Expression of the TM protein of Rous sarcoma virus in the absence of SU shows that this domain is capable of oligomerization and intracellular transport.

The SU and TM subunits of the Rous sarcoma virus glycoprotein, which are derived from a single polypeptide precursor, have been expressed independently with a simian virus 40 vector. The TM protein retains the ability to form an oligomer which resembles the TM oligomer derived from the wild-type glycoprotein complex present in virions. Oligomerization of the recombinant TM protein is more rapid than that observed for the intact glycoprotein expressed from the simian virus 40 vector and is required for its transport out of the endoplasmic reticulum. Oligomeric TM is terminally glycosylated in the Golgi complex but is less stable than the intact wild-type protein and does not accumulate at the cell surface. The SU protein, in contrast, does not form detectable oligomers but is efficiently secreted into the culture medium. These observations suggest that the oligomerization domain of the Rous sarcoma virus glycoprotein lies in the TM protein and that it can function independently of SU.     

The transmembrane glycoprotein of human immunodeficiency virus type 1 induces syncytium formation in the absence of the receptor binding glycoprotein.

To study the intracellular transport and biological properties of the human immunodeficiency virus type 1 (HIV-1) transmembrane glycoprotein (TM; gp41), we constructed a truncated envelope gene in which the majority of the coding sequences for the surface glycoprotein (SU; gp120) were deleted. Transient expression of this truncated env gene in primate cells resulted in the biosynthesis of two proteins with M(r)s of 52,000 and 41,000, respectively. Immunofluorescence studies with antibodies to the HIV-1 TM protein indicated that the intracellular and surface localization of these proteins were indistinguishable from those of the native HIV-1 gp120-gp41 complex. These results indicate that the oligosaccharide processing and cell surface transport of the HIV-1 TM protein were not dependent on the presence of the receptor binding subunit, gp120. Syncytium formation was readily detected upon expression of the deleted HIV-1 env gene into COS and CD4+ HeLa cell lines, suggesting that in the absence of gp120, the TM protein retained biological activity. This observation was confirmed by infection of primate and mouse cell lines with a recombinant vaccinia virus (vvgp41) expressing the truncated HIV-1 env gene. These results strongly suggest that (i) the two biological activities of the HIV-1 envelope glycoprotein can occur independently and (ii) the association of the two glycoprotein subunits may restrict the fusion activity of the transmembrane component to CD4+ cells.     

The transmembrane domain sequence affects the structure and function of the Newcastle disease virus fusion protein

The role of specific sequences in the transmembrane (TM) domain of Newcastle disease virus (NDV) fusion (F) protein in the structure and function of this protein was assessed by replacing this domain with the F protein TM domains from two other paramyxoviruses, Sendai virus (SV) and measles virus (MV), or the TM domain of the unrelated glycoprotein (G) of vesicular stomatitis virus (VSV). Mutant proteins with the SV or MV F protein TM domains were expressed, transported to cell surfaces, and proteolytically cleaved at levels comparable to that of the wild-type protein, while mutant proteins with the VSV G protein TM domain were less efficiently expressed on cell surfaces and proteolytically cleaved. All mutant proteins were defective in all steps of membrane fusion, including hemifusion. In contrast to the wild-type protein, the mutant proteins did not form detectable complexes with the NDV hemagglutinin-neuraminidase (HN) protein. As determined by binding of conformation-sensitive antibodies, the conformations of the ectodomains of the mutant proteins were altered. These results show that the specific sequence of the TM domain of the NDV F protein is important for the conformation of the preactivation form of the ectodomain, the interactions of the protein with HN protein, and fusion activity.     

Mutation of the dominant endocytosis motif in human immunodeficiency virus type 1 gp41 can complement matrix mutations without increasing Env incorporation

The human immunodeficiency virus type 1 transmembrane glycoprotein (TM) is efficiently endocytosed in a clathrin-dependent manner. Internalization is mediated by a tyrosine-containing motif within the cytoplasmic domain, and replacement of the cytoplasmic tyrosine by cysteine or phenylalanine increased expression of mutant glycoprotein on the surface of transfected cells by as much as 2.5-fold. Because interactions between the cytoplasmic domain of Env and the matrix protein (MA) have been suggested to mediate incorporation of Env in virus particles, we examined whether perturbation of endocytosis would alter incorporation. Proviruses were constructed to contain the wild-type or mutant Env in conjunction with point mutations in MA that had previously been shown to block Env incorporation. These constructs were used to evaluate the effect of glycoprotein endocytosis on incorporation into virus particles and to test the necessity for a specific interaction between Env and MA to mediate incorporation. Viruses produced from transfected 293T cells were used to infect various cell lines, including MAGI, H9, and CEMx174. Viruses encoding both a disrupted endocytosis motif signal and mutations within MA were significantly more infectious in MAGI cells than their counterparts encoding a mutant MA and wild-type Env. This complementation of infectivity for the MA incorporation mutant viruses was not due to increased glycoprotein incorporation into particles but instead reflected an enhanced fusogenicity of the mutated Env proteins. Our findings further support the concept that a specific interaction between the long cytoplasmic domain of TM and MA is required for efficient incorporation of Env into assembling virions. Alteration of the endocytosis signal of Env, and the resulting increase in cell surface glycoprotein, has no effect on incorporation despite demonstrable effects on fusion, virus entry, and infectivity.     

Endocytosis of chimeric influenza virus hemagglutinin proteins that lack a cytoplasmic recognition feature for coated pits

The influenza virus A/Japan/305/57 hemagglutinin (HA) can be converted from a protein that is essentially excluded from coated pits into one that is internalized at approximately the rate of uptake of bulk membrane by replacing the HA transmembrane and cytoplasmic sequences with those of either of two other glycoproteins (Roth et al., 1986. J. Cell Biol. 102:1271-1283). To identify more precisely the foreign amino acid sequences responsible for this change in HA traffic, DNA sequences encoding the transmembrane (TM) or cytoplasmic (CD) domains of either the G glycoprotein of vesicular stomatitis virus (VSV) or the gC glycoprotein of herpes simplex virus were exchanged for those encoding the analogous regions of wild type HA (HA wt). HA-HA-G and HA-HA-gC, chimeras that contain only a foreign CD, resembled HA wt in having a long residence on the cell surface and were internalized very slowly. HA-HA-gC was indistinguishable from HA in our assays, whereas twice as much HA-HA-G was internalized as was HA wt. However, HA-G-HA, containing only a foreign TM, was internalized as efficiently as was HA- G-G, a chimeric protein with transmembrane and cytoplasmic sequences of VSV G protein. Conditions that blocked internalization through coated pits also inhibited endocytosis of the chimeric proteins. Although the external domains of the chimeras were less well folded than that of the wild type HA, denaturation of the wild type HA external domain by treatment with low pH did not increase the interaction of HA with coated pits. However, mutation of four amino acids in the TM of HA allowed the protein to be internalized, indicating that the property that allows HA to escape endocytosis resides in its TM. These results indicate that possession of a cytoplasmic recognition feature is not required for the internalization of all cell surface proteins and suggest that multiple mechanisms for internalization exist that operate at distinctly different rates.     

Postassembly cleavage of a retroviral glycoprotein cytoplasmic domain removes a necessary incorporation signal and activates fusion activity.

Viral protease-mediated cleavage within the cytoplasmic domain of the transmembrane (TM) glycoprotein of the type D retrovirus, Mason-Pfizer monkey virus, removes approximately 16 amino acids from the carboxy terminus of the protein. To determine the functional significance of this cleavage in the virus life cycle, we introduced premature stop codons into the TM coding domain, resulting in the production of truncated glycoproteins. Progressive truncated of the cytoplasmic domain identified the carboxy-terminal third as being required for efficient incorporation of the glycoprotein complex into budding virions and profoundly increased the fusogenic capability of the TM glycoprotein. These results, together with the ability of matrix protein mutations to suppress TM cleavage, imply that this portion of the glycoprotein interacts specifically with the capsid proteins during budding, suppressing glycoprotein fusion function until virus maturation has occurred.     

Truncations of the simian immunodeficiency virus transmembrane protein confer expanded virus host range by removing a block to virus entry into cells.

We have investigated how truncation of the cytoplasmic domain of the transmembrane (TM) glycoprotein of simian immunodeficiency virus (SIV) modulates the host range of this virus. Termination codons were introduced into the env gene of SIVmac239 which resulted in the truncation of the transmembrane protein from a wild-type 354 amino acids (TM354) to 207 (TM207) and 193 (TM193) amino acids. Expression of the wild-type and mutant env genes from a simian virus 40-based vector resulted in normal biosynthesis and processing of the glycoproteins to gp130 and gp41 or the truncated TM proteins (gp28 and gp27). When expressed on the surface of COS-1 cells, all three glycoproteins mediated fusion of both CEMX174 and HUT78 cells. Virions containing the wild-type and mutant glycoproteins were capable of efficient replication in macaque peripheral blood lymphocytes and CEMX174 cells; in contrast, only virions that contained TM207 were capable of rapid infection of HUT78 cells. Both truncated glycoproteins were capable of efficiently mediating infection of both CEMX174 and HUT78 cells by an env-deficient human immunodeficiency virus. The wild-type SIV glycoprotein, however, was unable to mediate human immunodeficiency virus infection of HUT78 cells when assayed with this system. An analysis of the protein composition of SIV released from infected CEMX174 cells showed that the mutant virions contained significantly higher levels of glycoprotein compared with the wild type. These results demonstrate that truncation of the SIV cytoplasmic domain removes a block at the level of glycoprotein-mediated virus entry into HUT78 cells and points to a role for glycoprotein density in determining virus tropism.     

A tyrosine motif in the cytoplasmic domain of mason-pfizer monkey virus is essential for the incorporation of glycoprotein into virions

Mason-Pfizer monkey virus (M-PMV) encodes a transmembrane (TM) glycoprotein with a 38-amino-acid-long cytoplasmic domain. After the release of the immature virus, a viral protease-mediated cleavage occurs within the cytoplasmic domain, resulting in the loss of 17 amino acids from the carboxy terminus. This maturational cleavage occurs between a histidine at position 21 and a tyrosine at position 22 in the cytoplasmic domain of the TM protein. We have demonstrated previously that a truncated TM glycoprotein with a 21-amino-acid-long cytoplasmic tail showed enhanced fusogenicity but could not be incorporated into virions. These results suggest that postassembly cleavage of the cytoplasmic domain removes a necessary incorporation signal and activates fusion activity. To investigate the contribution of tyrosine residues to the function of the glycoprotein complex and virus replication, we have introduced amino acid substitutions into two tyrosine residues found in the cytoplasmic domain. The effects of these mutations on glycoprotein biosynthesis and function, as well as on virus infectivity, have been examined. Mutation of tyrosine 34 to alanine had little effect on glycoprotein function. In contrast, substitutions at tyrosine 22 modulated fusion activity in either a positive or negative manner, depending on the substituting amino acid. Moreover, any nonaromatic substitution at this position blocked glycoprotein incorporation into virions and abolished infectivity. These results demonstrate that M-PMV employs a tyrosine signal for the selective incorporation of glycoprotein into budding virions. Antibody uptake studies show that tyrosine 22 is part of an efficient internalization signal in the cytoplasmic domain of the M-PMV glycoprotein that can also be positively and negatively influenced by changes at this site.     

Lysis of target cells infected with vesicular stomatitis virus (VSV) in the presence of tunicamycin by anti-VSV cytotoxic T lymphocytes

We have analyzed the requirement for the expression of the major surface glycoprotein (G protein) of vesicular stomatitis virus (VSV) on target cells for recognition and lysis by anti-VSV cytotoxic T lymphocytes (CTL). In addition, we have attempted to determine if the carbohydrate moieties on the G protein are required for recognition and lysis by anti-VSV CTL. When VSV (Orsay) is grown at 30 degrees C in the presence of tunicamycin (TM), glycosylation of G protein is inhibited; however, nonglycosylated G protein is found on the surface of the cell and active virus particles are produced. In contrast, VSV (Orsay) grown at 39 degrees C in the presence of TM produces low titers of virus and the presence of G protein on the surface of cells is not detectable. The susceptibility of these target cells to lysis by anti-VSV CTL was analyzed. The results suggest that expression of the G protein is required for target cell lysis by anti-VSV CTL. However, the presence of the carbohydrate moieties on the G protein are nt an absolute requirement for recognition by anti-VSV CTL. VSV-infected target cells incubated in the presence of TM were lysed by anti-VSV CTL up to 50 to 80% of the infected target cell control. This result suggests either that some clones of anti-VSV CTL recognize carbohydrate moieties or that carbohydrate moieties play some as yet undefined nonantigenic role in the recognition of the target antigen by the CTL receptor.     

Truncation of the cytoplasmic domain of the simian immunodeficiency virus envelope glycoprotein alters the conformation of the external domain.

We previously reported that truncation of the cytoplasmic domain of the macaque simian immunodeficiency virus SIVmac239 envelope glycoprotein enhanced its ability to induce cell fusion in a variety of cell lines. In the present study, we examined the expression of the full-length and truncated SIVmac239 envelope glycoprotein complex on cell surfaces. Using a membrane-impermeable reagent to biotinylate proteins on cell surfaces followed by immunoprecipitation, we found that under conditions in which the full-length TM protein could not be detected on the surfaces of CD4-positive or CD4-negative cell lines, the truncated TM protein was detected efficiently. In contrast, using a membrane-impermeable iodination reagent to label proteins on cell surfaces, we could detect both the full-length and truncated TM proteins. No difference between the full-length and truncated proteins was observed in the detection of the SU proteins in the biotinylation assay. Additionally, we used an assay in which SIV-specific antibodies are prebound to the native envelope proteins expressed on the cell surface and then the proteins are immunoprecipitated. Using this assay, we could not detect the truncated or full-length TM protein on the cell surface, whereas we could detect the SU subunits of both proteins. We also observed that the truncated TM protein formed more stable sodium dodecyl sulfate-resistant oligomers than the full-length TM protein did. These results indicate that truncation of the cytoplasmic domain of the SIVmac239 envelope glycoprotein affects the conformation of the external domain of the TM protein on the cell surface, even though the two proteins have no differences in the amino acid sequences of their external domains. This altered conformation could play a role in the enhanced fusion activity of the truncated SIV glycoprotein.     

Analysis of the cleavage site of the human immunodeficiency virus type 1 glycoprotein: requirement of precursor cleavage for glycoprotein incorporation.

Endoproteolytic cleavage of the glycoprotein precursor to the mature SU and TM proteins is an essential step in the maturation of retroviral glycoproteins. Cleavage of the precursor polyprotein occurs at a conserved, basic tetrapeptide sequence and is carried out by a cellular protease. The glycoprotein of the human immunodeficiency virus type 1 contains two potential cleavage sequences immediately preceding the N terminus of the TM protein. To determine the functional significance of these two potential cleavage sites, a series of mutations has been constructed in each site individually, as well as in combinations that altered both sites simultaneously. A majority of the mutations in either potential cleavage site continued to allow efficient cleavage when present alone but abrogated cleavage of the precursor when combined. Despite being transported efficiently to the cell surface, these cleavage-defective glycoproteins were unable to initiate cell-cell fusion and viruses containing them were not infectious. Viruses that contained glycoproteins with a single mutation, and that retained the ability to be processed, were capable of mediating a productive infection, although infectivity was impaired in several of these mutants. Protein analyses indicated that uncleaved glycoprotein precursors were inefficiently incorporated into virions, suggesting that cleavage of the glycoprotein may be a prerequisite to incorporation into virions. The substitution of a glutamic acid residue for a highly conserved lysine residue in the primary cleavage site (residue 510) had no effect on glycoprotein cleavage or function, even though it removed the only dibasic amino acid pair in this site. Peptide sequencing of the N terminus of gp41 produced from this mutant glycoprotein demonstrated that cleavage continued to take place at this site. These results, demonstrating that normal cleavage of the human immunodeficiency virus type 1 glycoprotein can occur when no dibasic sequence is present at the cleavage site, raise questions about the specificity of the cellular protease that mediates this cleavage and suggest that cleavage of the glycoprotein is required for efficient incorporation of the glycoprotein into virions.     

Caprine arthritis-encephalitis virus envelope surface glycoprotein regions interacting with the transmembrane glycoprotein: structural and functional parallels with human immunodeficiency virus type 1 gp120

A sequence similarity between surface envelope glycoprotein (SU) gp135 of the lentiviruses maedi-visna virus and caprine arthritis-encephalitis virus (CAEV) and human immunodeficiency virus type 1 (HIV-1) gp120 has been described. The regions of sequence similarity are in the second and fifth conserved regions of gp120, and the similarity is highest in sequences coinciding with beta-strands 4 to 8 and 25, which are located in the most virion-proximal region of the gp120 inner domain. A subset of this structure, formed by gp120 beta-strands 4, 5, and 25, is conserved in most or all lentiviruses. Because of the orientation of gp120 on the virion, this highly conserved virion-proximal region of the gp120 core may interact with the transmembrane glycoprotein (TM) together with the amino and carboxy termini of full-length gp120. Therefore, interactions between SU and TM of lentiviruses may be structurally related. Here we tested whether the amino acid residues in the putative virion-proximal region of CAEV gp135 comprising putative beta-strands 4, 5, and 25, as well as its amino and carboxy termini, are important for stable interactions with TM. An amino acid change at gp135 position 119 or 521, located in the turn between putative beta-strands 4 and 5 and near beta-strand 25, respectively, specifically disrupted the epitope recognized by monoclonal antibody 29A. Thus, similar to the corresponding gp120 regions, these gp135 residues are located in close proximity to each other in the folded protein, supporting the hypothesis of a structural similarity between the gp120 virion-proximal inner domain and gp135. Amino acid changes in the amino- and carboxy-terminal and putative virion-proximal regions of gp135 increased gp135 shedding from the cell surface, indicating that these gp135 regions are involved in interactions with TM. Our results indicate structural and functional parallels between CAEV gp135 and HIV-1 gp120 that may be more broadly applicable to the SU of other lentiviruses.     

Mutations in the leucine zipper of the human immunodeficiency virus type 1 transmembrane glycoprotein affect fusion and infectivity.

Many retroviruses, including the human and simian immunodeficiency viruses, contain a leucine zipper-like repeat in a highly conserved region of the external domain of the transmembrane (TM) glycoprotein. This region has been postulated to play a role in stabilizing the oligomeric form of these molecules. To determine what role this region might play in envelope structure and function, several mutations were engineered into the middle isoleucine of the leucine zipper-like repeat of the human immunodeficiency virus type 1 (HIV-1) TM protein. A phenotypic analysis of these mutants demonstrated that conservative mutations (Ile to Val or Leu) did not block the ability of the viral glycoprotein to mediate cell-cell fusion or affect virus infectivity. In contrast, each of the other mutations, except for the Ile-to-Ala change, completely inhibited the ability of the glycoprotein to fuse HeLa-T4 cells and of mutant virions to infect H9 cells. The alanine mutation produced an intermediate phenotype in which both cell fusion and infectivity were significantly reduced. Thus, the biological activity of the glycoprotein titrates with the hydrophobicity of the residue in this position. None of the mutations affected the synthesis, oligomer formation, transport, or processing of the HIV glycoprotein complex. Although these results do not rule out a role for the leucine zipper region in glycoprotein oligomerization, they clearly point to a critical role for it in a post-CD4 binding step in HIV membrane fusion and virus entry.     

Progressive truncations C terminal to the membrane-spanning domain of simian immunodeficiency virus Env reduce fusogenicity and increase concentration dependence of Env for fusion

The simian immunodeficiency virus (SIV) transmembrane (TM) protein, gp41, has multiple functions, which include anchoring the glycoprotein complex in the lipid envelope of the virus and mediating fusion of the virus and host cell membranes. Recently, a series of mutants of the SIVmac239 TM protein that have truncations at the carboxyl terminus of the membrane-spanning domain (MSD) have been characterized (J. T. West, P. Johnston, S. R. Dubay, and E. Hunter, J. Virol. 75:9601-9612, 2001). These mutants retained membrane anchorage but demonstrated reduced fusogenicity and infectivity as the MSD length was shortened. We have established a novel three-color fluorescence assay, which allows qualitative confocal and quantitative flow cytometric analyses, to further characterize the nature of the fusion defect in five of the MSD mutants: TM185, TM186, TM187, TM188, and TM189. Our analysis showed that each mutant could mediate complete lipid and aqueous dye transfer at early time points after effector and target cell mixing. No hemifusion with only lipid dye flux was detected. However, another intermediate fusion stage, which appears to involve small-fusion-pore formation that allowed small aqueous dye transfer but prevented the exchange of large cytoplasmic components, was identified infrequently in mutant-Env-expressing cell and target cell mixtures. Quantitative flow cytometric analysis of these mutants demonstrated that the TM187, TM188, and TM189 mutants were significantly more fusogenic than TM185 and TM186 but remained significantly impaired compared to the wild type. Moreover, fusion efficiency showed an increased dependence on the expression level of glycoproteins, suggesting that, for these mutants, formation of an active fusion complex was an increasingly stochastic event.     

Effect of epitope position on neutralization by anti-human immunodeficiency virus monoclonal antibody 2F5

The membrane-proximal region of the human immunodeficiency virus type 1 (HIV-1) transmembrane protein (TM) is critical for envelope (Env)-mediated membrane fusion and contains the target for broadly reactive neutralizing antibody 2F5. It has been proposed that 2F5 neutralization might involve interaction of its long, hydrophobic, complementarity-determining region (CDR) H3, with adjacent viral membrane. Using Moloney murine leukemia virus (MLV) as a tool, we examined the effect of epitope position on 2F5 neutralization. When the 2F5 epitope was inserted in the proline-rich region of MLV Env surface protein (SU), 2F5 blocked cell fusion and virus infection, whereas MLV with a hemagglutinin (HA) epitope at the same position was not neutralized by anti-HA, even though the antibodies bound their respective Envs on the surface of infected cells and viruses equally well. When the 2F5 epitope was inserted in the MLV Env TM at a position comparable to its natural position in HIV-1 TM, 2F5 antibody blocked Env-mediated cell fusion. Epitope position had subtle effects on neutralization by 2F5: the antibody concentration for 50% inhibition of cell fusion was more than 10-fold lower when the 2F5 epitope was in SU than in TM, and inhibition was less complete at high concentrations of antibody; we discuss possible explanations for these effects of epitope position. Since membrane proximity was not required for neutralization by 2F5 antibody, we speculate that the CDR H3 of 2F5 contributes to neutralization by destabilizing an adjacent protein rather than by inserting into an adjacent membrane.     

Effect of retroviral proteinase inhibitors on Mason-Pfizer monkey virus maturation and transmembrane glycoprotein cleavage.

Mason-Pfizer monkey virus (M-PMV) is the prototype type D retrovirus which preassembles immature intracytoplasmic type A particles within the infected cell cytoplasm. Intracytoplasmic type A particles are composed of uncleaved polyprotein precursors which upon release are cleaved by the viral proteinase to their constituent mature proteins. This results in a morphological change in the virion described as maturation. We have investigated the role of the viral proteinase in virus maturation and infectivity by inhibiting the function of the enzyme through mutagenesis of the proteinase gene and by using peptide inhibitors originally designed to block human immunodeficiency virus type 1 proteinase activity. Mutation of the active-site aspartic acid, Asp-26, to asparagine abrogated the activity of the M-PMV proteinase but did not affect the assembly of noninfectious, immature virus particles. In mutant virions, the transmembrane glycoprotein (TM) of M-PMV, initially synthesized as a cell-associated gp22, is not cleaved to gp20, as is observed with wild-type virions. This demonstrates that the viral proteinase is responsible for this cleavage event. Hydroxyethylene isostere human immunodeficiency virus type 1 proteinase inhibitors were shown to block M-PMV proteinase cleavage of the TM glycoprotein and Gag-containing precursors in a dose-dependent manner. The TM cleavage event was more sensitive than cleavage of the Gag precursors to inhibition. The infectivity of treated particles was reduced significantly, but experiments showed that inhibition of precursor and TM cleavage may be at least partially reversible. These results demonstrate that the M-PMV aspartyl proteinase is activated in released virions and that the hydroxyethylene isostere proteinase inhibitors used in this study exhibit a broad spectrum of antiretroviral activity.     

Functional Murine Leukemia Virus Vectors Pseudotyped with the Visna Virus Envelope Show Expanded Visna Virus Cell Tropism

Pseudotype virus vectors serve as a powerful tool for the study of virus receptor usage and entry. We describe the development of murine leukemia virus (MuLV) particles pseudotyped with the visna virus envelope glycoprotein and encoding a green fluorescent protein reporter as a tool to study the expression of the visna virus receptor. Functional MuLV/visna virus pseudotypes were obtained when the cytoplasmic tail of the visna virus envelope TM protein was truncated to 3, 7, or 11 amino acids in length. MuLV/visna virus particles were used to transduce a panel of cell types from various organisms, including sheep, goat, human, hamster, mouse, monkey, and quail. The majority of the cells examined were susceptible to MuLV/visna pseudotype viruses, supporting the notion that the visna virus cellular receptor is a widely expressed protein found in many species. Of 16 different cell types tested, only mouse embryo fibroblast NIH 3T3 cells, hamster ovary CHO cells, and the human promonocyte cell line U937 cells were not susceptible to transduction by the pseudotyped virus. The production of functional MuLV/visna virus pseudotypes has provided a sensitive, biologically relevant system to study visna virus cell entry and envelope-receptor interactions.     

Preliminary characterization of an epitope involved in neutralization and cell attachment that is located on the major bovine rotavirus glycoprotein.

The 38,200-molecular weight (unreduced)/41,900-molecular-weight (reduced) glycoprotein of bovine rotavirus, isolate C486, was identified as the major neutralizing antigen. This glycoprotein as well as the corresponding glycoprotein of another bovine rotavirus serotype also specifically attached to cell monolayers under normal conditions for virus adsorption in vitro. Further support for this glycoprotein being directly responsible for virus attachment to cells was that (i) infectious virus of both serotypes could compete with the C486 glycoprotein for cell surface receptors, and (ii) neutralizing monospecific antiserum and neutralizing monoclonal antibodies directed toward the glycoprotein could block this virus-cell interaction. Preliminary epitope mapping of the glycoprotein with monoclonal antibodies further localized the neutralization-adsorption domain to a peptide with an approximate molecular weight of 14,000. The effect of two protein modifications, glycosylation and disulfide bridging, on the reactivity of this peptide with antibodies and cell surface receptors was investigated. It was demonstrated that, whereas glycosylation did not appear to affect these reactivities, disulfide bridging seemed to be essential.     

Human immunodeficiency virus type 1 and 2 envelope glycoproteins oligomerize through conserved sequences.

Hetero-oligomerization between human immunodeficiency virus type 2 (HIV-2) envelope glycoprotein (Env) truncation mutants and epitope-tagged gp160 is dependent on the presence of gp41 transmembrane protein (TM) amino acids 552 to 589, a putative amphipathic alpha-helical sequence. HIV-2 Env truncation mutants containing this sequence were also able to form cross-type hetero-oligomers with HIV-1 Env. HIV-2/HIV-1 hetero-oligomerization was, however, more sensitive to disruption by mutagenesis or increased temperature. The conservation of the Env oligomerization function of the HIV-1 and HIV-2 alpha-helical sequences suggests that retroviral TM alpha-helical motifs may have a universal role in oligomerization.     

Variable Constraints on the Principal Immunodominant Domain of the Transmembrane Glycoprotein of Human Immunodeficiency Virus Type 1

Lentiviruses have in their transmembrane glycoprotein (TM) a highly immunogenic structure referred to as the principal immunodominant domain (PID). The PID forms a loop of 5 to 7 amino acids between two conserved cysteines. Previous studies showed that envelope (Env) glycoprotein functions of feline immunodeficiency virus (FIV) could be retained after extensive mutation of the PID loop sequence, in spite of its high conservation. In order to compare Env function in different lentiviruses, either random mutations were introduced in the PID loop sequence of human immunodeficiency virus type 1 (HIV-1) or the entire HIV-1 PID loop was replaced by the corresponding PID loop of FIV or simian immunodeficiency virus (SIV). In the macrophage-tropic HIV-1 ADA Env, mutations impaired the processing of the gp160 Env precursor, thereby abolishing viral infectivity. However, 6 of the 108 random Env mutants that were screened retained the capacity to induce cell membrane fusion. The SIV and FIV sequences and five random mutations were then introduced in the context of T-cell-line-adapted HIV-1 LAI which, although phenotypically distant from HIV-1 ADA, has an identical PID loop sequence. In contrast to the situation for HIV-1 ADA mutants, the cleavage of the Env precursor was unaffected in most HIV-1 LAI mutants. Such mutations, however, resulted in increased shedding of the gp120 surface glycoprotein (SU) from the gp41 TM. The HIV-1 LAI Env mutants showed high fusogenic efficiency. Three Env mutants retained the capacity to mediate virus entry in target cells, although less efficiently than the wild-type Env, and allowed the reconstitution of infectious molecular clones. These results indicated that in HIV-1, like FIV, the conserved PID sequence can be changed without impairing Env function. However, functional constraints on the PID of HIV-1 vary depending on the structural context of Env, presumably in relation to the role of the PID in the interaction of the SU and TM subunits and the stability of the Env complex.