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

Structural determinants of virion assembly and release in the C-terminus of the M-PMV capsid protein

The transition from an immature to a fully infectious mature retrovirus particle is associated with molecular switches that trigger dramatic conformational changes in the structure of the Gag proteins. A dominant maturation switch that stabilizes the immature capsid lattice is located downstream of the capsid (CA) protein in many retroviral Gags. The HIV-1 Gag contains a stretch of five amino acid residues termed the 'clasp motif', important for the organization of the hexameric subunits that provide stability to the overall immature HIV-1 shell. Sequence alignment of the CA C-terminal domains (CTDs) of the HIV-1 and Mason-Pfizer Monkey Virus (M-PMV) highlighted a spacer-like domain in M-PMV that may provide comparable function. The importance of the sequences spanning the CA-NC cleavage has been demonstrated by mutagenesis, but the specific requirements for the clasp motif in several steps of M-PMV particle assembly and maturation have not been determined in detail. In the present study we report an examination of the role of the clasp motif in the M-PMV life cycle. We generated a series of M-PMV Gag mutants and assayed for assembly of the recombinant protein in vitro, and for the assembly, maturation, release, genomic RNA packaging, and infectivity of the mutant virus in vivo. The mutants revealed major defects in virion assembly and release in 293T and HeLa cells, and even larger defects in infectivity. Our data identifies the clasp motif as a fundamental contributor to CA-CTD interactions necessary for efficient viral infection. Importance The C-terminal domain of the capsid protein of many retroviruses has been shown to be critical for virion assembly and maturation, but the functions of this region of M-PMV are uncertain. We show that a short 'clasp' motif in the capsid domain of the M-PMV Gag protein plays a key role in M-PMV virion assembly, genome packaging, and infectivity.

Foamy virus budding and release

Like all other viruses, a successful egress of functional particles from infected cells is a prerequisite for foamy virus (FV) spread within the host. The budding process of FVs involves steps, which are shared by other retroviruses, such as interaction of the capsid protein with components of cellular vacuolar protein sorting (Vps) machinery via late domains identified in some FV capsid proteins. Additionally, there are features of the FV budding strategy quite unique to the spumaretroviruses. This includes secretion of non-infectious subviral particles and a strict dependence on capsid-glycoprotein interaction for release of infectious virions from the cells. Virus-like particle release is not possible since FV capsid proteins lack a membrane-targeting signal. It is noteworthy that in experimental systems, the important capsid-glycoprotein interaction could be bypassed by fusing heterologous membrane-targeting signals to the capsid protein, thus enabling glycoprotein-independent egress. Aside from that, other systems have been developed to enable envelopment of FV capsids by heterologous Env proteins. In this review article, we will summarize the current knowledge on FV budding, the viral components and their domains involved as well as alternative and artificial ways to promote budding of FV particle structures, a feature important for alteration of target tissue tropism of FV-based gene transfer systems.

Murine leukemia viruses: objects and organisms

Murine leukemia viruses (MLVs) are among the simplest retroviruses. Prototypical gammaretroviruses encode only the three polyproteins that will be used in the assembly of progeny virus particles. These are the Gag polyprotein, which is the structural protein of a retrovirus particle, the Pol protein, comprising the three retroviral enzymes—protease, which catalyzes the maturation of the particle, reverse transcriptase, which copies the viral RNA into DNA upon infection of a new host cell, and integrase, which inserts the DNA into the chromosomal DNA of the host cell, and the Env polyprotein, which induces the fusion of the viral membrane with that of the new host cell, initiating infection. In general, a productive MLV infection has no obvious effect upon host cells. Although gammaretroviral structure and replication follow the same broad outlines as those of other retroviruses, we point out a number of significant differences between different retroviral genera.

The impact of altered polyprotein ratios on the assembly and infectivity of Mason-Pfizer monkey virus

Most retroviruses employ a frameshift mechanism during polyprotein synthesis to balance appropriate ratios of structural proteins and enzymes. To investigate the requirements for individual precursors in retrovirus assembly, we modified the polyprotein repertoire of Mason-Pfizer monkey virus (M-PMV) by mutating the frameshift sites to imitate the polyprotein organization of Rous sarcoma virus (Gag-Pro and Gag-Pro-Pol) or Human immunodeficiency virus (Gag and Gag-Pro-Pol). For the "Rous-like" virus, assembly was impaired with no incorporation of Gag-Pro-Pol into particles and for the "HIV-like" virus an altered morphogenesis was observed. A mutant expressing Gag and Gag-Pro polyproteins and lacking Gag-Pro-Pol assembled intracellular particles at a level similar to the wild-type. Gag-Pro-Pol polyprotein alone neither formed immature particles nor processed the precursor. All the mutants were non-infectious except the "HIV-like", which retained fractional infectivity.

Amino acid residues in the cytoplasmic domain of the Mason-Pfizer monkey virus glycoprotein critical for its incorporation into virions

Assembly of an infectious retrovirus requires the incorporation of the envelope glycoprotein complex during the process of particle budding. We have recently demonstrated that amino acid substitutions of a tyrosine residue in the cytoplasmic domain block glycoprotein incorporation into budding Mason-Pfizer monkey virus (M-PMV) particles and abrogate infectivity (C. Song, S. R. Dubay, and E. Hunter, J. Virol. 77:5192-5200, 2003). To investigate the contribution of other amino acids in the cytoplasmic domain to the process of glycoprotein incorporation, we introduced alanine-scanning mutations into this region of the transmembrane protein. The effects of the mutations on glycoprotein biosynthesis and function, as well as on virus infectivity, have been examined. Mutation of two cytoplasmic residues, valine 20 and histidine 21, inhibits viral protease-mediated cleavage of the cytoplasmic domain that is observed during virion maturation, but the mutant virions show only moderately reduced infectivity. We also demonstrate that the cytoplasmic domain of the M-PMV contains three amino acid residues that are absolutely essential for incorporation of glycoprotein into virions. In addition to the previously identified tyrosine at residue 22, an isoleucine at position 18 and a leucine at position 25 each mediate the process of incorporation and efficient release of virions. While isoleucine 18 may be involved in direct interactions with immature capsids, antibody uptake studies showed that leucine 25 and tyrosine 22 are part of an efficient internalization signal in the cytoplasmic domain of the M-PMV glycoprotein. These results demonstrate that the cytoplasmic domain of M-PMV Env, in part through its YXXL-mediated endocytosis and intracellular trafficking signals, plays a critical role in the incorporation of glycoprotein into virions.

Activity of the Mason-Pfizer monkey virus fusion protein is modulated by single amino acids in the cytoplasmic tail

Mason-Pfizer monkey virus (M-PMV) encodes a transmembrane glycoprotein with a 38-amino-acid-long cytoplasmic tail. After the release of the immature virus, a viral protease-mediated cleavage of the cytoplasmic tail (CT) results in the loss of 17 amino acids from the carboxy terminus and renders the envelope protein fusion competent. To investigate the role of individual amino acid residues in the CT in fusion, a series of mutations was introduced, and the effects of these mutations on glycoprotein biosynthesis and fusion were examined. Most of the alanine-scanning mutations in the CT had little effect on fusion activity. However, four amino acid substitutions (threonine 4, lysine 7, glutamine 9, and isoleucine 10) resulted in substantially increased fusogenicity, while six (leucine 2, phenylalanine 5, isoleucine 13, lysine 16, proline 17, and glycine 31) resulted in much-reduced fusion. Interestingly, the bulk of these mutations are located upstream of the CT cleavage site in a region that has the potential to form a coiled-coil in the Env trimer. Substitutions at glutamine 9 and isoleucine 10 with alanine had the most dramatic positive effect and resulted in the formation of large syncytia. Taken together, these data demonstrate that individual residues within the cytoplasmic domain of M-PMV Env can modulate, in both a positive and negative manner, biological functions that are associated with the extracellular domains of the glycoprotein complex.

Switching between allosteric and dimerization inhibition of HIV-1 protease

Refining the functional groups on a phenethylamine moiety within an inhibitor of HIV-1 protease led to a switch in the mechanism of inhibition from competitive and allosteric to dimerization inhibition. Phenylether extensions to the phenethylamine group led to agents that target the dimerization interface of HIV-1 protease with high potency.

Crucial amides for dimerization inhibitors of HIV-1 protease

An inhibitor based on crosslinked peptides from the interfacial region of HIV-1 protease, previously shown to act by dimerization inhibition, was modified by N-methylation to ascertain the importance of the amide hydrogens on inhibition. The effects of N-methylation on HIV-1 protease inhibition, as well as the effects on degradation by proteases are described.

Three-dimensional structure of a monomeric form of a retroviral protease

The assembly of Mason-Pfizer monkey virus Gag polyproteins into immature capsids and their cleavage by the encoded protease are temporally and spatially separated processes, making the virus a particularly useful model for investigation of protease activation. Here we present a high resolution NMR structure of a fully folded monomer of a 12 kDa M-PMV protease (wt 12 PR) and of a Cys7Ala/Asp26Asn/Cys106Ala mutant (12 PR(D26N/C7A/C106A)). The overall structures of both wt 12 PR and 12 PR(D26N/C7A/C106A) follow the conservative structural motif of other retroviral proteases. The most prominent difference from the canonical fold of retroviral proteases is the absence of the interfacial beta-sheet, which leads to the loss of the principal force stabilizing the dimer of M-PMV PR. The monomer-dimer equilibrium can be shifted in favor of the dimer by adding a substrate or an inhibitor, partially compensating for the missing role of the beta-sheet. We also show that cysteines C7 and C106 play a crucial role in stabilizing the dimer and consequently increasing the proteolytic activity of M-PMV PR. This is consistent with the role of reversible oxidative modification of the cysteine residues in the regulation of the maturation of assembled M-PMV capsids in the cytoplasm.

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.

Rabbit endogenous retrovirus-H encodes a functional protease

Recent studies have revealed that 'human retrovirus-5' sequences found in human samples belong to a rabbit endogenous retrovirus family named RERV-H. A part of the gag-pro region of the RERV-H genome was amplified by PCR from DNA in human samples and several forms of RERV-H protease were expressed in bacteria. The RERV-H protease was able to cleave itself from a precursor protein and was also able to cleave the RERV-H Gag polyprotein precursor in vitro whereas a form of the protease with a mutation engineered into the active site was inactive. Potential N- and C-terminal autocleavage sites were characterized. The RERV-H protease was sensitive to pepstatin A, showing it to be an aspartic protease. Moreover, it was strongly inhibited by PYVPheStaAMT, a pseudopeptide inhibitor specific for Mason-Pfizer monkey virus and avian myeloblastosis-associated virus. A structural model of the RERV-H protease was constructed that, together with the activity data, confirms that this is a retroviral aspartic protease.

Regulation of fusion activity by the cytoplasmic domain of a paramyxovirus F protein

SER virus is a member of the family Paramyxoviridae, genus Rubulavirus, which has been isolated from pigs. It is very closely related to SV5 virus serologically, in protein profile, and in nucleotide sequence. However, unlike SV5, SER induces minimal syncytium formation in infected CV-1 or BHK cells. Fluorescence transfer experiments between labeled erythrocytes and infected MDBK cells revealed that SER also induces hemifusion and pore formation with reduced efficiency. The virion polypeptide profiles of SER and SV5 are very similar, except that the SER F1 subunit shows an apparent molecular weight that is about 2 kDa higher than that of SV5. Comparison of the deduced amino acid sequences revealed the SER F (551 aa) to be longer than SV5 F (529 aa) by 22 residues in the cytoplasmic tail (CT) domain. The HN and M gene sequences of the viruses were found to be very similar. The SER F showed minimal fusion activity when coexpressed with either SV5 or SER HN. In contrast, SV5 F was highly fusogenic when coexpressed with either HN protein, indicating that the restricted fusion capacity of SER virus is a property of its F protein. Truncation in the CT of SER F by 22 residues completely rescued its ability to cause syncytium formation, whereas other truncations rescued syncytium formation partially. These results demonstrate that an elongated CT of a paramyxovirus F protein suppresses its membrane fusion activity.

Activation of the Mason-Pfizer monkey virus protease within immature capsids in vitro

For all retroviruses, the completion of the viral budding process correlates with the activation of the viral protease by an unknown mechanism, and, as the structural (Gag) polyproteins are cleaved by the viral protease, maturation of the immature virus-like particle into an infectious virion. Unlike most retroviruses, the Mason-Pfizer monkey virus Gag polyproteins assemble into immature capsids within the cytoplasm of the cell before the viral budding event. The results reported here describe a unique experimental system in which Mason-Pfizer monkey virus immature capsids are removed from the cell, and the protease is activated in vitro by the addition of a reducing agent. The cleavage of the protease from the precursor form is a primary event, which proceeds with a half time of 14 min, and is followed by authentic processing of the Gag polyproteins. Activity of the viral protease in vitro depends on pH, with an increase in catalytic rates at acidic and neutral pH. The initiation of protease activity within immature capsids in vitro demonstrates that viral protease activity is sensitive to oxidation-reduction conditions, and that the viral protease can be activated in the absence of viral budding.

Sequences in the cytoplasmic tail of the gibbon ape leukemia virus envelope protein that prevent its incorporation into lentivirus vectors

Pseudotyping retrovirus and lentivirus vectors with different viral fusion proteins is a useful strategy to alter the host range of the vectors. Although lentivirus vectors are efficiently pseudotyped by Env proteins from several different subtypes of murine leukemia virus (MuLV), the related protein from gibbon ape leukemia virus (GaLV) does not form functional pseudotypes. We have determined that this arises because of an inability of GaLV Env to be incorporated into lentivirus vector particles. By exploiting the homology between the GaLV and MuLV Env proteins, we have mapped the determinants of incompatibility in the GaLV Env. Three modifications that allowed GaLV Env to pseudotype human immunodeficiency virus type 1 particles were identified: removal of the R peptide (C-terminal half of the cytoplasmic domain), replacement of the whole cytoplasmic tail with the corresponding MuLV region, and mutation of two residues upstream of the R peptide cleavage site. In addition, we have previously proposed that removal of the R peptide from MuLV Env proteins enhances their fusogenicity by transmitting a conformational change to the ectodomain of the protein (Y. Zhao et al., J. Virol. 72:5392-5398, 1998). Our analysis of chimeric MuLV/GaLV Env proteins provides further evidence in support of this model and suggests that proper Env function involves both interactions within the cytoplasmic tail and more long-range interactions between the cytoplasmic tail, the membrane-spanning region, and the ectodomain of the protein.

Aspartic proteases from the nematode Caenorhabditis elegans. Structural organization and developmental and cell-specific expression of asp-1

A Caenorhabditis elegans gene (asp-1) and cDNA that encode a homologue of cathepsin D aspartic protease were cloned and characterized. The asp-1 mRNA is transcribed from a single exon, and it begins with the SL1 trans-splice leader sequence. The protein (ASP-1) is expressed as a 396-amino acid, 42.7-kDa pre-pro-peptide that is post-translationally processed into a approximately 40-kDa lysosomal protein. ASP-1 shares approximately 60% sequence identity with the aspartic protease precursor from the nematode Strongyloides stercoralis. The amino acid sequences adjacent to the two active site aspartic acid residues in ASP-1 are 100% identical to those in other eukaryotic aspartic proteases. In addition, ASP-1 contains conserved, potential disulfide bond-forming cysteine residues and N-glycosylation sites. The asp-1 gene is exclusively transcribed in the intestinal cells, with the highest levels of expression observed at late embryonic and early larval stages of development. asp-1 transcription is not observed in adult nematodes or mature larvae. Furthermore, transcription predominantly occurs in eight anterior cells of the intestine (int6-int8). Analyses of ASP-1 nucleotide and amino acid sequences revealed the presence of five additional C. elegans aspartic proteases.

Coreceptor-dependent inhibition of the cell fusion activity of simian immunodeficiency virus Env proteins

The cytoplasmic tail (R peptide) sequence is able to regulate the fusion activity of the murine leukemia virus (MuLV) envelope (Env) protein. We have previously shown that this sequence exerts a profound inhibitory effect on the fusion activity of simian immunodeficiency virus (SIV)-MuLV chimeric Env proteins which contain the extracellular and transmembrane domains of the SIV Env protein. Recent studies have shown that SIV can utilize several alternative cellular coreceptors for its fusion and entry into the cell. We have investigated the fusion activity of SIV and SIV-MuLV chimeric Env proteins using cells that express different coreceptors. HeLa cells were transfected with plasmid constructs that carry the SIV or SIV-MuLV chimeric Env protein genes and were overlaid with either CEMx174 cells or Ghost Gpr15 cells, which express the Gpr15 coreceptor for SIV, or Ghost CCR5 cells, which express CCR5, an alternate coreceptor for SIV. The R-peptide sequence in the SIV-MuLV chimeric proteins was found to inhibit the fusion with CEMx174 cells or Ghost Gpr15 cells. However, a significant level of fusion was still observed when HeLa cells expressing the chimeric Env proteins were cocultivated with Ghost CCR5 cells. These results show that the R-peptide sequence exerts differential effects on the fusion activity of SIV Env proteins using target cells that express alternative coreceptors.

Induction of syncytia by neuropathogenic murine leukemia viruses depends on receptor density, host cell determinants, and the intrinsic fusion potential of envelope protein

Infection by the neuropathogenic murine leukemia virus (MLV) TR1.3 results in hemorrhagic disease that correlates directly to in vivo syncytium formation of brain capillary endothelial cells (BCEC). This phenotype maps to amino acid 102 in the envelope (Env) protein of TR1.3. Substitution of glycine (G) for tryptophan (W) at this position (W102G Env) in the nonpathogenic MLV FB29 induces both syncytium formation and neurologic disease in vivo. Using an in vitro gene reporter cell fusion assay, we showed that fusion either with murine NIH 3T3 cells or with nonmurine target cells that expressed receptors at or below endogenous murine levels mirrored that seen in BCEC in vivo. In these instances only TR1.3 and W102G Env induced cell fusion. In contrast, when receptor levels on nonmurine cells were raised above endogenous murine levels, FB29 Env was as fusogenic as the neuropathogenic TR1.3 and W102G Env. These results indicate that TR1.3 Env and W102G Env are intrinsically more fusogenic than FB29 Env, that the induction of fusion requires a threshold number of receptors that is greater for FB29 Env than for TR1.3 or W102G Env, and that receptor density on murine NIH 3T3 cells and BCEC is below the threshold for FB29-dependent fusion. Surprisingly, receptor density on NIH 3T3 cells could not be increased by stable expression of exogenous receptors, and FB29-dependent fusion was not observed in NIH 3T3 cells that transiently expressed elevated receptor numbers. These results suggest that an additional undefined host cell factor(s) may limit both receptor expression and fusion potential in murine cells.

The YXXL sequences of a transmembrane protein of bovine leukemia virus are required for viral entry and incorporation of viral envelope protein into virions

The cytoplasmic domain of an envelope transmembrane glycoprotein (gp30) of bovine leukemia virus (BLV) has two overlapping copies of the (YXXL)2 motif. The N-terminal motif has been implicated in in vitro signal transduction pathways from the external to the intracellular compartment and is also involved in infection and maintenance of high viral loads in sheep that have been experimentally infected with BLV. To determine the role of YXXL sequences in the replication of BLV in vitro, we changed the tyrosine or leucine residues of the N-terminal motif in an infectious molecular clone of BLV, pBLV-IF, to alanine to produce mutated proviruses designated Y487A, L490A, Y498A, L501A, and Y487/498A. Transient transfection of African green monkey kidney COS-1 cells with proviral DNAs that encoded wild-type and mutant sequences revealed that all of the mutated proviral DNAs synthesized mature envelope proteins and released virus particles into the growth medium. However, serial passages of fetal lamb kidney (FLK) cells, which are sensitive to infection with BLV, after transient transfection revealed that mutation of a second tyrosine residue in the N-terminal motif completely prevented the propagation of the virus. Similarly, Y498A and Y487/498A mutant BLV that was produced by the stably transfected COS-1 cells exhibited significantly reduced levels of cell-free virion-mediated transmission. Analysis of the protein compositions of mutant viruses demonstrated that lower levels of envelope protein were incorporated by two of the mutant virions than by wild-type and other mutant virions. Furthermore, a mutation of a second tyrosine residue decreased the specific binding of BLV particles to FLK cells and the capacity for viral penetration. Our data indicate that the YXXL sequences play critical roles in both viral entry and the incorporation of viral envelope protein into the virion during the life cycle of BLV.

Cleavage of the murine leukemia virus transmembrane env protein by human immunodeficiency virus type 1 protease: transdominant inhibition by matrix mutations

We have identified mutations in the human immunodeficiency virus type 1 (HIV-1) matrix protein (MA) which block infectivity of virions pseudotyped with murine leukemia virus (MuLV) envelope (Env) glycoproteins without affecting infectivity conferred by HIV-1 Env or vesicular stomatitis virus G glycoproteins. This inhibition is very potent and displays a strong transdominant effect; infectivity is reduced more than 100-fold when wild-type and mutant molecular clones are cotransfected at a 1:1 ratio. This phenomenon is observed with both ecotropic and amphotropic MuLV Env. The MA mutations do not affect the incorporation of MuLV Env into virions. We demonstrate that in HIV-1 virions pseudotyped with MuLV Env, the HIV-1 protease (PR) efficiently catalyzes the cleavage of the p15(E) transmembrane (TM) protein to p12(E). Immunoprecipitation analysis of pseudotyped virions reveals that the mutant MA blocks this HIV-1 PR-mediated cleavage of MuLV TM. Furthermore, the transdominant inhibition exerted by the mutant MA on wild-type infectivity correlates with the relative level of p15(E) cleavage. Consistent with the hypothesis that abrogation of infectivity imposed by the mutant MA is due to inhibition of p15(E) cleavage, mutant virions are significantly more infectious when pseudotyped with a truncated p12(E) form of MuLV Env. These results indicate that HIV-1 Gag sequences can influence the viral PR-mediated processing of the MuLV TM Env protein p15(E). These findings have implications for the development of HIV-1-based retroviral vectors pseudotyped with MuLV Env, since p15(E) cleavage is essential for activating membrane fusion and virus infectivity.

Three active forms of aspartic proteinase from Mason-Pfizer monkey virus

Mason-Pfizer monkey virus (M-PMV) proteinase, released by the autocatalytic cleavage of Gag-Pro and Gag-Pro-Pol polypeptide precursors, catalyzes the processing of viral precursors to yield the structural proteins and enzymes of the virion. In retroviruses, usually only one proteolytically active form of proteinase exists. Here, we describe an unusual feature of M-PMV, the existence of three active forms of a retroviral proteinase with molecular masses of 17, 13, and 12 kDa as determined by mass spectroscopy. These forms arise in vitro by self-processing of a 26-kDa proteinase precursor. We have developed a process for isolation of each truncated product and demonstrate that all three forms display proteolytic activity. Amino acid analyses, as well as the determination of N- and C-terminal sequences, revealed that the N-termini of all three forms are identical, confirming that in vitro autoprocessing of the 17-kDa form occurs at the C-terminus to yield the truncated forms. The 17-kDa form and the newly described 13-kDa form of proteinase were identified in virions collected from the rhesus monkey CMMT cell line chronically infected with M-PMV, confirming that multiple forms exist in vivo.

Elongation of the cytoplasmic tail interferes with the fusion activity of influenza virus hemagglutinin

The hemagglutinin (HA) of fowl plague virus was lengthened and shortened by site-specific mutagenesis at the cytoplasmic tail, and the effects of these modifications on HA functions were analyzed after expression from a simian virus 40 vector. Elongation of the tail by the addition of one to six histidine (His) residues did not interfere with intracellular transport, glycosylation, proteolytic cleavage, acylation, cell surface expression, and hemadsorption. However, the ability to induce syncytia at a low pH decreased dramatically depending on the number of His residues added. Partial fusion (hemifusion), assayed by fluorescence transfer from octadecylrhodamine-labeled erythrocyte membranes, was also reduced, but even with the mutant carrying six His residues, significant transfer was observed. However, when the formation of fusion pores was examined with hydrophilic fluorescent calcein, transfer from erythrocytes to HA-expressing cells was not observed with the mutant carrying six histidine residues. The addition of different amino acids to the cytoplasmic tail of HA caused an inhibitory effect similar to that caused by the addition of His. On the other hand, a mutant lacking the cytoplasmic tail was still able to fuse at a reduced level. These results demonstrate that elongation of the cytoplasmic tail interferes with the formation and enlargement of fusion pores. Thus, the length of the cytoplasmic tail plays a critical role in the fusion process.

A proline-rich motif (PPPY) in the Gag polyprotein of Mason-Pfizer monkey virus plays a maturation-independent role in virion release

Virus assembly represents one of the last steps in the retrovirus life cycle. During this process, Gag polyproteins assemble at specific sites within the cell to form viral capsids and induce membrane extrusion (viral budding) either as assembly progresses (type C virus) or following formation of a complete capsid (type B and type D viruses). Finally, the membrane must undergo a fusion event to pinch off the particle in order to release a complete enveloped virion. Structural elements within the MA region of the Gag polyprotein define the route taken to the plasma membrane and direct the process of virus budding. Results presented here suggest that a distinct region of Gag is necessary for virus release. The pp24 and pp16 proteins of the type D retrovirus Mason-Pfizer monkey virus (M-PMV) are phosphoproteins that are encoded in the gag gene of the virus. The pp16 protein is a C-terminally located cleavage product of pp24 and contains a proline-rich motif (PPPY) that is conserved among the Gag proteins of a wide variety of retroviruses. By performing a functional analysis of this coding region with deletion mutants, we have shown that the pp16 protein is dispensable for capsid assembly but essential for virion release. Moreover, additional experiments indicated that the virus release function of pp16 was abolished by the deletion of only the PPPY motif and could be restored when this motif alone was reinserted into a Gag polyprotein lacking the entire pp16 domain. Single-amino-acid substitutions for any of the residues within this motif confer a similar virion release-defective phenotype. It is unlikely that the function of the proline-rich motif is simply to inhibit premature activation of protease, since the PPPY deletion blocked virion release in the context of a protease-defective provirus. These results demonstrate that in type D retroviruses a PPPY motif plays a key role in a late stage of virus budding that is independent of and occurs prior to virion maturation.

Evidence for cooperation between murine leukemia virus Env molecules in mixed oligomers

A retroviral Env molecule consists of a surface glycoprotein (SU) complexed with a transmembrane protein (TM). In turn, these complexes are grouped into oligomers on the surfaces of the cell and of the virion. In the case of murine leukemia viruses (MuLVs), the SU moieties are polymorphic, with SU proteins of different viral isolates directed towards different cell surface receptors. During maturation of the released virus particle, the 16 C-terminal residues of TM (the R peptide or p2E) are removed from the protein by the viral protease; this cleavage is believed to activate the membrane-fusing potential of MuLV Env. We have tested the possibility that different MuLV Env proteins in the same cell can interact with each other, both physically and functionally, in mixed oligomers. We found that coexpressed Env molecules can be precipitated out of cell lysates by antiserum which reacts with only one of them. Furthermore, they can evidently cooperate with each other: if one Env species lacks the R peptide, then it can apparently induce fusion if the SU protein of the other Env species encounters its cognate receptor on the surface of another cell. This functional interaction between different Env molecules has a number of implications with respect to the mechanism of induction of membrane fusion, for the genetic analysis of Env function, and for the design of targeted retroviral vectors for gene therapy.

Analysis of the murine leukemia virus R peptide: delineation of the molecular determinants which are important for its fusion inhibition activity

In previous studies, the C-terminal R peptide of the murine leukemia virus (MuLV) Env protein was shown to be a potent inhibitor of viral fusion activity. In the present study, we investigated the molecular determinants in the MuLV Env protein cytoplasmic tail which are important for the fusion inhibition activity of the R peptide. We constructed a series of mutant MuLV env genes which express Env proteins with serial truncations, internal deletions, or amino acid substitutions in the cytoplasmic tail. To analyze their cell fusion activity, we employed a quantitative fusion assay. We found that truncations of up to 7 amino acids from the C terminus of the cytoplasmic tail had no detectable effect on the lack of fusion activity of the full-length Env protein; however, further truncations resulted in a progressive increase in cell fusion activity. Studies of mutant proteins with amino acid substitutions in the cytoplasmic tail showed that Leu-627 plays an important role in fusion inhibition by the R peptide, while most of the other amino acids in the R peptide were not essential for fusion inhibition. Studies of mutant proteins with internal deletions upstream of the cleavage site in the cytoplasmic tail showed that this region is also involved in fusion inhibition by the R peptide, although only to a limited extent. The results are consistent with a model in which the MuLV R peptide exhibits its fusion inhibition activity through interaction with a cellular factor(s).

Functional analysis of the cytoplasmic tail of Moloney murine leukemia virus envelope protein

The cytoplasmic tail of the immature Moloney murine leukemia virus (MoMuLV) envelope protein is approximately 32 amino acids long. During viral maturation, the viral protease cleaves this tail to release a 16-amino-acid R peptide, thereby rendering the envelope protein fusion competent. A series of truncations, deletions, and amino acid substitutions were constructed in this cytoplasmic tail to examine its role in fusion and viral transduction. Sequential truncation of the cytoplasmic tail revealed that removal of as few as 11 amino acids resulted in significant fusion when the envelope protein was expressed in NIH 3T3 cells, similar to that seen following expression of an R-less envelope (truncation of 16 amino acids). Further truncation of the cytoplasmic tail beyond the R-peptide cleavage site toward the membrane-spanning region had no additional effect on the level of fusion observed. In contrast, some deletions and nonconservative amino acid substitutions in the membrane-proximal region of the cytoplasmic tail (residues L602 to F605) reduced the amount of fusion observed in XC cell cocultivation assays, suggesting that this region influences the fusogenicity of full-length envelope protein. Expression of the mutant envelope proteins in a retroviral vector system revealed that decreased envelope-mediated cell-cell fusion correlated with a decrease in infectivity of the resulting virions. Additionally, some mutant envelope proteins which were capable of mediating cell-cell fusion were not efficiently incorporated into retroviral particles, resulting in defective virions. The cytoplasmic tail of MoMuLV envelope protein therefore influences both the fusogenicity of the envelope protein and its incorporation into virions.

Analysis of the cell fusion activities of chimeric simian immunodeficiency virus-murine leukemia virus envelope proteins: inhibitory effects of the R peptide

It was previously reported that truncation or proteolytic removal of the C-terminal 16 amino acids (the R peptide) from the cytoplasmic tail of the murine leukemia virus (MuLV) envelope protein greatly increases its fusion activity. In this study, to investigate the specificity of the effect of the R peptide on the fusion activity of viral envelope proteins, we expressed simian immunodeficiency virus (SIV)-MuLV chimeric proteins in which the entire cytoplasmic tail of the SIV envelope protein was replaced by either the full-length MuLV cytoplasmic tail or a truncated MuLV cytoplasmic tail with the R peptide deleted. Extensive fusion of CD4-positive cells with the chimeric protein containing a truncated MuLV cytoplasmic tail was observed. In contrast, no cell fusion activity was found for the chimeric protein with a full-length MuLV cytoplasmic tail. We constructed another SIV-MuLV chimeric protein in which the MuLV R peptide was added to an SIV envelope protein cytoplasmic tail 17 amino acids from its membrane-spanning domain. No fusion activity was observed within this construct, while the corresponding truncated SIV envelope protein lacking the R peptide showed extensive fusion activity. No significant difference in the transport or surface expression was observed among the various SIV-MuLV chimeric proteins and the truncated SIV envelope protein. Our results thus demonstrate that the MuLV R peptide has profound inhibitory effects on virus-induced cell fusion, not only with MuLV but also in a distantly related retroviral envelope protein which utilizes a different receptor and fuses different cell types.

p6Gag is required for particle production from full-length human immunodeficiency virus type 1 molecular clones expressing protease

The human immunodeficiency virus type 1 (HIV-1) Gag protein precursor, Pr55Gag, contains at its C-terminal end a proline-rich, 6-kDa domain designated p6. Two functions have been proposed for p6: incorporation of the HIV-1 accessory protein Vpr into virus particles and virus particle production. To characterize the role of p6 in the HIV-1 life cycle and to map functional domains within p6, we introduced a number of nonsense and single and multiple amino acid substitution mutations into p6. Following the introduction of the mutations into the full-length HIV-1 molecular clone pNL4-3, the effects on Gag protein expression and processing, virus particle production, and virus infectivity were analyzed. The production of mutant virus particles was also examined by transmission electron microscopy. The results indicate that (i) p6 is required for efficient virus particle production from a full-length HIV-1 molecular clone; (ii) a Pro-Thr-Ala-Pro sequence, located between residues 7 and 10 of p6, is critical for virus particle production; (iii) mutations outside the Pro-Thr-Ala-Pro motif have little or no effect on virus assembly and release; (iv) the p6 defect is manifested at a late stage in the budding process; and (v) mutations in p6 that severely reduce virion production in HeLa cells also block or significantly delay the establishment of a productive infection in the CEM (12D-7) T-cell line. We further demonstrate that mutational inactivation of the viral protease reverses the p6 defect, suggesting a functional linkage between p6 and the proteolytic processing of the Gag precursor protein during the budding of progeny virions.

Cloning, bacterial expression, and characterization of the Mason-Pfizer monkey virus proteinase

We have cloned and expressed the 3' region of the Mason-Pfizer monkey virus pro gene in Escherichia coli. The recombinant 26-kDa precursor undergoes rapid self-processing both in E. coli and in vitro at the NH2 terminus, yielding a proteolytically active 17-kDa protein, p17. This initial cleavage is followed in vitro by a much slower self-processing that leads to emergence of proteolytically active p12 and a COOH-terminal cleavage product p5. We have found the NH2-terminal processing site of both the p17 and p12 to be identical and similar to the amino terminus of the mouse mammary tumor virus proteinase. We have also identified the COOH-terminal processing site of the p12 form. Using purified recombinant proteins and synthetic oligopeptide substrates based on naturally occurring retroviral processing sites, we have determined the enzymatic activity and specificity of the Mason-Pfizer monkey virus proteinase to be more closely related to that of myeloblastosis-associated virus proteinase rather than that of the Human immunodeficiency virus type 1 proteinase. Inhibition studies using peptide inhibitors support these results.

Human immunodeficiency virus type 1 virions composed of unprocessed Gag and Gag-Pol precursors are capable of reverse transcribing viral genomic RNA

The structural proteins and enzymes of the human immunodeficiency virus type 1 core are translated as part of two polyprotein precursors, Gag and Gag-Pol, which are cleaved by a virally encoded protease. Viruses grown in the presence of inhibitors of the protease contain core particles that are aberrantly assembled, and upon infection of susceptible cells, they do not synthesize viral DNA. Through the use of a proteinase inhibitor (A77003), we determined that the viral reverse transcriptase can efficiently synthesize viral DNA as part of the unprocessed Gag-Pol precursor. We also found that the stabilities of core particles composed of unprocessed precursors were considerably enhanced. These observations suggest that for viruses composed of unprocessed precursors, replication is interrupted before the reverse transcription step.

The activity of the protease of human immunodeficiency virus type 1 is initiated at the membrane of infected cells before the release of viral proteins and is required for release to occur with maximum efficiency

The final steps in the production of the type C retroviruses include assembly of the viral core particle and release of virions from the surface of the infected cell. The core proteins are translated as part of one of two precursors, Gag and Gag/Pol, which are cleaved by a virally encoded protease. We examined the interaction between the processing of the human immunodeficiency virus type 1 Gag precursor and the membrane-based assembly and budding of virions. Our results indicate that cleavage by the viral protease is initiated at the membrane of the infected cell during virus release and that protease activity is required for virion release to occur with maximum efficiency.

Function of the cytoplasmic domain of a retroviral transmembrane protein: p15E-p2E cleavage activates the membrane fusion capability of the murine leukemia virus Env protein

In the murine leukemia viruses (MuLVs), the Env complex is initially cleaved by a cellular protease into gp70SU and pre15ETM. After the virus particle is released from the cell, the C-terminal 16 residues are removed from the cytoplasmic domain of pre15E by the viral protease, yielding the mature p15ETM and p2E. We have investigated the function of this cleavage by generating a Moloney MuLV mutant, termed p2E-, in which the Env coding region terminates at the cleavage site. This mutant synthesizes only the truncated, mature form of TM rather than its extended precursor. When cells expressing this truncated Env protein are cocultivated with NIH 3T3 cells, they induce rapid cell-cell fusion. Thus, the truncated form, which is normally found in virions but not in virus-producing cells, is capable of causing membrane fusion. We conclude that the 16-residue p2E tail inhibits this activity of Env until the virus has left the cell. p2E- virions were found to be infectious, though with a lower specific infectivity than that of the wild type, showing that p2E does not play an essential role in the process of infection. Fusion was also observed with a chimeric p2E- virus in which gp70SU and nearly all of p15ETM are derived from amphotropic, rather than Moloney, MuLV. In a second mutant, an amino acid at the cleavage site was changed. The pre15E protein in this mutant is not cleaved. While the mutant Env complex is incorporated into virions, these particles have a very low specific infectivity. This result suggests that the cleavage event is essential for infectivity, in agreement with the idea that removal of p2E activates the membrane fusion capability of the Env complex.

Human endogenous retroviral element K10 (HERV-K10) encodes a full-length gag homologous 73-kDa protein and a functional protease

The gag-homologous region of the human endogenous retrovirus K10 (HERV-K10) was amplified by PCR from human genomic DNA and was analyzed by DNA cloning, sequencing, and expression of open reading frames in the prokaryotic pATH expression system. The analysis of genomic DNA of three donors provided evidence that HERV-K10 genes contain an open reading frame of 1966 bp spanning the entire gag-homologous region. In the prokaryotic system the entire reading frame of the HERV-K10 gag gene could be expressed as a fusion protein exhibiting a molecular weight of about 110,000. In addition, when the gag-homologous region and the adjacent HERV-K10 protease gene were prokaryotically expressed, we observed a Gag-protease fusion protein that exhibited specific autoproteolytic activities and processing of HERV-K10 Gag protein. By introducing deletions on the right end of the putative protease gene an autocatalytic site could be localized within 300 bp of the putative HERV-K10 protease gene. For the first time, these results provide evidence that the HERV-K10 encodes a full-length Gag protein and a functional protease.

Recent advances in retrovirus vector technology

Retroviral vectors are widely used for the study of retroviral replication and to introduce DNA into somatic cells. An exciting new approach in retroviral vector technology is the use of internal ribosome entry sites from picornaviruses to provide stable expression of multiple genes. In addition, strategies are being developed that target the expression of retroviral vectors to specific cell populations.

Importance of p12 protein in Mason-Pfizer monkey virus assembly and infectivity

Mason-Pfizer monkey virus (M-PMV) represents the prototype type D retrovirus, characterized by the assembly of intracytoplasmic A-type particles within the infected-cell cytoplasm. These immature particles migrate to the plasma membrane, where they are released by budding. The gag gene of M-PMV encodes a novel protein, p12, just 5' of the major capsid protein (CA) p27 on the polyprotein precursor. The function of p12 is not known, but an equivalent protein is found in mouse mammary tumor virus and is absent from the type C retroviruses. In order to determine whether the p12 protein plays a role in the intracytoplasmic assembly of capsids, a series of in-frame deletion mutations were constructed in the p12 coding domain. The mutant gag genes were expressed by a recombinant vaccinia virus-T7 polymerase-based system in CV-1 cells or in the context of the viral genome in COS-1 cells. In both of these high-level expression systems, mutant Gag precursors were competent to assemble but were not infectious. In contrast, when stable transfectant HeLa cell lines were established, assembly of the mutant precursors into capsids was drastically reduced. Instead, the polyprotein precursors remained predominantly soluble in the cytoplasm. These results show that while p12 is not required for the intracytoplasmic assembly of M-PMV capsids, under the conditions of low-level protein biosynthesis seen in virus-infected cells, it may assist in the stable association of polyprotein precursors for capsid assembly. Moreover, the presence of the p12 coding domain is absolutely required for the infectivity of M-PMV virions.