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

Conserved residues of the immunosuppressive domain of MLV are essential for regulating the fusion-critical SU-TM disulfide bond

The Env protein of murine leukemia virus (MLV) is the prototype of a large clade of retroviral fusogens, collectively known as gamma-type Envs. Gamma-type Envs are found in retroviruses and endogenous retroviruses (ERVs) representing a broad range of vertebrate hosts. All gamma-type Envs contain a highly conserved stretch of 26-residues in the transmembrane subunit (TM) comprising two motifs, a putative immunosuppressive domain (ISD) and a CX6CC motif. Extraordinary conservation of the ISD and its invariant association with the CX6CC suggests a fundamental contribution to Env function. To investigate ISD function, we characterized several mutants with single amino acid substitutions at conserved positions in the MLV ISD. A majority abolished infectivity, although we did not observe a corresponding loss in intrinsic ability to mediate membrane fusion. Ratios of the surface subunit (SU) to capsid protein (CA) in virions were diminished for a majority of the ISD mutants, while TM:CA ratios were similar to wild type. Specific loss of SU reflected premature isomerization of the labile disulfide bond that links SU and TM prior to fusion. Indeed, all non-infectious mutants displayed significantly lower disulfide stability than wild-type Env. These results reveal a role for ISD positions 2, 3, 4, 7, and 10 in regulating a late step in entry after fusion peptide insertion but prior to creation of the fusion pore. This implies that the ISD is part of a larger domain, comprising the ISD and CX6CC motifs, that is critical for the formation and regulation of the metastable, intersubunit disulfide bond.IMPORTANCEThe gamma-type Env is a prevalent viral fusogen, found within retroviruses and endogenous retroviruses across vertebrate species and in filoviruses such as Ebolavirus. The fusion mechanism of gamma-type Envs is unique from other Class I fusogens such as those of influenza A virus and HIV-1. Gamma-type Envs contain a hallmark feature known as the immunosuppressive domain (ISD) that has been the subject of some controversy in the literature surrounding its putative immunosuppressive effects. Despite the distinctive conservation of the ISD, little has been done to investigate the role of this region for the function of this widespread fusogen. Our work demonstrates the importance of the ISD for the function of gamma-type Envs in infection, particularly in regulating the intermediate steps of membrane fusion. Understanding the fusion mechanism of gamma-type Envs has broad implications for understanding the entry of extant viruses and aspects of host biology connected to co-opted endogenous gamma-type Envs.

Unique Structure and Distinctive Properties of the Ancient and Ubiquitous Gamma-Type Envelope Glycoprotein

After the onset of the AIDS pandemic, HIV-1 (genus Lentivirus) became the predominant model for studying retrovirus Env glycoproteins and their role in entry. However, HIV Env is an inadequate model for understanding entry of viruses in the Alpharetrovirus, Gammaretrovirus and Deltaretrovirus genera. For example, oncogenic model system viruses such as Rous sarcoma virus (RSV, Alpharetrovirus), murine leukemia virus (MLV, Gammaretrovirus) and human T-cell leukemia viruses (HTLV-I and HTLV-II, Deltaretrovirus) encode Envs that are structurally and functionally distinct from HIV Env. We refer to these as Gamma-type Envs. Gamma-type Envs are probably the most widespread retroviral Envs in nature. They are found in exogenous and endogenous retroviruses representing a broad spectrum of vertebrate hosts including amphibians, birds, reptiles, mammals and fish. In endogenous form, gamma-type Envs have been evolutionarily coopted numerous times, most notably as placental syncytins (e.g., human SYNC1 and SYNC2). Remarkably, gamma-type Envs are also found outside of the Retroviridae. Gp2 proteins of filoviruses (e.g., Ebolavirus) and snake arenaviruses in the genus Reptarenavirus are gamma-type Env homologs, products of ancient recombination events involving viruses of different Baltimore classes. Distinctive hallmarks of gamma-type Envs include a labile disulfide bond linking the surface and transmembrane subunits, a multi-stage attachment and fusion mechanism, a highly conserved (but poorly understood) "immunosuppressive domain", and activation by the viral protease during virion maturation. Here, we synthesize work from diverse retrovirus model systems to illustrate these distinctive properties and to highlight avenues for further exploration of gamma-type Env structure and function.

A Novel Retrovirus (Gunnison's Prairie Dog Retrovirus) Associated With Thymic Lymphoma in Gunnison's Prairie Dogs in Colorado, USA

As part of research and wildlife disease surveillance efforts, we performed necropsy examinations of 125 free-ranging (n = 114) and captive (n = 11) prairie dogs in Colorado from 2009 to 2017. From these cases, we identified three cases of thymic lymphoma in free-ranging Gunnison's prairie dogs (Cynomys gunnisoni), and we identified a novel retroviral sequence associated with these tumors. The viral sequence is 7700 nucleotides in length and exhibits a genetic organization that is consistent with the characteristics of a type D betaretrovirus. The proposed name of this virus is Gunnison's prairie dog retrovirus (GPDRV). We screened all 125 prairie dogs for the presence of GPDRV using PCR with envelope-specific primers and DNA extracted from spleen samples. Samples were from Gunnison's prairie dogs (n = 59), black-tailed prairie dogs (Cynomys ludovicianus) (n = 40), and white-tailed prairie dogs (Cynomys leucurus) (n = 26). We identified GPDRV in a total of 7/125 (5.6%) samples including all three of the prairie dogs with thymic lymphoma, as well as spleen from an additional four Gunnison's prairie dogs with no tumors recognized at necropsy. None of the GPDRV-negative Gunnison's prairie dogs had thymic lymphomas. We also identified a related, apparently endogenous retroviral sequence in all prairie dog samples. These results suggest that GPDRV infection may lead to development of thymic lymphoma in Gunnison's prairie dogs.

Adenovirus based virus-like-vaccines targeting endogenous retroviruses can eliminate growing colorectal cancers in mice

Endogenous retroviruses (ERVs) that make up 8% of the human genome have been associated with the development and progression of cancer. The murine model system of the melanoma associated retrovirus (MelARV), which is expressed in different murine cancer cell lines, can be used to study mechanisms and therapeutic approaches against ERVs in cancer. We designed a vaccine strategy (Ad5-MelARV) of adenoviruses encoding the MelARV proteins Gag and Env that assemble in vivo into virus-like particles displaying the cancer-associated MelARV Env to the immune system. The novel vaccine was designed to induce both humoral as well as cellular immune responses in order to attack ERV expressing tumor cells. Despite a lack of antibody induction, we found that T cell responses were strong enough to prevent colorectal CT26 tumor growth and progression in BALB/c mice after a single vaccination before or after tumor challenge. A combination with the checkpoint inhibitor anti-PD-1 further increased the efficacy of the vaccination leading to complete tumor regression. Furthermore, immune responses in vaccinated mice were not restricted to only one cancer cell line but vaccinated animals were also protected from a rechallenge with the distinct breast cancer cell line 4T1. Thus, the developed vaccine strategy could represent a novel tool to successfully target diverse ERV-bearing tumors in cancer patients.

Sequential activation of the three protomers in the Moloney murine leukemia virus Env

Viral membrane fusion proteins of class I are trimers in which the protomeric unit is a complex of a surface subunit (SU) and a fusion active transmembrane subunit (TM). Here we have studied how the protomeric units of Moloney murine leukemia virus envelope protein (Env) are activated in relation to each other, sequentially or simultaneously. We followed the isomerization of the SU-TM disulfide and subsequent SU release from Env with biochemical methods and found that this early activation step occurred sequentially in the three protomers, generating two asymmetric oligomer intermediates according to the scheme (SU-TM)₃ → (SU-TM)₂TM → (SU-TM)TM₂ → TM₃ This was the case both when activation was triggered in vitro by depleting stabilizing Ca²⁺ from solubilized Env and when viral Env was receptor triggered on rat XC cells. In the latter case, the activation reaction was too fast for direct observation of the intermediates, but they could be caught by alkylation of the isomerization active thiol.

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.

Characterizing the Murine Leukemia Virus Envelope Glycoprotein Membrane-Spanning Domain for Its Roles in Interface Alignment and Fusogenicity

The membrane-proximal region of murine leukemia virus envelope (Env) is a critical modulator of its functionality. We have previously shown that the insertion of one amino acid (+1 leucine) within the membrane-spanning domain (MSD) abolished protein functionality in infectivity assays. However, functionality could be restored to this +1 leucine mutant by either inserting two additional amino acids (+3 leucine) or by deleting the cytoplasmic tail domain (CTD) in the +1 leucine background. We inferred that the ectodomain and CTD have protein interfaces that have to be in alignment for Env to be functional. Here, we made single residue deletions to the Env mutant with the +1 leucine insertion to restore the interface alignment (gain of functionality) and therefore define the boundaries of the two interfaces. We identified the glycine-proline pairs near the N terminus (positions 147 and 148) and the C terminus (positions 159 and 160) of the MSD as being the boundaries of the two interfaces. Deletions between these pairs restored function, but deletions outside of them did not. In addition, the vast majority of the single residue deletions regained function if the CTD was deleted. The exceptions were four hydroxyl-containing amino acid residues (T139, T140, S143, and T144) that reside in the ectodomain interface and the proline at position 148, which were all indispensable for functionality. We hypothesize that the hydroxyl-containing residues at positions T139 and S143 could be a driving force for stabilizing the ectodomain interface through formation of a hydrogen-bonding network.

IMPORTANCE

The membrane-proximal external region (MPER) and membrane-spanning domains (MSDs) of viral glycoproteins have been shown to be critical for regulating glycoprotein fusogenicity. However, the roles of these two domains are poorly understood. We report here that point deletions and insertions within the MPER or MSD result in functionally inactive proteins. However, when the C-terminal tail domain (CTD) is deleted, the majority of the proteins remain functional. The only residues that were found to be critical for function regardless of the CTD were four hydroxyl-containing amino acids located at the C terminus of the MPER (T139 and T140) and at the N terminus of the MSD (S143 and T144) and a proline near the beginning of the MSD (P148). We demonstrate that hydrogen-bonding at positions T139 and S143 is critical for protein function. Our findings provide novel insights into the role of the MPER in regulating fusogenic activity of viral glycoproteins.

Maintaining therapeutic activity in the operating room: compatibility of a gamma-retroviral replicating vector with clinical materials and biofluids

Toca 511 is a novel retroviral replicating vector, encoding a modified yeast cytosine deaminase, administered to recurrent high grade glioma patients in Phase 1 trials by stereotactic, transcranial injection into the tumor or into the walls of the resection cavity. A key issue, with little published data, is vector biocompatibility with agents likely to be encountered in a neurosurgical setting. We tested biocompatibility of Toca 511 with: delivery devices; MRI contrast agents, including ProHance supporting coinjection for real time MRI-guided intratumoral delivery; hemostatic agents; biofluids (blood and cerebrospinal fluid); potential adjuvants; and a needleless vial adapter that reduces risk of accidental needle sticks. Toca 511 is stable upon thawing at ambient temperature for at least 6 hours, allowing sufficient time for administration, and its viability is not reduced in the presence of: stainless steel and silica-based delivery devices; the potential MRI contrast agent, Feraheme; ProHance at several concentrations; the hemostatic agent SURGIFOAM; blood; cerebrospinal fluid; and the needleless vial adapter. Toca 511 is not compatible with the hemostatic agent SURGICEL or with extended exposures to titanium-based biopsy needles.

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.

Retrovirus glycoprotein functionality requires proper alignment of the ectodomain and the membrane-proximal cytoplasmic tail

Nonnative viral glycoproteins, including Friend murine leukemia virus envelope (F-MLV Env) are actively recruited to HIV-1 assembly sites by an unknown mechanism. Because interactions with the lipid microenvironment at budding sites could contribute to recruitment, we examined the contribution of the hydrophobicity of the F-MLV Env membrane-spanning domain (MSD) to its incorporation into HIV-1 particles. A series of F-MLV Env mutants that added or deleted one, two, or three leucines in the MSD were constructed. All six mutants retained the ability to be incorporated into HIV-1 particles, but the -1L, -2L, -3L, +1L, and +2L mutants were not capable of producing infectious particles. Surprisingly, the +3L Env glycoprotein was able to produce infectious particles and was constitutively fusogenic. However, when the cytoplasmic tail domains (CTDs) in the Env constructs were deleted, all six of the MSD mutants were able to produce infectious particles. Further mutational analyses revealed that the first 10 amino acids of the CTD is a critical regulator of infectivity. A similar phenotype was observed in HIV-1 Env upon addition of leucines in the MSD, with +1 and +2 leucine mutations greatly reducing Env activity, but +3 leucine mutations behaving similar to the wild type. Unlike F-MLV Env (+1L and +2L), HIV-1 Env (+1L and +2L) infectivity was not restored by deletion of the CTD. We hypothesize that the CTD forms a coiled-coil that disrupts the protein's functionality if it is not in phase with the trimer interface of the ectodomain.

Pushing the endogenous envelope

The majority of retroviral envelope glycoproteins characterized to date are typical of type I viral fusion proteins, having a receptor binding subunit associated with a fusion subunit. The fusion subunits of lentiviruses and alpha-, beta-, delta- and gammaretroviruses have a very conserved domain organization and conserved features of secondary structure, making them suitable for phylogenetic analyses. Such analyses, along with sequence comparisons, reveal evidence of numerous recombination events in which retroviruses have acquired envelope glycoproteins from heterologous sequences. Thus, the envelope gene (env) can have a history separate from that of the polymerase gene (pol), which is the most commonly used gene in phylogenetic analyses of retroviruses. Focusing on the fusion subunits of the genera listed above, we describe three distinct types of retroviral envelope glycoproteins, which we refer to as gamma-type, avian gamma-type and beta-type. By tracing these types within the ‘fossil record’ provided by endogenous retroviruses, we show that they have surprisingly distinct evolutionary histories and dynamics, with important implications for cross-species transmissions and the generation of novel lineages. These findings validate the utility of env sequences in contributing phylogenetic signal that enlarges our understanding of retrovirus evolution.

Betaretroviral envelope subunits are noncovalently associated and restricted to the mammalian class

The structure of the transmembrane subunit (TM) of the retroviral envelope glycoprotein (Env) is highly conserved among most retrovirus genera and includes a pair of cysteines that forms an intramolecular disulfide loop within the ectodomain. Alpha-, gamma-, and deltaretroviruses have a third cysteine, adjacent to the loop, which forms a disulfide bond between TM and the surface subunit (SU) of Env, while lentiviruses, which have noncovalently associated subunits, lack this third cysteine. The Betaretrovirus genus includes Jaagsiekte sheep retrovirus (JSRV) and mouse mammary tumor virus (MMTV), as well as many endogenous retroviruses. Envelope subunit association had not been characterized in the betaretroviruses, but lack of a third cysteine in the TM ectodomain suggested noncovalently associated subunits. We tested the Env proteins of JSRV and MMTV, as well as human endogenous retrovirus K (HERV-K)108--a betaretrovirus-like human endogenous retrovirus--for intersubunit bonding and found that, as in the lentiviruses, the Env subunits lack an intersubunit disulfide bond. Since these results suggest that the number of cysteines in the TM loop region readily distinguishes between covalent and noncovalent structure, we surveyed endogenous retroviral TM sequences in the genomes of vertebrates represented in public databases and found that (i) retroviruses with noncovalently associated subunits have been present during all of anthropoid evolution and (ii) the noncovalent env motif is limited to mammals, while the covalent type is found among five vertebrate classes. We discuss implications of these findings for retroviral evolution, cross-species transmissions, and recombination events involving the env gene.

Maturation cleavage of the murine leukemia virus Env precursor separates the transmembrane subunits to prime it for receptor triggering

The Env protein of murine leukemia virus matures by two cleavage events. First, cellular furin separates the receptor binding surface (SU) subunit from the fusion-active transmembrane (TM) subunit and then, in the newly assembled particle, the viral protease removes a 16-residue peptide, the R-peptide from the endodomain of the TM. Both cleavage events are required to prime the Env for receptor-triggered activation. Cryoelectron microscopy (cryo-EM) analyses have shown that the mature Env forms an open cage-like structure composed of three SU-TM complexes, where the TM subunits formed separated Env legs. Here we have studied the structure of the R-peptide precursor Env by cryo-EM. TM cleavage in Moloney murine leukemia virus was inhibited by amprenavir, and the Envs were solubilized in Triton X-100 and isolated by sedimentation in a sucrose gradient. We found that the legs of the R-peptide Env were held together by trimeric interactions at the very bottom of the Env. This suggested that the R-peptide ties the TM legs together and that this prevents the activation of the TM for fusion. The model was supported by further cryo-EM studies using an R-peptide Env mutant that was fusion-competent despite an uncleaved R-peptide. The Env legs of this mutant were found to be separated, like in the mature Env. This shows that it is the TM leg separation, normally caused by R-peptide cleavage, that primes the Env for receptor triggering.

Human T-cell leukemia viruses are highly unstable over a wide range of temperatures

The biological properties of human T-cell leukemia virus type I (HTLV-I) and HTLV type II (HTLV-II) are not well elucidated as cell-free viruses. We established new assay systems to detect the infectivity of cell-free HTLVs and examined the stability of cell-free HTLVs at different temperatures. HTLVs lost infectivity more rapidly than did bovine leukemia virus (BLV), which is genetically related to HTLVs. The half-lives of three HTLV-I strains (two cosmopolitan strains and one Melanesian strain) at 37 °C were approximately 0.6 h, whereas the half-life of a BLV strain was 8.5 h. HTLV-I rapidly lost infectivity unexpectedly at 0 and 4 °C. We examined the stability of vesicular stomatitis virus pseudotypes with HTLV-I, HTLV-II or BLV Env proteins, and the Env proteins of HTLVs were found to be more unstable at 4 and 25 °C than the Env proteins of the BLV. Over the course of the viral life cycle, heat treatment inhibited HTLV-I infection at the phase of attachment to the host cells, and inhibition was more marked upon entry into the cells. The HTLV-I Env surface (SU) protein (gp46) was easily released from virions during incubation at 37 °C. However, this release was inhibited by pre-treatment of the virions with N-ethylmaleimide, suggesting that the inter-subunit bond between gp46 SU and gp21 transmembrane (TM) proteins is rearranged by disulfide bond isomerization. HTLVs are highly unstable over a wide range of temperatures because the disulfide bonds between the SU and TM proteins are labile.

Cooperative cleavage of the R peptide in the Env trimer of Moloney murine leukemia virus facilitates its maturation for fusion competence

The spike protein of murine leukemia virus, MLV, is made as a trimer of the Env precursor. This is primed for receptor-induced activation of its membrane fusion function first by cellular furin cleavage in the ectodomain and then by viral protease cleavage in the endodomain. The first cleavage separates the peripheral surface (SU) subunit from the transmembrane (TM) subunit, and the latter releases a 16-residue-long peptide (R) from the TM endodomain. Here, we have studied the distribution of R peptide cleavages in the spike TM subunits of Moloney MLV preparations with partially R-peptide-processed spikes. The spikes were solubilized as trimers and separated with an R peptide antibody. This showed that the spikes were either uncleaved or cleaved in all of its TM subunits. Further studies showed that R peptide cleavage-inhibited Env mutants, L(649)V and L(649)I, were rescued by wild-type (wt) Env in heterotrimeric spikes. These findings suggested that the R peptide cleavages in the spike are facilitated through positive allosteric cooperativity; i.e., the cleavage of the TM subunit in one Env promoted the cleavages of the TMs in the other Envs. The mechanism ensures that protease cleavage in newly released virus will generate R-peptide-cleaved homotrimers rather than heterotrimeric intermediates. However, using a cleavage site Env mutant, L(649)R, which was not rescued by wt Env, it was possible to produce virus with heterotrimers. These were shown to be less fusion active than the R-peptide-cleaved homotrimers. Therefore, the cooperative cleavage will speed up the maturation of released virus for fusion competence.

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.

Role of ubiquitin C-terminal hydrolase-L1 in antipolyspermy defense of mammalian oocytes

The ubiquitin-proteasome system regulates many cellular processes through rapid proteasomal degradation of ubiquitin-tagged proteins. Ubiquitin C-terminal hydrolase-L1 (UCHL1) is one of the most abundant proteins in mammalian oocytes. It has weak hydrolytic activity as a monomer and acts as a ubiquitin ligase in its dimeric or oligomeric form. Recently published data show that insufficiency in UCHL1 activity coincides with polyspermic fertilization; however, the mechanism by which UCHL1 contributes to this process remains unclear. Using UCHL1-specific inhibitors, we induced a high rate of polyspermy in bovine zygotes after in vitro fertilization. We also detected decreased levels in the monomeric ubiquitin and polyubiquitin pool. The presence of UCHL1 inhibitors in maturation medium enhanced formation of presumptive UCHL1 oligomers and subsequently increased abundance of K63-linked polyubiquitin chains in oocytes. We analyzed the dynamics of cortical granules (CGs) in UCHL1-inhibited oocytes; both migration of CGs toward the cortex during oocyte maturation and fertilization-induced extrusion of CGs were impaired. These alterations in CG dynamics coincided with high polyspermy incidence in in vitro-produced UCHL1-inhibited zygotes. These data indicate that antipolyspermy defense in bovine oocytes may rely on UCHL1-controlled functioning of CGs.

Turning of the receptor-binding domains opens up the murine leukaemia virus Env for membrane fusion

The activity of the membrane fusion protein Env of Moloney mouse leukaemia virus is controlled by isomerization of the disulphide that couples its transmembrane (TM) and surface (SU) subunits. We have arrested Env activation at a stage prior to isomerization by alkylating the active thiol in SU and compared the structure of isomerization-arrested Env with that of native Env. Env trimers of respective form were isolated from solubilized particles by sedimentation and their structures were reconstructed from electron microscopic images of both vitrified and negatively stained samples. We found that the protomeric unit of both trimers formed three protrusions, a top, middle and a lower one. The atomic structure of the receptor-binding domain of SU fitted into the upper protrusion. This was formed similar to a bent finger. Significantly, in native Env the tips of the fingers were directed against each other enclosing a cavity below, whereas they had turned outward in isomerization-arrested Env transforming the cavity into an open well. This might subsequently guide the fusion peptides in extended TM subunits into the target membrane.

Intersubunit disulfide isomerization controls membrane fusion of human T-cell leukemia virus Env

Human T-cell leukemia virus (HTLV-1) Env carries a typical disulfide isomerization motif, C(225)XXC, in the C-terminal domain SU. Here we have tested whether this motif is used for isomerization of the intersubunit disulfide of Env and whether this rearrangement is required for membrane fusion. We introduced the C225A and C228A mutations into Env and found that the former but not the latter mutant matured into covalently linked SU-TM complexes in transfected cells. Next, we constructed a secreted Env ectodomain and showed that it underwent incubation-dependent intersubunit disulfide isomerization on target cells. However, the rearrangement was blocked by the C225A mutation, suggesting that C(225) carried the isomerization-active thiol. Still, it was possible to reduce the intersubunit disulfide of the native C225A ectodomain mutant with dithiothreitol (DTT). The importance of the CXXC-mediated disulfide isomerization for infection was studied using murine leukemia virus vectors pseudotyped with wild-type or C225A HTLV-1 Env. We found that the mutant Env blocked infection, but this could be rescued with DTT. The fusion activity was tested in a fusion-from-within assay using a coculture of rat XC target and transfected BHK-21 effector cells. We found that the mutation blocked polykaryon formation, but this could be reversed with DTT. Similar DTT-reversible inhibition of infection and fusion was observed when a membrane-impermeable alkylator was present during the infection/fusion incubation. We conclude that the fusion activity of HTLV-1 Env is controlled by an SU CXXC-mediated isomerization of the intersubunit disulfide. Thus, this extends the applicability of the isomerization model from gammaretroviruses to deltaretroviruses.

Structures and mechanisms of viral membrane fusion proteins: multiple variations on a common theme

Recent work has identified three distinct classes of viral membrane fusion proteins based on structural criteria. In addition, there are at least four distinct mechanisms by which viral fusion proteins can be triggered to undergo fusion-inducing conformational changes. Viral fusion proteins also contain different types of fusion peptides and vary in their reliance on accessory proteins. These differing features combine to yield a rich diversity of fusion proteins. Yet despite this staggering diversity, all characterized viral fusion proteins convert from a fusion-competent state (dimers or trimers, depending on the class) to a membrane-embedded homotrimeric prehairpin, and then to a trimer-of-hairpins that brings the fusion peptide, attached to the target membrane, and the transmembrane domain, attached to the viral membrane, into close proximity thereby facilitating the union of viral and target membranes. During these conformational conversions, the fusion proteins induce membranes to progress through stages of close apposition, hemifusion, and then the formation of small, and finally large, fusion pores. Clearly, highly divergent proteins have converged on the same overall strategy to mediate fusion, an essential step in the life cycle of every enveloped virus.

Stabilization of TM trimer interactions during activation of moloney murine leukemia virus Env

The transmembrane subunit (TM) of the trimeric retrovirus Env complex is thought to direct virus-cell membrane fusion by refolding into a cell membrane-interacting, extended form that subsequently folds back on itself into a very stable trimer of hairpin-like TM polypeptides. However, so far there is only limited evidence for the formation of a stable TM trimer during Env activation. Here we have studied the oligomer composition and stability of an intermediate and the fully activated form of Moloney murine leukemia virus (Mo-MLV) Env. Activation of Mo-MLV Env is controlled by isomerization of its intersubunit disulfide. This results in surface subunit (SU) dissociation and TM refolding. If activation is done in the presence of an alkylator, this will modify the isomerization-active thiol in the SU of Env and arrest Env at an intermediate stage, the isomerization-arrested state (IAS) of its activation pathway. We generated IAS and fully activated Envs in vitro and in vivo and studied their states of oligomerization by two-dimensional blue native polyacrylamide gel electrophoresis (PAGE) and nonreducing sodium dodecyl sulfate (SDS)-PAGE. The IAS Env was composed of trimers of SU-TM complexes, whereas the activated Env consisted of SU monomers and TM trimers. When the oligomers were subjected to mild SDS treatment the TM trimer was found to be 3.5 times more resistant than the IAS oligomer. Thus, this demonstrates that a structural conversion of TM takes place during activation, which results in the formation of a stable TM trimer.

R-Peptide cleavage potentiates fusion-controlling isomerization of the intersubunit disulfide in Moloney murine leukemia virus Env

Fusion of the membrane of the Moloney murine leukemia virus (Mo-MLV) Env protein is facilitated by cleavage of the R peptide from the cytoplasmic tail of its TM subunit, but the mechanism for this effect has remained obscure. The fusion is also controlled by the isomerization of the intersubunit disulfide of the Env SU-TM complex. In the present study, we used several R-peptide-cleavage-inhibited virus mutants to show that the R peptide suppresses the isomerization reaction in both in vitro and in vivo assays. Thus, the R peptide affects early steps in the activation pathway of murine leukemia virus Env.

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.

The conserved His8 of the Moloney murine leukemia virus Env SU subunit directs the activity of the SU-TM disulphide bond isomerase

Murine leukemia virus (MLV) fusion is controlled by isomerization of the disulphide bond between the receptor-binding surface (SU) and fusion-active transmembrane subunits of the Env-complex. The bond is in SU linked to a CXXC motif. This carries a free thiol that upon receptor binding can be activated (ionized) to attack the disulphide and rearrange it into a disulphide isomer within the motif. To find out whether His8 in the conserved SPHQ sequence of Env directs thiol activation, we analyzed its ionization in MLV vectors with wtEnv and Env with His8 deleted or substituted for Tyr or Arg, which partially or completely arrests fusion. The ionization was monitored by following the pH effect on isomerization in vitro by Ca2+ depletion or in vivo by receptor binding. We found that wtEnv isomerized optimally at slightly basic pH whereas the partially active mutant required higher and the inactive mutants still higher pH. This suggests that His8 directs the ionization of the CXXC thiol.

Kinetic analyses of the surface-transmembrane disulfide bond isomerization-controlled fusion activation pathway in Moloney murine leukemia virus

The surface (SU) and transmembrane (TM) subunits of Moloney murine leukemia virus (Mo-MLV) Env are disulfide linked. The linking cysteine in SU is part of a conserved CXXC motif in which the other cysteine carries a free thiol. Recently, we showed that receptor binding activates its free thiol to isomerize the intersubunit disulfide bond into a disulfide within the motif instead (M. Wallin, M. Ekström and H. Garoff, EMBO J. 23:54-65, 2004). This facilitated SU dissociation and activation of TM for membrane fusion. The evidence was mainly based on the finding that alkylation of the CXXC-thiol prevented isomerization. This arrested membrane fusion, but the activity could be rescued by cleaving the intersubunit disulfide bond with dithiothreitol (DTT). Here, we demonstrate directly that receptor binding causes SU-TM disulfide bond isomerization in a subfraction of the viral Envs. The kinetics of the isomerization followed that of virus-cell membrane fusion. Arresting the fusion with lysophosphatidylcholine did not arrest isomerization, suggesting that isomerization precedes the hemifusion stage of fusion. Our earlier finding that native Env was not possible to alkylate but required isomerization induction by receptor binding intimated that alkylation trapped an intermediate form of Env. To further clarify this possibility, we analyzed the kinetics by which the alkylation-sensitive Env was generated during fusion. We found that it followed the fusion kinetics. In contrast, the release of fusion from alkylated, isomerization-blocked virus by DTT reduction of the SU-TM disulfide bond was much faster. These results suggest that the alkylation-sensitive form of Env is a true intermediate in the fusion activation pathway of Env.

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.

The fusion-controlling disulfide bond isomerase in retrovirus Env is triggered by protein destabilization

The membrane fusion function of murine leukemia virus (MLV) is carried by the Env protein. This protein is composed of three SU-TM subunit complexes. The fusion activity is loaded into the transmembrane TM subunit and controlled by the peripheral, receptor-binding SU subunit. It is assumed that TM adopts a metastable conformation in the native Env and that fusion activation involves the folding of TM into a stable form. Activation is suppressed by the associated SU and triggered by its dissociation, which follows receptor binding. Recently we showed that the two subunits are disulfide linked and that SU dissociation and triggering of the fusion function are caused by a switch of the intersubunit disulfide into an intrasubunit disulfide isomer using an isomerization-active CWLC motif in SU (M. Wallin, M. Ekstrom, and H. Garoff, EMBO J. 23:54-65, 2004). In the present work we address how the SU disulfide isomerase is activated. Using Moloney MLV, we show that isomerization of the SU-TM disulfide bond can be triggered by heat, urea, or guanidinium hydrochloride. Such protein perturbation treatments also significantly increase the kinetics and efficiency of viral fusion. The threshold conditions for the effects on isomerization and fusion are virtually the same. This finding indicates that destabilization of interactions in the SU oligomer induces the disulfide bond isomerase and the subsequent activation of the fusion function in TM.

Folding and dimerization of hepatitis C virus E1 and E2 glycoproteins in stably transfected CHO cells

The recombinant E1E2 heterodimer of the hepatitis C virus is a candidate for a subunit vaccine. Folding analysis of E1 and E2 glycoproteins, stably expressed in CHO cells, showed that E1 folding was faster and more efficient than E2. The oxidized DTT-resistant conformation of E1 was completed within 2 h post-synthesis, while E2 not only required up to 6 h but also generated non-native species. Calnexin was found to assist E1 folding, whereas no chaperone association was found with E2. The assembly of E1 and E2 was assessed by co-immunoprecipitation and sedimentation velocity analysis. We found that the formation of native E1E2 heterodimers paralleled E2 oxidation kinetics, suggesting that E2 completed its folding process after association with E1. Once formed, sedimentation of the native E1E2 heterodimers was consistent with the absence of additional associated factors. Taken together, our data strongly suggest that productive folding of the major HCV spike protein E2 is assisted by E1.

Oxidative stress/damage induces multimerization and interaction of Fanconi anemia proteins

Fanconi anemia (FANC) is a heterogeneous genetic disorder characterized by a hypersensitivity to DNA-damaging agents, chromosomal instability, and defective DNA repair. Eight FANC genes have been identified so far, and five of them (FANCA, -C, -E, -F, and -G) assemble in a multinuclear complex and function at least in part in a complex to activate FANCD2 by monoubiquitination. Here we show that FANCA and FANCG are redox-sensitive proteins that are multimerized and/or form a nuclear complex in response to oxidative stress/damage. Both FANCA and FANCG proteins exist as monomers under non-oxidizing conditions, whereas they become multimers following H2O2 treatment. Treatment of cells with oxidizing agent not only triggers the multimeric complex of FANCA and FANCG in vivo but also induces the interaction between FANCA and FANCG. N-Ethylmaleimide treatment abolishes multimerization and interaction of FANCA and FANCG in vitro. Taken together, our results lead us to conclude that FANCA and FANCG uniquely respond to oxidative damage by forming complex(es) via intermolecular disulfide linkage(s), which may be crucial in forming such complexes and in determining their function.

Evolution of the HIV-1 envelope glycoproteins with a disulfide bond between gp120 and gp41

BACKGROUND

We previously described the construction of an HIV-1 envelope glycoprotein complex (Env) that is stabilized by an engineered intermolecular disulfide bond (SOS) between gp120 and gp41. The modified Env protein antigenically mimics the functional wild-type Env complex. Here, we explore the effects of the covalent gp120 - gp41 interaction on virus replication and evolution.

RESULTS

An HIV-1 molecular clone containing the SOS Env gene was only minimally replication competent, suggesting that the engineered disulfide bond substantially impaired Env function. However, virus evolution occurred in cell culture infections, and it eventually always led to elimination of the intermolecular disulfide bond. In the course of these evolution studies, we identified additional and unusual second-site reversions within gp41.

CONCLUSIONS

These evolution paths highlight residues that play an important role in the interaction between gp120 and gp41. Furthermore, our results suggest that a covalent gp120 - gp41 interaction is incompatible with HIV-1 Env function, probably because this impedes conformational changes that are necessary for fusion to occur, which may involve the complete dissociation of gp120 from gp41.

Isomerization of the intersubunit disulphide-bond in Env controls retrovirus fusion

The membrane fusion activity of murine leukaemia virus Env is carried by the transmembrane (TM) and controlled by the peripheral (SU) subunit. We show here that all Env subunits of the virus form disulphide-linked SU-TM complexes that can be disrupted by treatment with NP-40, heat or urea, or by Ca(2+) depletion. Thiol mapping indicated that these conditions induced isomerization of the disulphide-bond by activating a thiol group in a Cys-X-X-Cys (CXXC) motif in SU. This resulted in dissociation of SU from the virus. The active thiol was hidden in uninduced virus but became accessible for alkylation by either Ca(2+) depletion or receptor binding. The alkylation inhibited isomerization, virus fusion and infection. DTT treatment of alkylated Env resulted in cleavage of the SU-TM disulphide-bond and rescue of virus fusion. Further studies showed that virus fusion was specifically inhibited by high and enhanced by low concentrations of Ca(2+). These results suggest that Env is stabilized by Ca(2+) and that receptor binding triggers a cascade of reactions involving Ca(2+) removal, CXXC-thiol exposure, SU-TM disulphide-bond isomerization and SU dissociation, which lead to fusion activation.

Structural criteria for regulation of membrane fusion and virion incorporation by the murine leukemia virus TM cytoplasmic domain

The cytoplasmic domains of viral glycoproteins influence the trafficking and subcellular localization of the glycoproteins and their incorporation into virions. They also promote correct virus morphology and viral budding. The cytoplasmic domains of murine-leukemia-virus envelope-protein TM subunits regulate membrane fusion. During virion maturation the carboxy-terminal 16 amino acid residues of the TM protein are removed by the retroviral protease. Deletion of these residues activates envelope-protein-mediated membrane fusion. Our quantitative analysis of the effects of Moloney murine leukemia virus TM mutations on envelope-protein function support the proposition that a trimeric coiled coil in the TM cytoplasmic domain inhibits fusion. The data demonstrate that cleavage of the TM cytoplasmic domain is not required for viral entry and provide evidence for a model in which fusogenic and nonfusogenic conformations of the envelope protein exists in an equilibrium that is regulated by the cytoplasmic domain. In addition, a conserved tyrosine residue in the TM cytoplasmic domain was shown to play an important role in envelope-protein incorporation into retroviral particles.

The temperature stability of mouse retroviruses depends on the cholesterol levels of viral lipid shell and cellular plasma membrane

To delineate parameters contributing to the extracellular lifetime of retroviral vectors, we carried out stability tests of retroviruses derived from cell lines of different origin and kept under different cultivation conditions. Results show that amphotropic mouse retroviruses (MLV-A) derived from human and hamster cells exhibit 2- to 3-fold higher half-lives compared to retroviruses from mouse cells. Cultivation at 32 degrees C has been reported to yield high virus titers. However, the benefit of virus production in mouse cells at 32 degrees C is controversial. In our hands the cultivation temperature affected, hitherto not noticed, the half-life time of MLV-A. The 37/32 degrees C shift resulted in a 3-fold decrease of viral half-lifes compared to MLV-A released from mouse cells at 37 degrees C. Thus, MLV-A released at 37 degrees C is phenotypically different from MLV-A synthesized at 32 degrees C. Increased virus stability was inversely correlated with the level of cholesterol in the viral membrane. Finally, depletion of viral cholesterol in vitro resulted in intact virus with increased thermal stability. Thus, retrovirus lability depends on the host cell and parallels the cholesterol amount in the viral lipid shell.

Cytoplasmic tail of Moloney murine leukemia virus envelope protein influences the conformation of the extracellular domain: implications for mechanism of action of the R Peptide

The envelope (Env) protein of Moloney murine leukemia virus (MoMuLV) is a homotrimeric complex whose monomers consist of linked surface (SU) and transmembrane (TM) proteins cleaved from a precursor protein by a cellular protease. In addition, a significant fraction of virion-associated TM is further processed by the viral protease to remove the C-terminal 16 amino acids of the cytoplasmic domain, the R peptide. This cleavage greatly enhances the fusogenicity of the protein and is necessary for the formation of a fully functional Env protein complex. We have previously proposed that R peptide cleavage enhances fusogenicity by altering the conformation of the ectodomain of the protein (Y. Zhao et al., J. Virol. 72:5392-5398, 1998). Using a series of truncation and point mutants of MoMuLV Env, we now provide direct biochemical and immunological evidence that the cytoplasmic tail and the membrane-spanning region of Env can influence the overall structure of the ectodomain of the protein and alter the strength of the SU-TM interaction. The R-peptide-truncated form of the protein, in particular, exhibits a markedly different conformation than the full-length protein.

Disulfide-linked integrase oligomers involving C280 residues are formed in vitro and in vivo but are not essential for human immunodeficiency virus replication

The human immunodeficiency virus type 1 integrase (IN) forms an oligomer that integrates both ends of the viral DNA. The nature of the active oligomer is unclear. Recombinant IN obtained under reducing conditions is always in the form of noncovalent oligomers. However, disulfide-linked oligomers of IN were recently observed within viral particles. We show that IN produced from a baculovirus expression system can form disulfide-linked oligomers. We investigated which residues are responsible for the disulfide bridges and the relationship between the ability to form covalent dimers and IN activity. Only the mutation of residue C280 was sufficient to prevent the formation of intermolecular disulfide bridges in oligomers of recombinant IN. IN activity was studied under and versus nonreducing conditions: the formation of disulfide bridges was not required for the in vitro activities of the enzyme. Moreover, the covalent dimer does not dissociate into individual protomers on disulfide bridge reduction. Instead, IN undergoes a spontaneous multimerization process that yields a homogenous noncovalent tetramer. The C280S mutation also completely abolished the formation of disulfide bonds in the context of the viral particle. Finally, the replication of the mutant virus was investigated in replicating and arrested cells. The infectivity of the virus was not affected by the C280S IN mutation in either dividing or nondividing cells. The disulfide-linked form of the IN oligomers observed in the viral particles is thus not required for viral replication.

No false start for novel pseudotyped vectors

Pseudotyped vectors can be used to introduce genes into cells or to study the entry process of the virus from which the outer shell of the recombinant virus is derived. Recently, several novel pseudotyped retroviruses and lentiviruses have been constructed. Virus vectors pseudotyped with an alphavirus glycoprotein hold special promise. The increasing diversity of the available pseudotyped vectors offers expanded opportunities for gene transfer to specific cells.

A chimeric human T-cell lymphotropic virus type 1 with the envelope glycoprotein of Moloney murine leukemia virus is infectious for murine cells

We constructed a chimeric human T-cell lymphotropic virus type 1 (HTLV-1) provirus in which the original envelope precursor sequence was replaced by that of ecotropic Moloney murine leukemia virus (Mo-MuLV). Chimeric particles produced by transient transfection of this chimeric provirus were infectious for murine cells, such as NIH 3T3 fibroblasts, lymphoid EL4 cells, and primary CD4(+) T lymphocytes, whereas HTLV-1 particles were not. The infectivity of chimeric particles increased 10 times when the R peptide located at the carboxy terminus of the MuLV envelope glycoprotein was deleted. Primary murine CD4(+) T lymphocytes, infected by the Delta R chimeric virus, released particles that could spread the infection to other naive murine lymphoid cells. This chimeric virus, with the Mo-MuLV envelope glycoprotein and the replication characteristics of HTLV-1, should be useful in studying the pathogenesis of HTLV-1 in a mouse model.

IFN-gamma can promote tumor evasion of the immune system in vivo by down-regulating cellular levels of an endogenous tumor antigen

Although IFN-gamma has been generally thought to enhance antitumor immune responses, we found that IFN-gamma can promote tumor escape in the CT26 colon carcinoma by down-regulating the protein expression of an endogenous tumor Ag. gp70, the env product of the endogenous ecotropic murine leukemia virus, has been reported to be the immunodominant Ag of CT26. We show that IFN-gamma down-regulates intracellular and surface levels of gp70 protein resulting in a reduced lysis by CTL, which is restored by pulsing IFN-gamma-treated CT26 with the L(d)-restricted immunodominant AH1 epitope derived from gp70. To investigate the role of CT26 sensitivity to IFN-gamma in vivo, we constructed two variants of CT26, CT26.mugR and CT26.IFN, that are unresponsive to IFN-gamma or express IFN-gamma, respectively. We demonstrate using these variants that tumor responsiveness to IFN-gamma promotes a reduction in tumor immunogenicity in vivo that is correlated with an increased tumor incidence in immune mice. Analysis of the tumors from mice challenged with CT26 or CT26.mugR revealed infiltration of CD8 T cells secreting IFN-gamma. We conclude that IFN-gamma secreted by tumor-infiltrating T cells promotes tumor escape through the down-regulation of the endogenous tumor Ag gp70. These findings have impact on the design of effective antitumor vaccine strategies.

Efficient cell infection by Moloney murine leukemia virus-derived particles requires minimal amounts of envelope glycoprotein

Retrovirus entry into cells is mediated by specific interactions between the retrovirally encoded Env envelope glycoprotein and a host cell surface receptor. Though a number of peptide motifs responsible for the structure as well as for the binding and fusion activities of Env have been identified, only a few quantitative data concerning the infection process are available. Using an inducible expression system, we have expressed various amounts of ecotropic and amphotropic Env at the surfaces of Moloney murine leukemia virus-derived vectors and assayed for the infectivity of viral particles. Contrary to the current view that numerous noncooperative Env-viral receptor interactions are required for cell infection, we report here that very small amounts of Env are sufficient for optimal infection. However, increasing Env density clearly accelerates the rate at which infectious attachment to cells occurs. Moreover, our data also show that a surprisingly small number of Env molecules are sufficient to drive infection, albeit at a reduced efficiency, and that, under conditions of low expression, Env molecules act cooperatively. These observations have important consequences for our understanding of natural retroviral infection as well as for the design of cell-targeted infection techniques involving retroviral vectors.

The SU and TM envelope protein subunits of bovine leukemia virus are linked by disulfide bonds, both in cells and in virions

After the polyprotein precursor of retroviral envelope proteins is proteolytically cleaved, the surface (SU) and transmembrane (TM) subunits remain associated with each other by noncovalent interactions or by disulfide bonds. Disulfide linkages confer a relatively stable association between the SU and TM envelope protein subunits of Rous sarcoma virus and murine leukemia virus. In contrast, the noncovalent association between SU and TM of human immunodeficiency virus leads to significant shedding of SU from the surface of infected cells. The SU and TM proteins of bovine leukemia virus (BLV) initially were reported to be disulfide linked but later were concluded not to be, since TM is often lost during purification of SU protein. Here, we show that SU and TM of BLV do, indeed, associate through disulfide bonds, whether the envelope proteins are overexpressed in transfected cells, are produced in virus-infected cells, or are present in newly produced virions.

A recombinant human immunodeficiency virus type 1 envelope glycoprotein complex stabilized by an intermolecular disulfide bond between the gp120 and gp41 subunits is an antigenic mimic of the trimeric virion-associated structure

The few antibodies that can potently neutralize human immunodeficiency virus type 1 (HIV-1) recognize the limited number of envelope glycoprotein epitopes exposed on infectious virions. These native envelope glycoprotein complexes comprise three gp120 subunits noncovalently and weakly associated with three gp41 moieties. The individual subunits induce neutralizing antibodies inefficiently but raise many nonneutralizing antibodies. Consequently, recombinant envelope glycoproteins do not elicit strong antiviral antibody responses, particularly against primary HIV-1 isolates. To try to develop recombinant proteins that are better antigenic mimics of the native envelope glycoprotein complex, we have introduced a disulfide bond between the C-terminal region of gp120 and the immunodominant segment of the gp41 ectodomain. The resulting gp140 protein is processed efficiently, producing a properly folded envelope glycoprotein complex. The association of gp120 with gp41 is now stabilized by the supplementary intermolecular disulfide bond, which forms with approximately 50% efficiency. The gp140 protein has antigenic properties which resemble those of the virion-associated complex. This type of gp140 protein may be worth evaluating for immunogenicity as a component of a multivalent HIV-1 vaccine.

Oligomerization within virions and subcellular localization of human immunodeficiency virus type 1 integrase

Previous biochemical and genetic evidence indicated that the functional form of retroviral integrase protein (IN) is a multimer. A direct demonstration of IN oligomerization during the infectious cycle was, however, missing, due to the absence of a sensitive detection method. We describe here the generation of infectious human immunodeficiency virus type 1 (HIV-1) viral clones carrying IN protein tagged with highly antigenic epitopes. In this setting, we could readily visualize IN both in producer cells and in viral particles. More interestingly, we detected IN oligomers, the formation of which was dependent on disulfide bridges and took place inside virions. Additionally, expression of a tagged HIV-1 IN in the absence of other viral components resulted in almost exclusive nuclear accumulation of the protein. Mutation of a conserved cysteine in the proposed dimer interface determined the loss of viral infectivity, associated with a reduction of IN oligomer formation and the redistribution of the mutated protein in the nucleus and cytoplasm. Epitope tagging of HIV-1 IN expressed alone or in the context of a replication-competent viral clone provides powerful tools to validate debated issues on the implication of this enzyme in different steps of the viral cycle.

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.