A novel intermediate in processing of murine leukemia virus envelope glycoproteins. Proteolytic cleavage in the late Golgi region

Retroviral env glycoprotein trafficking and incorporation into virions

Together with the Gag protein, the Env glycoprotein is a major retroviral structural protein and is essential for forming infectious virus particles. Env is synthesized, processed, and transported to certain microdomains at the plasma membrane and takes advantage of the same host machinery for its trafficking as that used by cellular glycoproteins. Incorporation of Env into progeny virions is probably mediated by the interaction between Env and Gag, in some cases with the additional involvement of certain host factors. Although several general models have been proposed to explain the incorporation of retroviral Env glycoproteins into virions, the actual mechanism for this process is still unclear, partly because structural data on the Env protein cytoplasmic tail is lacking. This paper presents the current understanding of the synthesis, trafficking, and virion incorporation of retroviral Env proteins.

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.

Vpu-dependent block to incorporation of GaLV Env into lentiviral vectors

Background

The gibbon ape leukemia virus (GaLV) Env protein mediates entry into a wide range of human cells and is frequently used to pseudotype retroviral vectors. However, an incompatibility exists between GaLV Env and lentiviral vectors that results in decreased steady-state levels of the mature GaLV Env in cells and prevents its incorporation into lentiviral vector particles.

Results

We identified the HIV-1 Vpu protein as the major cause of the depletion in GaLV Env levels that occurs when lentiviral vector components are present. This activity of Vpu targeted the mature (cleaved) form of the GaLV Env that exists within or beyond the trans-Golgi. The activity required two conserved phospho-serines in the cytoplasmic tail of Vpu that are known to recruit β TrCP, a substrate adaptor for an SCF E3 ubiquitin ligase complex, and could be blocked by mutation of lysine 618 in the GaLV Env tail. Moreover, the Vpu-mediated decrease of GaLV Env levels was inhibited by the lysosomal inhibitor, bafilomycin A1. Interestingly, this activity of Vpu was only observed in the presence of other lentiviral vector components.

Conclusions

Similar to the mechanism whereby Vpu targets BST-2/tetherin for degradation, these findings implicate β-TrCP-mediated ubiquitination and the endo-lysosomal pathway in the degradation of the GaLV Env by lentiviral vector components. Possibly, the cytoplasmic tail of the GaLV Env contains features that mimic bona fide targets of Vpu, important to HIV-1 replication. Furthermore, the lack of effect of Vpu on GaLV Env in the absence of other HIV-1 proteins, suggests that a more complex interaction may exist between Vpu and its target proteins, with the additional involvement of one or more component(s) of the HIV-1 replication machinery.

Full genome sequence and some biological properties of reticuloendotheliosis virus strain APC-566 isolated from endangered Attwater's prairie chickens

Reticuloendotheliosis virus (REV) causes runting, high mortality, immunosuppression, and chronic neoplasia associated with T and/or B cell lymphomas in a variety of domestic and wild birds, including Attwater's prairie chickens (APC) (Tympanuchus cupido attwateri). The complete proviral sequence of a recent REV isolate from APC (REV APC-566) was determined. This virus was isolated from an APC maintained in captivity in a reproduction program intended to avoid its extinction. REV APC-566 was determined to be oncogenic in Japanese quail (Coturnix coturnix japonica), chickens (Gallus gallus) and turkeys (Meleagris gallopavo). Immune responses against bacteria and viruses were significantly reduced in turkeys infected with REV APC-566. The proviral genome is 8286 nucleotides in length and exhibits a genetic organization characteristic of replication-competent gammaretroviruses. The REV APC-566 provirus contains two identical long terminal repeats (LTR) and a complete set of genes including gag, gag-pol and env. As previously reported, alignments with other REV sequences showed high similarity with sequences found in the gag and pol genes from other REVs. The REV APC-566 env gene showed high nucleotide sequence homology with REV sequences inserted in fowl poxvirus (99.8%), and with spleen necrosis virus (SNV) (95.1%). Sequences coding for a previously reported immunosuppressive peptide contained in the transmembrane region of the env gene are well conserved among all REV sequences analyzed. The LTR was the most divergent region, exhibiting various deletions and insertions. REV APC-566 has a unique insertion of 23 bp in U3 and shares deletions of 19 and 5 bp with chicken syncytial virus and REV inserts in fowlpox virus.

Synthesis, assembly, and processing of the Env ERVWE1/syncytin human endogenous retroviral envelope

Syncytin is a fusogenic protein involved in the formation of the placental syncytiotrophoblast layer. This protein is encoded by the envelope gene of the ERVWE1 proviral locus belonging to the human endogenous retrovirus W (HERV-W) family. The HERV-W infectious ancestor entered the primate lineage 25 to 40 million years ago. Although the syncytin fusion property has been clearly demonstrated, little is known about this cellular protein maturation process with respect to classical infectious retrovirus envelope proteins. Here we show that the cellular syncytin protein is synthesized as a glycosylated gPr73 precursor cleaved into two mature proteins, a gp50 surface subunit (SU) and a gp24 transmembrane subunit (TM). These SU and TM subunits are found associated as homotrimers. The intracytoplasmic tail is critical to the fusogenic phenotype, although its cleavage requirements seem to have diverged from those of classical retroviral maturation.

Abundant defective viral particles budding from microglia in the course of retroviral spongiform encephalopathy

A pathogenetic hallmark of retroviral neurodegeneration is the affinity of neurovirulent retroviruses for microglia cells, while degenerating neurons are excluded from retroviral infections. Microglia isolated ex vivo from rats peripherally infected with a neurovirulent retrovirus released abundant mature type C virions; however, infectivity associated with microglia was very low. In microglia, viral transcription was unaffected but envelope proteins were insufficiently cleaved into mature viral proteins and were not detected on the microglia cell surface. These microglia-specific defects in envelope protein translocation and processing not only may have prevented formation of infectious virus particles but also may have caused further cellular defects in microglia with the consequence of indirect neuronal damage. It is conceivable that similar events play a role in neuro-AIDS.

The role of the membrane-spanning domain sequence in glycoprotein-mediated membrane fusion

The role of glycoprotein membrane-spanning domains in the process of membrane fusion is poorly understood. It has been demonstrated that replacing all or part of the membrane-spanning domain of a viral fusion protein with sequences that encode signals for glycosylphosphatidylinositol linkage attachment abrogates membrane fusion activity. It has been suggested, however, that the actual amino acid sequence of the membrane-spanning domain is not critical for the activity of viral fusion proteins. We have examined the function of Moloney murine leukemia virus envelope proteins with substitutions in the membrane-spanning domain. Envelope proteins bearing substitutions for proline 617 are processed and incorporated into virus particles normally and bind to the viral receptor. However, they possess greatly reduced or undetectable capacities for the promotion of membrane fusion and infectious virus particle formation. Our results imply a direct role for the residues in the membrane-spanning domain of the murine leukemia virus envelope protein in membrane fusion and its regulation. They also support the thesis that membrane-spanning domains possess a sequence-dependent function in other protein-mediated membrane fusion events.

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

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

Retroviral vector-mediated expression in primary human T cells of an endoplasmic reticulum-retained CD4 chimera inhibits human immunodeficiency virus type-1 replication

Intracellular expression of genes that inhibit key steps in the human immunodeficiency virus (HIV-1) replicative cycle could offer an alternative therapy for AIDS treatment. One of these approaches involves the inhibition of env protein maturation through the expression of CD4 molecules with added exogenous sequences that promote their retention in the endoplasmic reticulum (ER). We have tested this strategy using a CD4 chimera (CD4epsilon10) containing an ER retention sequence derived from the TCR CD3-epsilon chain. Transfection of CD4epsilon10 in the human T cell line Jurkat made it resistant to infection with two different HIV-1 isolates, which was evaluated by measuring p24 antigen production, induction of apoptosis, and syncytia formation. Furthermore, polymerase chain reaction (PCR) analysis of genomic DNA showed no traces of the proviral HIV-1 genome in CD4epsilon10-transfected cells, suggesting it was not maintained latently in these cells. To facilitate the delivery of the CD4epsilon10 chimera to primary cells from AIDS patients, a Moloney-based retroviral vector was constructed that expresses CD4epsilon10 under the transcriptional control of the HIV-1 long terminal repeat (LTR) promoter. Transduction of the MT-2 human T cell line with this vector rendered it resistant to infection with HIV-1 by a process that involved the inhibition of gp160 proteolytic processing. Finally, transduction of the CD4epsilon10 chimera into T lymphoblasts derived from asymptomatic HIV-infected individuals demonstrated a protective effect, resulting in both an increased cellular proliferation rate and an increased percentage of CD4+ cells. These results suggest that it is feasible to use retroviral transduction of CD4epsilon10 as a gene therapy approach for AIDS treatment.

Megalomicin inhibits HIV-1 replication and interferes with gp160 processing

The inhibitory effects on HIV replication of megalomicin (MGM, an inhibitor of intra-Golgi vesicle transport, have been studied. In experiments at low multiplicity of infection on Jurkat and MT2 cell lines. MGM inhibited the production of p24 antigen, the formation of syncytia, and the induction of apoptosis at concentrations below 5 microM. Furthermore, PCR analysis of genomic DNA showed that, in the presence of MGM, HIV-1 had been eradicated from the culture. MGM also inhibited replication of primary isolates of HIV-1 in blood lymphoblasts and more importantly, at 1 microM, MGM inhibited depletion of CD4+ T cells in cultures of blood lymphocytes from seropositive patients. Finally, MGM inhibited the generation of infectious virions and the processing of the envelope protein precursor gp160 to its mature forms, resulting in the rapid degradation of gp 160. These data suggest that MGM induces a powerful inhibitory effect on HIV-1 replication at nontoxic concentrations by preventing the processing of HIV-1 gp160 envelope protein and the subsequent formation of infectious viral particles.

Furin is important but not essential for the proteolytic maturation of gp160 of HIV-1

The envelope glycoproteins of HIV are required for viral infectivity. Proteolysis of the precursor envelope glycoprotein gp160 results in the formation of gp120 and gp41. Cleavage occurs after the sequence Arg-Glu-Lys-Arg. This sequence is expected to be a substrate for the cellular protease furin. We examined whether furin is responsible for cleavage of gp160 by using a furin-deficient CHO cell line and the same cell line transfected with furin cDNA. Data obtained from viral transmission assays suggested that furin increased viral infectivity but was not essential for the maturation of gp160, implying that other proprotein processing enzymes also recognize this putative furin cleavage site.

Maturation of mouse mammary tumor virus envelope protein is blocked by a specific point mutation

Expression of a mouse mammary tumor virus (MMTV) envelope gene, ENV2, in COS-7 cells resulted in the synthesis of a precursor protein Pr74 that underwent complete proteolytic processing to the mature envelope species gp52 and gp33. In contrast, expression of a second MMTV envelope gene, ENV1, in COS-7 cells resulted in the synthesis of a precursor protein Pr74 that failed to undergo proteolytic processing to the mature products. Expression and sequencing of various chimeric ENV1/ENV2 genes determined that a single glycine-to-glutamic acid mutation at position 54 was responsible for the nonprocessable phenotype of ENV1-encoded Pr74. The significance of this point mutation in relation to the requirements for precursor protein folding and maturation are discussed.