV-1 of MN-like V3 serotype. The reactive sera had significantly higher titers when assayed against gp(1-263):V3HXB2 than when assayed against matching V3 peptides. Immunoprecipitation of this fusion glycoprotein by the human serum was only partially blocked by V3 peptide, indicating that this infected individual produced antibodies against epitopes in V3 that were expressed on the fusion glycoprotein but not by synthetic peptides. These data demonstrated that the chimeric glycoproteins described here effectively present native epitopes present in the V1/V2 and V3 domains of gp120 and provide efficient methods for detection of antibodies directed against native epitopes in these regions and for characterization of such epitopes.

trans-dominant interference with virus infection at two different stages by a mutant envelope protein of Friend murine leukemia virus

A dominant negative mutant Friend murine leukemia virus (FMLV) env gene was cloned from an immunoselected Friend erythroleukemia cell. The mutant env had a point mutation which resulted in a Cys-to-Arg substitution at the 361st amino acid in the FMLV envelope protein (Env). The mutant Env was retained in the endoplasmic reticulum (ER) and accumulated because of its slow degradation. The NIH 3T3 cells expressing the mutant env were resistant to ecotropic Moloney MLV (MoMLV) penetration, suggesting that the mutant Env traps the ecotropic MLV receptors in the ER. When the mutant env gene was transfected into and expressed in the cells persistently infected with MoMLV, the wild-type Env was trapped in the ER, and the MoMLV production was suppressed. Thus, the mutant Env accumulating in the ER trans-dominantly and efficiently interfered with the ecotropic MLV infection at both the early and the late stages.

Erythropoietin receptor (EpoR)-dependent mitogenicity of spleen focus-forming virus correlates with viral pathogenicity and processing of env protein but not with formation of gp52-EpoR complexes in the endoplasmic reticulum

Recent evidence suggests that interactions between spleen focus-forming virus (SFFV) env products and the erythropoietin receptor (EpoR) are responsible for viral pathogenicity. Infection of factor-dependent cell lines expressing epoR (the cloned gene for EpoR) with SFFVP is mitogenic, generating cell lines that are no longer dependent on added growth factor, and an immunoprecipitable complex between EpoR and immature env protein in the endoplasmic reticulum has been identified. The dependence of these in vitro activities on env protein processing and their relationship to pathogenicity of SFFV were explored by using glycosylation site mutants of SFFV env. Mutants carrying Asn-->Asp mutations at each of the two consensus signals for N-linked glycosylation in the N-terminal domain of SFFVAP-L env (gs1 and gs2), the gs1-2- double mutant, and the gs0 quadruple mutant (mutated at all four signals utilized for N-linked glycosylation in SFFVAP-L env) were made. The primary translation products (gp52) of single-site mutant envs were processed into more highly glycosylated forms, and the corresponding viruses induced splenomegaly in susceptible mice, whereas the gs1-2- and gs0 proteins were not processed, and these viruses were not pathogenic. Unprocessed env proteins of both pathogenic and nonpathogenic mutants coprecipitated with EpoR. In the BaF3 cell assay for epoR-dependent mitogenicity, the pathogenic single mutants induced factor-independent growth efficiently whereas the nonpathogenic gs1-2- and gs0 mutants did not. These data demonstrate that the ability of gp52 to form complexes with EpoR in the endoplasmic reticulum is not sufficient for either mitogenicity in cell culture or induction of splenomegaly in mice while supporting the hypothesis that pathogenicity and mitogenicity of SFFV both result from an interaction between EpoR and SFFV env protein.

Conserved structural features in the interaction between retroviral surface and transmembrane glycoproteins?

Among the retroviruses, the surface (SU) and transmembrane (TM) glycoproteins of lentiviruses are linked exclusively by noncovalent bonds. For some C-type retroviruses, however, a small proportion of the SU proteins has been shown to be linked to their TM proteins by a disulfide bond, with the remainder being noncovalently associated. A region near the carboxyl terminus of the HIV-1 SU glycoprotein has been implicated in contacting the TM glycoprotein. Computer modelling indicates that this region of divergent lentivirus and oncovirus SU glycoproteins forms a structurally conserved "pocket" which could accommodate a "knob"-like protrusion formed by an immunodominant region in the TM protein containing the CxxxxxC (lentiviruses) or CxxxxxxCC (C- and D-type viruses) motif. An anti-idiotypic monoclonal antibody, raised against a monoclonal antibody reacting with a sequence in the "pocket" of HIV-1 gp120, was found to bind to synthetic peptides close to the CxxxxxC motif. It is suggested that part of the SU-TM linkage mechanism for the lentiviruses and oncoviruses is a 'knob and socket' structure and that the interaction between SU and TM proteins is similar in one region for lentiviruses and C-type as well as D-type viruses. The conserved knob and socket linkage may be relevant to a mechanism for viral-cell membrane fusion that is broadly common to all of these retroviruses.

Mutational analysis of the N-linked glycosylation sites of the SU envelope protein of Moloney murine leukemia virus

The role of the N-linked glycosylation sites in the major envelope glycoprotein, SU (gp70), of Moloney murine leukemia virus has been examined. By using site-specific oligonucleotide-directed mutagenesis, each of the seven glycan addition sites has been individually eliminated. Mutations resulting in the loss of a single glycosylation site produced, intracellularly, stable precursor SU-TM proteins which were 4 to 5 kDa smaller than the wild-type virus SU-TM protein. Mutant delta 1,4,7, a trimutant lacking three N-linked glycan addition sites, resulted in a viable, infectious virus with a stable SU-TM protein approximately 12 to 15 kDa smaller than the wild-type SU-TM protein. Five of the seven single-site mutations resulted in viable virus as judged by the release of reverse transcriptase in transient-expression assays and XC syncytium assays. Mutations at two of the sites resulted in a detectable phenotype. Virus mutated at position 2 was temperature sensitive in Rat2 cells; viable virus was produced at 32 degrees C but not at 37 degrees C. Virus mutated at position 3 was noninfectious and yielded virions lacking detectable mature SU protein. The mutation results in the block of transport of the protein to the cell surface and assembly into virion particles.

A 585-bp deletion found in the spleen focus-forming virus (SFFV) env gene is responsible for the defective intracellular transport of SFFV gp52

The Friend spleen focus-forming virus (F-SFFV) codes for a transport defective, leukemogenic envelope glycoprotein designated as gp52. Gp52 closely resembles the envelope glycoproteins (gp70-p15E) encoded by the mink cell focus-forming viruses (MCFV). The major differences between SFFV and MCFV include a 585-bp deletion and a frame-shift mutation near the 3' end of the SFFV env gene. We have constructed a mutant MCFV env gene, which contains a 585-bp deletion like that found in the SFFV env gene, and expressed this gene using recombinant vaccinia vectors or retroviral vectors. The mutant MCFV env gene expressed a truncated, transport defective glycoprotein (gp57). Only a small proportion of gp57 underwent further oligosaccharide processing. Intracellular gp57 remained predominantly monomeric and only a small proportion of gp57 (and its processed forms) formed disulfide-linked dimers and trimers which resembled those formed by SFFV gp52. Processed forms of gp57 were found on the cell surfaces and in culture fluids. The extracellular forms had a faster electrophoretic mobility than the intracellular-processed forms of gp57. These results indicate that the 585-bp deletion found in SFFV env gene is responsible for the folding, transport, and secretion of gp52. Retroviral vectors carrying the mutant MCFV env gene were nonpathogenic (or weakly pathogenic) in adult mice. The results indicate that the 585-bp deletion, although essential, is not the sole determinant of SFFV-induced disease in adult mice.

Glucosidase inhibitors for treatment of HIV-1 infection

The HIV-1 envelope protein is a glycoprotein composed of 120 kD and 41 kD subunits. It contains 30-38 potential asparagine-linked glycosylation sites which have been shown to play a role in CD4 binding, virus uptake, and cytopathogenicity. Several inhibitors of oligosaccharide attachment or modification have been tested. An agent which inhibits glucosidases, N-butyl deoxynojirimycin was found to inhibit HIV-1 and SIVmac infectivity, and is currently in clinical trials.