Mutational analysis of the human immunodeficiency virus type 1 env gene product proteolytic cleavage site

Functional role of the glycan cluster of the human immunodeficiency virus type 1 transmembrane glycoprotein (gp41) ectodomain

To examine the role of the glycans of human immunodeficiency virus type 1 transmembrane glycoprotein gp41, conserved glycosylation sites within the env sequence (Asn-621, Asn-630, and Asn-642) were mutated to Gln. The mutated and control wild-type env genes were introduced into recombinant vaccinia virus and used to infect BHK-21 or CD4+ CEM cells. Mutated gp41 appeared as a 35-kDa band in a Western blot (immunoblot), and it comigrated with the deglycosylated form of wild-type gp41. Proteolytic cleavage of the recombinant wild-type and mutant forms of the gp160 envelope glycoprotein precursor was analyzed by pulse-chase experiments and enzyme-linked immunosorbent assay: gp160 synthesis was similar whether cells were infected with control or mutated env-expressing recombinant vaccinia virus, but about 10-fold less cleaved gp120 and gp41 was produced by the mutated construct than the control construct. The rates of gp120-gp41 cleavage at each of the two potential sites appeared to be comparable in the two constructs. By using a panel of antibodies specific for gp41 and gp120 epitopes, it was shown that the overall immunoreactivities of control and mutated gp41 proteins were similar but that reactivity to epitopes at the C and N termini of gp120, as present on gp160 produced by the mutated construct, was enhanced. This was no longer observed for cleaved gp120 in supernatants. Both gp120 proteins, from control and mutated env, were expressed on the cell surface under a cleaved form and could bind to membrane CD4, as determined by quantitative immunofluorescence assay. In contrast, and despite sufficient expression of env products at the cell membrane, gp41 produced by the mutated construct was unable to induce membrane fusion. Therefore, while contradictory results reported in the literature suggest that gp41 individual glycosylation sites are dispensable for the bioactivity and conformation of env products, it appears that such is not the case when the whole gp41 glycan cluster is removed.

Inhibition of furin-mediated cleavage activation of HIV-1 glycoprotein gp160

The envelope glycoprotein of human immunodeficiency virus (HIV) initiates infection by mediating fusion of the viral envelope with the cell membrane. Fusion activity requires proteolytic cleavage of the gp160 protein into gp120 and gp41 at a site containing several arginine and lysine residues. Activation at basic cleavage sites is observed with many membrane proteins of cellular and viral origin. We have recently found that the enzyme activating the haemagglutinin of fowl plague virus (FPV), an avian influenza virus, is furin. Furin, a subtilisin-like eukaryotic endoprotease, has a substrate specificity for the consensus amino-acid sequence Arg-X-Lys/Arg-Arg at the cleavage site. We show here that the glycoprotein of HIV-1, which has the same protease recognition motif as the FPV haemagglutinin, is also activated by furin.

Properties of HIV membrane reconstituted from its recombinant gp160 envelope glycoprotein

Human immunodeficiency virus (HIV) membrane has been reconstituted from the recombinant envelope glycoprotein precursor (gp160) by a detergent dialysis technique. Electron microscopy shows that gp160-virosomes are spherical vesicles with a mean diameter identical to that of viral particles. Enzyme-linked immunosorbent assay and immunogold labeling demonstrate efficient association of gp160 with lipid vesicles and proteolysis treatment reveals an asymmetric insertion with about 90% of glycoproteins having their gp120-moiety pointing outside. Glycoproteins are organized as dimers and tetramers and gp160 retains its ability to specifically bind CD4 receptor after reconstitution into virosome.

HIV-1-induced cytopathogenicity in cell culture despite very decreased amounts of fusion-competent viral glycoprotein

In order to examine the potential role of env-induced membrane fusion in the cytopathogenic properties of HIV-1 in cell culture, the effects of mutations within the proteolytic cleavage site of gp160, which result in a reduction but not a complete absence of proteolytic processing have been further studied. Cells expressing the mutant glycoproteins were shown to be severely reduced in their capacity to form syncytia. However, viruses encoding these glycoproteins could infect cell culture cells, albeit with delayed kinetics, and, at late infection time points, resulted in complete cytolysis of the infected culture. Since amplification by polymerase chain reaction and direct sequencing of the DNA in the infected cultures confirmed the presence of the mutant and the absence of revertant DNA, this shows that the amount of fusion competent viral glycoprotein does not influence HIV-1 cytopathogenicity, but rather that other parameters must be involved in inducing cell death.

Identification and characterization of fusion and processing domains of the human immunodeficiency virus type 2 envelope glycoprotein

The envelope glycoprotein of the human immunodeficiency virus type 2 (HIV-2) is synthesized as a polyprotein precursor which is proteolytically processed to produce the mature surface and transmembrane envelope glycoproteins. The processed envelope glycoprotein species are responsible for the fusion between the viral envelope and the host cell membrane during the infection process. The envelope glycoprotein also induces syncytium formation between envelope-expressing cells and receptor-bearing cells. To characterize domains of the HIV-2 envelope glycoprotein involved in membrane fusion and in proteolytic processing, we introduced single amino acid mutations into the region of the HIV-2 surface glycoprotein corresponding to the principal neutralizing determinant (the V3 loop) of HIV-1, the putative HIV-2 envelope precursor-processing sequence, and the hydrophobic amino terminus of the HIV-2 transmembrane envelope glycoprotein. The effects of these mutations on syncytium formation, virus infectivity, envelope expression, envelope processing, and CD4 binding were analyzed. Our results suggest that the V3-like region of the HIV-2 surface glycoprotein and the hydrophobic amino terminus of the transmembrane glycoprotein are HIV-2 fusion domains and characterize the effects of mutations in the HIV-2 envelope glycoprotein precursor-processing sequence.

Expression of biologically active HIV glycoproteins using a T7 RNA polymerase-based eucaryotic vector system

Bacteriophage T7 RNA polymerase and a derivative containing a nuclear localization signal were transiently expressed in CV-1 cells and were shown to localize to the cytoplasm and nucleus, respectively. A vector was constructed containing T7 promoter and transcription terminator sequences flanking a picornaviral 5' untranslated sequence for cap-independent translation and a polyA signal. Expression of the HIV-1 envelope glycoproteins in this vector system gave high levels of specific transcripts and translation products, independent of the subcellular localization of T7 RNA polymerase. The synthesis of HIV glycoproteins was also completely independent of the coexpression of the HIV rev protein, which is normally required for the expression of HIV structural proteins. In addition, a polyA signal was not required, whereas the presence of the picornaviral 5' untranslated region was necessary for efficient expression. Different possibilities to account for these findings are discussed. The HIV glycoproteins synthesized in this system were normally processed and assembled; they could induce syncytium formation and complement an env-deletion mutant of HIV-1.

Differential transport and processing of variant mouse mammary tumor virus glycoproteins

The transport and proteolytic processing of two individual gene isolates of the mouse mammary tumor virus (MMTV) glycoprotein were compared in transfected rat HTC hepatoma cells. Plasmids were constructed such that the MMTV glycoprotein genes were constitutively expressed from the promoter within the Rous Sarcoma Virus 5' Long Terminal Repeat in the absence of other MMTV proteins. An isolate of the GR strain MMTV glycoprotein was efficiently transported and processed resulting in the localization of MMTV glycoproteins at the cell surface and in the extracellular environment. Moreover, the kinetics of acquisition of endoglycosidase H resistant oligosaccharide side chains and the rate of endoproteolytic cleavage of the glycosylated polyprotein expressed in transfected cells were virtually identical to that observed in viral-infected rat hepatoma cells. In contrast, a natural variant of the C3H strain MMTV glycoprotein expressed in transfected cells was retained in an intracellular compartment by a heavy chain binding protein (BiP)-independent pathway in an endoglycosidase H sensitive and uncleaved form. This MMTV glycoprotein isolate was retained early in the exocytic pathway and displayed a half-life of approximately 45 min in transfected cells. Only a minor fraction of the expressed C3H variant glycoprotein was detected at the cell surface but was not externalized. Our results suggest that the variant C3H MMTV glycoprotein contains one or more mutations that preclude its efficient transport through the exocytic pathway.

Retained in vitro infectivity and cytopathogenicity of HIV-1 despite truncation of the C-terminal tail of the env gene product

Five in-frame stop mutations in the HIV-1 env gene, which lead to the production of env gene products truncated within the cytoplasmic C-terminal tail, have been generated and their effects on membrane fusion capacity, glycoprotein incorporation into virus particles, infectivity, and cytopathogenicity were analyzed. The resulting truncated glycoproteins were processed normally, were transported to the cell surface, and were able to induce CD4-dependent membrane fusion. The membrane fusion capacity of one of the mutant glycoproteins with a truncation of 144 amino acids was increased to about double of that induced by wild-type glycoprotein. With a single exception, the truncated viral glycoproteins were incorporated into virus particles which were infectious and cytopathic for permissive MT-4 cells. The infection kinetics with the mutated viruses were, however, delayed to varying degrees in comparison to infection with wild-type virus. Nevertheless, in each case, PCR amplification and direct sequencing of viral DNA in the infected cultures confirmed the presence of the mutant and the absence of revertant DNA. The mutant virus encoding a viral glycoprotein with the longest truncation (144 amino acids), in which only 7 cytoplasmic C-terminal amino acids in gp41 remain, resulted in infection kinetics in MT-4 cells which were only marginally delayed in comparison to those induced by wild-type virus. This means that these C-terminal 144 amino acids of gp41 are not necessary for glycoprotein incorporation into virus particles nor do they significantly contribute to the infectivity nor the cytopathogenicity of HIV-1 in MT-4 cells.

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.

Immunological analysis of human immunodeficiency virus type 1 envelope glycoprotein proteolytic cleavage

Two potential cleavage sites have been identified on precursor gp 160 of human immunodeficiency virus type 1. Using antibodies directed against the C-terminus of gp 120, including the sequence between the two sites, we have shown that nonmutated viral and recombinant gp 160 are cleaved at both sites: the great majority of molecules are cleaved at site 1 (Arg-Glu-Lys-Arg), and gp41 can then associate as an oligomer; a minority of molecules are cleaved at site 2 (Lys-Ala-Lys-Arg-Arg) and the corresponding gp41 appears to present as a monomer. This could reflect two different processing pathways for gp41 biosynthesis, one of which only may result in biologically active molecules according to the literature.

Novel human endogenous sequences related to human immunodeficiency virus type 1

Endogenous retrovirus-related sequences exist within the normal genomic DNA of all eukaryotes, and these endogenous sequences have been shown to be important to the nature and biology of related exogenous retroviruses and may also play a role in cellular functions. To date, no endogenous sequences related to human immunodeficiency virus type 1 (HIV-1) have been reported. Herein we describe the first report of the presence of nucleotide sequences related to HIV-1 in human, chimpanzee, and rhesus monkey DNAs from normal uninfected individuals. We also present the isolation and characterization of two of these endogenous HIV-1-related sequences, EHS-1 and EHS-2. With use of low-stringency Southern blot hybridization, complex banding patterns were detected in human DNA with 5' and 3' HIV-1-derived probes. When an HIV-1 env region probe was used, we detected a less complex, conserved banding pattern in human DNA as well as a related but distinct banding pattern in chimpanzee and rhesus monkey DNAs. EHS-1 and -2 were cloned from normal human genomic DNA libraries by using the env region probe. Clone EHS-1 shows sequence similarity with the domain of the envelope cellular protease cleavage site of HIV-1, while EHS-2 has sequence similarity to the overlapping reading frame for Rev and gp41. Stringent hybridization of EHS-1 back to primate genomic DNA indicates two distinct EHS-1 loci in normal human DNA, an identical band pattern in chimpanzee DNA, and a single locus in rhesus monkey DNA. Likewise, EHS-2 is present as a single highly conserved locus in all three species. An oligonucleotide derived from EHS-2 across a region of near identity to HIV-1 detects a complex banding pattern in all primates tested similar to that seen with the 3' HIV-1 probe. These data suggest that most of the HIV-1-related sequences identified in primate DNA share a common core of nucleic acid sequence found in both EHS-2 and rev and that some of these HIV-1-related sequences have additional larger regions of sequence similarity to HIV-1.

Mutations within the proteolytic cleavage site of the Rous sarcoma virus glycoprotein define a requirement for dibasic residues for intracellular cleavage

We investigated the amino acid sequence requirements for intracellular cleavage of the Rous sarcoma virus glycoprotein precursor by introducing mutations into the region encoding the cleavage recognition site (Arg-Arg-Lys-Arg). In addition to mutants G1 (Arg-Arg-Glu-Arg) and Dr1 (deletion of all four codons) that we have reported on previously (L. G. Perez and E. Hunter, J. Virol. 61:1609-1614, 1987), we constructed two additional mutants, AR1 (Arg-Arg-Arg-Arg), in which the highly conserved lysine is replaced by an arginine, and S19 (Ser-Arg-Glu-Arg), in which no dibasic pairs remain. The results of these studies demonstrate that when the cleavage sequence is deleted (Dr1) or modified to contain unpaired basic residues (S19), intracellular cleavage of the glycoprotein precursor is completely blocked. This demonstrates that the cellular endopeptidase responsible for cleavage has a stringent requirement for the presence of a pair of basic residues (Arg-Arg or Lys-Arg). Furthermore, it implies that the cleavage enzyme is not trypsinlike, since it is unable to recognize arginine residues that are sensitive to trypsin action. Substitution of the mutated genes into a replication-competent avian retrovirus genome showed that cleavage of the glycoprotein precursor was not required for incorporation into virions but was necessary for infectivity. Treatment of BH-RCAN-S19-transfected turkey cells with low levels of trypsin resulted in the release of infectious virus, demonstrating that exogenous cleavage could generate a biologically active glycoprotein molecule.

Inhibition of HIV and SIV infectivity by blockade of alpha-glucosidase activity

Processing of HIV and SIV envelope oligosaccharides is critical for proper intracellular trafficking and function. An inhibitor of alpha-glucosidases I and II, N-butyl deoxynojirimycin (N-BuDNJ), retards HIV-1 and SIVmac spread in lymphocytes and monocytes by diminishing virus infectivity, and also causes a reduction in syncytia formation between infected cells and uninfected lymphocytes. N-BuDNJ retards envelope processing from the precursor form to the mature surface (SU) and transmembrane proteins in HIV-1- and SIVmac-infected cells, as well as in cells infected with vaccinia-HIV-1 envelope recombinant virus. However, no significant reduction is seen in the amount of SU in released virus particles, though the virus particle-associated SU from N-BuDNJ-treated cells has an altered electrophoretic mobility. In contrast, N-BuDNJ had no effect on GAG protein synthesis and processing. These findings demonstrate a critical requirement for oligosaccharide processing by alpha-glucosidases I and II for HIV-1 and SIVmac envelope processing and fusogenicity.