Structural role of the matrix protein of type D retroviruses in gag polyprotein stability and capsid assembly

Direct evidence for intracellular anterograde co-transport of M-PMV Gag and Env on microtubules

The intracellular transport of Mason-Pfizer monkey virus (M-PMV) assembled capsids from the pericentriolar region to the plasma membrane (PM) requires trafficking of envelope glycoprotein (Env) to the assembly site via the recycling endosome. However, it is unclear if Env-containing vesicles play a direct role in trafficking capsids to the PM. Using live cell microscopy, we demonstrate, for the first time, anterograde co-transport of Gag and Env. Nocodazole disruption of microtubules had differential effects on Gag and Env trafficking, with pulse-chase assays showing a delayed release of Env-deficient virions. Particle tracking demonstrated an initial loss of linear movement of GFP-tagged capsids and mCherry-tagged Env, followed by renewed movement of Gag but not Env at 4h post-treatment. Thus, while delayed capsid trafficking can occur in the absence of microtubules, efficient anterograde transport of capsids appears to be mediated by microtubule-associated Env-containing vesicles.

Virus-producing cells determine the host protein profiles of HIV-1 virion cores

Background

Upon HIV entry into target cells, viral cores are released and rearranged into reverse transcription complexes (RTCs), which support reverse transcription and also protect and transport viral cDNA to the site of integration. RTCs are composed of viral and cellular proteins that originate from both target and producer cells, the latter entering the target cell within the viral core. However, the proteome of HIV-1 viral cores in the context of the type of producer cells has not yet been characterized.

Results

We examined the proteomic profiles of the cores purified from HIV-1 NL4-3 virions assembled in Sup-T1 cells (T lymphocytes), PMA and vitamin D₃ activated THP1 (model of macrophages, mMΦ), and non-activated THP1 cells (model of monocytes, mMN) and assessed potential involvement of identified proteins in the early stages of infection using gene ontology information and data from genome-wide screens on proteins important for HIV-1 replication. We identified 202 cellular proteins incorporated in the viral cores (T cells: 125, mMΦ: 110, mMN: 90) with the overlap between these sets limited to 42 proteins. The groups of RNA binding (29), DNA binding (17), cytoskeleton (15), cytoskeleton regulation (21), chaperone (18), vesicular trafficking-associated (12) and ubiquitin-proteasome pathway-associated proteins (9) were most numerous. Cores of the virions from SupT1 cells contained twice as many RNA binding proteins as cores of THP1-derived virus, whereas cores of virions from mMΦ and mMN were enriched in components of cytoskeleton and vesicular transport machinery, most probably due to differences in virion assembly pathways between these cells. Spectra of chaperones, cytoskeletal proteins and ubiquitin-proteasome pathway components were similar between viral cores from different cell types, whereas DNA-binding and especially RNA-binding proteins were highly diverse. Western blot analysis showed that within the group of overlapping proteins, the level of incorporation of some RNA binding (RHA and HELIC2) and DNA binding proteins (MCM5 and Ku80) in the viral cores from T cells was higher than in the cores from both mMΦ and mMN and did not correlate with the abundance of these proteins in virus producing cells.

Conclusions

Profiles of host proteins packaged in the cores of HIV-1 virions depend on the type of virus producing cell. The pool of proteins present in the cores of all virions is likely to contain factors important for viral functions. Incorporation ratio of certain RNA- and DNA-binding proteins suggests their more efficient, non-random packaging into virions in T cells than in mMΦ and mMN.

The G-patch domain of Mason-Pfizer monkey virus is a part of reverse transcriptase

Mason-Pfizer monkey virus (M-PMV), like some other betaretroviruses, encodes a G-patch domain (GPD). This glycine-rich domain, which has been predicted to be an RNA binding module, is invariably localized at the 3' end of the pro gene upstream of the pro-pol ribosomal frameshift sequence of genomic RNAs of betaretroviruses. Following two ribosomal frameshift events and the translation of viral mRNA, the GPD is present in both Gag-Pro and Gag-Pro-Pol polyproteins. During the maturation of the Gag-Pro polyprotein, the GPD transiently remains a C-terminal part of the protease (PR), from which it is then detached by PR itself. The destiny of the Gag-Pro-Pol-encoded GPD remains to be determined. The function of the GPD in the retroviral life cycle is unknown. To elucidate the role of the GPD in the M-PMV replication cycle, alanine-scanning mutational analysis of its most highly conserved residues was performed. A series of individual mutations as well as the deletion of the entire GPD had no effect on M-PMV assembly, polyprotein processing, and RNA incorporation. However, a reduction of the reverse transcriptase (RT) activity, resulting in a drop in M-PMV infectivity, was determined for all GPD mutants. Immunoprecipitation experiments suggested that the GPD is a part of RT and participates in its function. These data indicate that the M-PMV GPD functions as a part of reverse transcriptase rather than protease.

Expression and purification of myristoylated matrix protein of Mason-Pfizer monkey virus for NMR and MS measurements

Matrix proteins play multiple roles both in early and late stages of the viral replication cycle. Their N-terminal myristoylation is important for interaction with the host cell membrane during virus budding. We used Escherichia coli, carrying N-myristoyltransferase gene, for the expression of the myristoylated His-tagged matrix protein of Mason-Pfizer monkey virus. An efficient, single-step purification procedure eliminating all contaminating proteins including, importantly, the non-myristoylated matrix protein was designed. The comparison of NMR spectra of matrix protein with its myristoylated form revealed substantial structural changes induced by this fatty acid modification.

Mechanisms of late restriction induced by an endogenous retrovirus

The host has developed during evolution a variety of "restriction factors" to fight retroviral infections. We investigated the mechanisms of a unique viral block acting at late stages of the retrovirus replication cycle. The sheep genome is colonized by several copies of endogenous retroviruses, known as enJSRVs, which are highly related to the oncogenic jaagsiekte sheep retrovirus (JSRV). enJS56A1, one of the enJSRV proviruses, can act as a restriction factor by blocking viral particles release of the exogenous JSRV. We show that in the absence of enJS56A1 expression, the JSRV Gag (the retroviral internal structural polyprotein) targets initially the pericentriolar region, in a dynein and microtubule-dependent fashion, and then colocalizes with the recycling endosomes. Indeed, by inhibiting the endocytosis and trafficking of recycling endosomes we hampered JSRV exit from the cell. Using a variety of approaches, we show that enJS56A1 and JSRV Gag interact soon after synthesis and before pericentriolar/recycling endosome targeting of the latter. The transdominant enJS56A1 induces intracellular Gag accumulation in microaggregates that colocalize with the aggresome marker GFP-250 but develop into bona fide aggresomes only when the proteasomal machinery is inhibited. The data argue that dominant-negative proteins can modify the overall structure of Gag multimers/viral particles hampering the interaction of the latter with the cellular trafficking machinery.

Potential role of N-myristoyltransferase in cancer

Colorectal cancer is the second leading cause of malignant death, and better preventive strategies are needed. The treatment of colonic cancer remains difficult because of the lack of effective chemotherapeutic agents; therefore it is important to continue to search for cellular functions that can be disrupted by chemotherapeutic drugs resulting in the inhibition of the development and progression of cancer. The current knowledge of the modification of proteins by myristoylation involving myristoyl-CoA: protein N-myristoyltransferase (NMT) is in its infancy. This process is involved in the pathogenesis of cancer. We have reported for the first time that NMT activity and protein expression were higher in human colorectal cancer, gallbladder carcinoma and brain tumors. In addition, an increase in NMT activity appeared at an early stage in colonic carcinogenesis. It is conceivable therefore that NMT can be used as a potential marker for the early detection of cancer. These observations lead to the possibility of developing NMT specific inhibitors, which may be therapeutically useful. We proposed that HSC70 and/or enolase could be used as an anticancer therapeutic target. This review summarized the status of NMT in cancer which has been carried in our laboratory.

Rescue of internal scaffold-deleted Mason-Pfizer monkey virus particle production by plasma membrane targeting

The Mason-Pfizer monkey virus (M-PMV) Gag protein follows a morphogenesis pathway in which immature capsids are preassembled within the cytoplasm before interaction with and budding through the plasma membrane. Intracytoplasmic assembly is facilitated by sequences within the p12 domain of Gag that we have termed the Internal Scaffold Domain (ISD). If M-PMV utilizes an ISD then what provides the equivalent function for most other retroviruses that assemble at the plasma membrane? To investigate the possibility that the membrane itself fulfills this role, we have combined functional deletion of the ISD with a mutation that disrupts intracellular targeting or with a plasma membrane targeting signal. By either modification, targeting of ISD-deleted Gag to the plasma membrane restores particle production. These results provide support for a model in which the plasma membrane and the D-type ISD provide an interchangeable scaffold-like function in retrovirus assembly.

The pp24 phosphoprotein of Mason-Pfizer monkey virus contributes to viral genome packaging

Background

The Gag protein of Mason-Pfizer monkey virus, a betaretrovirus, contains a phosphoprotein that is cleaved into the Np24 protein and the phosphoprotein pp16/18 during virus maturation. Previous studies by Yasuda and Hunter (J. Virology. 1998. 72:4095–4103) have demonstrated that pp16/18 contains a viral late domain required for budding and that the Np24 protein plays a role during the virus life cycle since deletion of this N-terminal domain blocked virus replication. The function of the Np24 domain, however, is not known.

Results

Here we identify a region of basic residues (KKPKR) within the Np24 domain that is highly conserved among the phosphoproteins of various betaretroviruses. We show that this KKPKR motif is required for virus replication yet dispensable for procapsid assembly, membrane targeting, budding and release, particle maturation, or viral glycoprotein packaging. Additional experiments indicated that deletion of this motif reduced viral RNA packaging 6–8 fold and affected the transient association of Gag with nuclear pores.

Conclusion

These results demonstrate that the Np24 domain plays an important role in RNA packaging and is in agreement with evidence that suggests that correct intracellular targeting of Gag to the nuclear compartment is an fundamental step in the retroviral life cycle.

N-terminal Gag domain required for foamy virus particle assembly and export

Among the Retroviridae, foamy viruses (FVs) exhibit an unusual way of particle assembly and a highly specific incorporation of envelope protein into progeny virions. We have analyzed deletions and point mutants of the prototypic FV gag gene for capsid assembly and egress, envelope protein incorporation, infectivity, and ultrastructure. Deletions introduced at the 3' end of gag revealed the first 297 amino acids (aa) to be sufficient for specific Env incorporation and export of particulate material. Deletions introduced at the 5' end showed the region between aa 6 and 200 to be dispensable for virus capsid assembly but required for the incorporation of Env and particle egress. Point mutations were introduced in the 5' region of gag to target residues conserved among FVs from different species. Alanine substitutions of residues in a region between aa 40 and 60 resulted in severe alterations in particle morphology. Furthermore, at position 50, this region harbors the conserved arginine that is presumably at the center of a signal sequence directing FV Gag proteins to a cytoplasmic assembly site.

Incorporation of pol into human immunodeficiency virus type 1 Gag virus-like particles occurs independently of the upstream Gag domain in Gag-pol

By using particle-associated reverse transcriptase (RT) activity as an assay for Pol incorporation into human immunodeficiency virus type 1 (HIV-1) Gag virus-like particles (VLPs), it has been found that truncated, protease-negative, Gag-Pol missing cis Gag sequences is still incorporated into Gag VLPs, albeit at significantly reduced levels (10 to 20% of the level of wild-type Gag-Pol). In this work, we have directly measured the incorporation of truncated Gag-Pol species into Gag VLPs and have found that truncated Gag-Pol that is missing all sequences upstream of RT is still incorporated into Gag VLPs at levels approximating 70% of that achieved by wild-type Gag-Pol. Neither protease nor integrase regions in Pol are required for its incorporation, implying an interaction between Gag and RT sequences in the Pol protein. While the incorporation of Gag-Pol into Gag VLPs is reduced 12-fold by the replacement of the nucleocapsid within Gag with a leucine zipper motif, this mutation does not affect Pol incorporation. However, the deletion of p6 in Gag reduces Pol incorporation into Gag VLPs four- to fivefold. Pol shows the same ability as Gag-Pol to selectively package tRNA(Lys) into Gag VLPs, and primer tRNA(3)(Lys) is found annealed to the viral genomic RNA. These data suggest that after the initial separation of Gag from Pol during cleavage of Gag-Pol by viral protease, the Pol species still retains the capacity to bind to both Gag and tRNA(3)(Lys), which may be required for Pol and tRNA(3)(Lys) to be retained in the assembling virion until budding is completed.

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.

Rapid localization of Gag/GagPol complexes to detergent-resistant membrane during the assembly of human immunodeficiency virus type 1

During human immunodeficiency virus type 1 (HIV-1) assembly in HIV-1-transfected COS7 cells, almost all steady-state Gag/Gag and Gag/GagPol complexes are membrane bound. However, exposure to 1% Triton X-100 gives results indicating that while all Gag/GagPol complexes remain associated with the detergent-resistant membrane (DRM), only 30% of Gag/Gag complexes are associated with the DRM. Analysis of the localization of newly synthesized Gag/Gag and Gag/GagPol to the membrane indicates that after a 10-min pulse with radioactive [(35)S]Cys-[(35)S]Met, all newly synthesized Gag/GagPol is found at the DRM. Only 30% of newly synthesized Gag/Gag moves to the membrane, and at 0 min of chase, only 38% of this membrane-bound Gag/Gag is associated with the DRM. During the first 30 min of chase, most membrane-bound Gag/Gag moves to the DRM, while between 30 and 60 min of chase, there is a significant decrease in membrane-bound Gag/Gag and Gag/GagPol. Since the localization of newly synthesized Gag/Gag to the DRM and the interaction of GagPol with Gag both depend upon Gag multimerization, the rapid localization of GagPol to the DRM probably reflects the interaction of all newly synthesized GagPol with the first newly synthesized polymeric Gag to associate with the DRM.

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.

Type D retrovirus Gag polyprotein interacts with the cytosolic chaperonin TRiC

The carboxy terminus-encoding portion of the gag gene of Mason-Pfizer monkey virus (M-PMV), the prototype immunosuppressive primate type D retrovirus, encodes a 36-amino-acid, proline-rich protein domain that, in the mature virion, becomes the p4 capsid protein. The p4 domain has no known role in M-PMV replication. We found that two mutants with premature termination codons that remove half or all of the p4 domain produced lower levels of stable Gag protein and of self-assembled capsids. Interestingly, yeast two-hybrid screening revealed that p4 specifically interacted with TCP-1gamma, a subunit of the chaperonin TRiC (TCP-1 ring complex). TRiC is a cytosolic chaperonin that is known to be involved in both folding and subunit assembly of a variety of cellular proteins. TCP-1gamma also associated with high specificity with the M-PMV pp24/16-p12 domain and human immunodeficiency virus p6. Moreover, in cells, Gag polyprotein associated with the TRiC chaperonin complex and this association depended on ATP hydrolysis. In the p4 truncation mutants, the Gag-TRiC association was significantly reduced. These results strongly suggest that cytosolic chaperonin TRiC is involved in Gag folding and/or capsid assembly. We propose that TRiC associates transiently with nascent M-PMV Gag molecules to assist in their folding. Consequently, properly folded Gag molecules carry out the intermolecular interactions involved in self-assembly of the immature capsid.

Analysis of Mason-Pfizer monkey virus Gag domains required for capsid assembly in bacteria: role of the N-terminal proline residue of CA in directing particle shape

Mason-Pfizer monkey virus (M-PMV) preassembles immature capsids in the cytoplasm prior to transporting them to the plasma membrane. Expression of the M-PMV Gag precursor in bacteria results in the assembly of capsids indistinguishable from those assembled in mammalian cells. We have used this system to investigate the structural requirements for the assembly of Gag precursors into procapsids. A series of C- and N-terminal deletion mutants progressively lacking each of the mature Gag domains (matrix protein [MA]-pp24/16-p12-capsid protein [CA]-nucleocapsid protein [NC]-p4) were constructed and expressed in bacteria. The results demonstrate that both the CA and the NC domains are necessary for the assembly of macromolecular arrays (sheets) but that amino acid residues at the N terminus of CA define the assembly of spherical capsids. The role of these N-terminal domains is not based on a specific amino acid sequence, since both MA-CA-NC and p12-CA-NC polyproteins efficiently assemble into capsids. Residues N terminal of CA appear to prevent a conformational change in which the N-terminal proline plays a key role, since the expression of a CA-NC protein lacking this proline results in the assembly of spherical capsids in place of the sheets assembled by the CA-NC protein.

Role of matrix protein in the type D retrovirus replication cycle: importance of the arginine residue at position 55

We previously reported that a mutant of Mason-Pfizer monkey virus (M-PMV), which has an amino acid substitution in the matrix (MA) protein at position 55, MA-R55W, showed altered viral morphogenesis, reduced glycoprotein incorporation, and loss of infectivity. In this report, we show that two additional amino acid substitutions at this site in MA, R55F and R55Y, also result in similar altered morphogenesis, Env incorporation, and infectivity, demonstrating that these changes are not specific for the substitution of tryptophan in place of arginine 55. Attempts to isolate second site infectious revertants from cells transfected with the R55W mutant genome resulted only in the recovery of infectious viruses in which the codon at position 55 had reverted to one encoding arginine. In contrast, no revertants were obtained from the phenylalanine and tyrosine mutants in which three nucleotide changes had been engineered into the arginine codon. These results confirm that the arginine residue at position 55 is critical for intracellular targeting of M-PMV Gag molecules and support the concept that as part of a cytoplasmic transport retention signal R55 interacts with cellular trafficking components rather than other regions of Gag.

A cell-line-specific defect in the intracellular transport and release of assembled retroviral capsids

Retrovirus assembly involves a complex series of events in which a large number of proteins must be targeted to a point on the plasma membrane where immature viruses bud from the cell. Gag polyproteins of most retroviruses assemble an immature capsid on the cytoplasmic side of the plasma membrane during the budding process (C-type assembly), but a few assemble immature capsids deep in the cytoplasm and are then transported to the plasma membrane (B- or D-type assembly), where they are enveloped. With both assembly phenotypes, Gag polyproteins must be transported to the site of viral budding in either a relatively unassembled form (C type) or a completely assembled form (B and D types). The molecular nature of this transport process and the host cell factors that are involved have remained obscure. During the development of a recombinant baculovirus/insect cell system for the expression of both C-type and D-type Gag polyproteins, we discovered an insect cell line (High Five) with two distinct defects that resulted in the reduced release of virus-like particles. The first of these was a pronounced defect in the transport of D-type but not C-type Gag polyproteins to the plasma membrane. High Five cells expressing wild-type Mason-Pfizer monkey virus (M-PMV) Gag precursors accumulate assembled immature capsids in large cytoplasmic aggregates similar to a transport-defective mutant (MA-A18V). In contrast, a larger fraction of the Gag molecules encoded by the M-PMV C-type morphogenesis mutant (MA-R55W) and those of human immunodeficiency virus were transported to the plasma membrane for assembly and budding of virions. When pulse-labeled Gag precursors from High Five cells were fractionated on velocity gradients, they sedimented more rapidly, indicating that they are sequestered in a higher-molecular-mass complex. Compared to Sf9 insect cells, the High Five cells also demonstrate a defect in the release of C-type virus particles. These findings support the hypothesis that host cell factors are important in the process of Gag transport and in the release of enveloped viral particles.

Identification of a cytoplasmic targeting/retention signal in a retroviral Gag polyprotein

Retroviral capsid assembly can occur by either of two distinct morphogenic processes: in type C viruses, the capsid assembles and buds at the plasma membrane, while in type B and D viruses, the capsid assembles within the cytoplasm and is then transported to the plasma membrane for budding. We have previously reported that a single-amino-acid substitution of a tryptophan for an arginine in the matrix protein (MA) of Mason-Pfizer monkey virus (MPMV) converts its capsid assembly from that of a type D retrovirus to that of the type C viruses (S. S. Rhee and E. Hunter, Cell 63:77-86, 1990). Here we identify a region of 18 amino acids within the MA of MPMV that is responsible for type D-specific morphogenesis. Insertion of these 18 amino acids into the MA of type C Moloney murine leukemia virus causes it to assemble an immature capsid in the cytoplasm. Furthermore, fusion of the MPMV MA to the green fluorescent protein resulted in altered intracellular targeting and a punctate accumulation of the fusion protein in the cytoplasm. These 18 amino acids, which are necessary and sufficient to target retroviral Gag polyproteins to defined sites in the cytoplasm, appear to define a novel mammalian cytoplasmic targeting/retention signal.

Polarized human immunodeficiency virus budding in lymphocytes involves a tyrosine-based signal and favors cell-to-cell viral transmission

Maturation and release of human immunodeficiency virus type 1 (HIV-1) is targeted at the pseudopod of infected mononuclear cells. However, the intracellular mechanism or targeting signals leading to this polarized viral maturation are yet to be identified. We have recently demonstrated the presence of a functional YXXL motif for specific targeting of HIV-1 virions to the basolateral membrane surface in polarized epithelial Madin-Darby canine kidney cells (MDCK). Site-directed mutagenesis was used to demonstrate that the membrane-proximal tyrosine in the intracytoplasmic tail of the HIV-1 transmembrane glycoprotein (gp41) is an essential component of this signal. In the present study, immunolocalization of viral budding allowed us to establish that this tyrosine-based signal is involved in determining the exact site of viral release at the surface of infected mononuclear cells. Substitution of the critical tyrosine residue was also shown to increase the amount of envelope glycoprotein at the cell surface, supporting previous suggestions that the tyrosine-based motif can promote endocytosis. Although alteration of the dual polarization-endocytosis motif did not affect the infectivity of cell-free virus, it could play a key role in cell-to-cell viral transmission. Accordingly, chronically infected lymphocytes showed a reduced ability to transmit the mutant virus to a cocultivated cell line. Overall, our data indicate that the YXXL targeting motif of HIV is active in various cell types and could play an important role in viral propagation; this may constitute an alternative target for HIV therapeutics and vaccine development.

Multiple functions for the basic amino acids of the human T-cell leukemia virus type 1 matrix protein in viral transmission

We studied the involvement of the human T-cell leukemia virus type 1 (HTLV-1) Gag matrix protein in the cell-to-cell transmission of the virus using missense mutations of the basic amino acids. These basic amino acids are clustered at the N terminus of the protein in other retroviruses and are responsible for targeting the Gag proteins to the plasma membrane. In the HTLV-bovine leukemia virus genus of retroviruses, the basic amino acids are distributed throughout the matrix protein sequence. The HTLV-1 matrix protein contains 11 such residues. A wild-type phenotype was obtained only for mutant viruses with mutations at one of two positions in the matrix protein. The phenotypes of the other nine mutant viruses showed that the basic amino acids are involved at various steps of the replication cycle, including some after membrane targeting. Most of these nine mutations allowed normal synthesis, transport, and cleavage of the Gag precursor, but particle release was greatly affected for seven of them. In addition, four mutated proteins with correct particle release and envelope glycoprotein incorporation did not however permit cell-to-cell transmission of HTLV-1. Thus, particle release, although required, is not sufficient for the cell-to-cell transmission of HTLV-1, and the basic residues of the matrix protein are involved in steps that occur after viral particle release.

The role of nucleocapsid of HIV-1 in virus assembly

The role of the nucleocapsid protein of HIV-1 Gag in virus assembly was investigated using Gag truncation mutants, a nucleocapsid deletion mutant, and point mutations in the nucleocapsid region of Gag, in transfected COS cells, and in stable T-cell lines. Consistent with previous investigations, a truncation containing only the matrix and capsid regions of Gag was unable to assemble efficiently into particles; also, the pelletable material released was lighter than the density of wild-type HIV-1. A deletion mutant lacking p7 nucleocapsid but containing the C-terminal p6 protein was also inefficient in particle release and released lighter particles, while a truncation containing only the first zinc finger of p7 could assemble more efficiently into virions. These results clearly show that p7 is indispensable for virus assembly and release. Some point mutations in the N-terminal basic domain and in the basic linker region between the two zinc fingers, which had been previously shown to have reduced RNA binding in vitro [Schmalzbauer, E., Strack, B., Dannull, J., Guehmann, S., and Moelling, K. (1996). J. Virol. 70: 771-777], were shown to reduce virus assembly dramatically when expressed in full-length viral clones. A fusion protein consisting of matrix and capsid fused to a heterologous viral protein known to have nonspecific RNA binding activity [Ribas, J. C., Fujimura, T., and Wickner, R. B. (1994) J. Biol. Chem. 269: 28420-28428] released pelletable material slightly more efficiently than matrix and capsid alone, and these particles had density higher than matrix and capsid alone. These results demonstrate the essential role of HIV-1 nucleocapsid in the virus assembly process and show that the positively charged N terminus of p7 is critical for this role.

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.

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.

Type D retrovirus capsid assembly and release are active events requiring ATP

Mason-Pfizer monkey virus (M-PMV), the prototype type D retrovirus, differs from most other retroviruses by assembling its Gag polyproteins into procapsids in the cytoplasm of infected cells. Once assembled, the procapsids migrate to the plasma membrane, where they acquire their envelope during budding. Because the processes of M-PMV protein transport, procapsid assembly, and budding are temporally and spatially unlinked, we have been able to determine whether cellular proteins play an active role during the different stages of procapsid morphogenesis. We report here that at least two stages of morphogenesis require ATP. Both procapsid assembly and procapsid transport to the plasma membrane were reversibly blocked by treating infected cells with sodium azide and 2-deoxy-D-glucose, which we show rapidly and reversibly depletes cellular ATP pools. Assembly of procapsids in vitro in a cell-free translation/assembly system was inhibited by the addition of nonhydrolyzable ATP analogs, suggesting that ATP hydrolysis and not just ATP binding is required. Since retrovirus Gag polyproteins do not bind or hydrolyze ATP, these results demonstrate that cellular components must play an active role during retrovirus morphogenesis.

Myristoylation

N-myristoylation is an acylation process absolutely specific to the N-terminal amino acid glycine in proteins. This maturation process concerns about a hundred proteins in lower and higher eukaryotes involved in oncogenesis, in secondary cellular signalling, in infectivity of retroviruses and, marginally, of other virus types. Thy cytosolic enzyme responsible for this activity, N-myristoyltransferase (NMT), studied since 1987, has been purified from different sources. However, the studies of the specificities of the various NMTs have not progressed in detail except for those relating to the yeast cytosolic enzyme. Still to be explained are differences in species specificity and between various putative isoenzymes, also whether the data obtained from the yeast enzyme can be transposed to other NMTs. The present review discusses data on the various addressing processes subsequent to myristoylation, a patchwork of pathways that suggests myristoylation is only the first step of the mechanisms by which a protein associates with the membrane. Concerning the enzyme itself, there are evidences that NMT is also present in the endoplasmic reticulum and that its substrate specificity is different from that of the cytosolic enzyme(s). These differences have major implications for their differential inhibition and for their respective roles in several pathologies. For instance, the NMTs from mammalians are clearly different from those found in several microorganisms, which raises the question whether the NMT may be a new targets for fungicides. Finally, since myristoylation has a central role in virus maturation and oncogenesis, specific NMT inhibitors might lead to potent antivirus and anticancer agents.

Three-dimensional structure of the HTLV-II matrix protein and comparative analysis of matrix proteins from the different classes of pathogenic human retroviruses

The matrix protein performs similar roles in all retroviruses, initially directing membrane localization of the assembling viral particle and subsequently forming a stable structural shell associated with the inner surface of the mature viral membrane. Although conserved structural elements are likely to perform these functions in all retroviral matrix proteins, invariant motifs are not evident at the primary sequence level and three-dimensional structures have been available for only the primate lentiviral matrix proteins. We have therefore used NMR spectroscopy to determine the structure of the matrix protein from human T-cell leukemia virus type II (HTLV-II), a member of the human oncovirus subclass of retroviruses. A total of 577 distance restraints were used to build 20 refined models that superimpose with an rmsd of 0.71 A for the backbone atoms of the structured regions. The globular HTLV-II matrix structure is composed of four alpha-helices and a 3(10) helix. Exposed basic residues near the C terminus of helix II form a putative membrane binding surface which could act in concert with the N-terminal myristoyl group to anchor the protein on the viral membrane surface. Clear structural similarities between the HTLV-II and HIV-1 matrix proteins suggest that the topology and exposed cationic membrane binding surface are likely to be conserved features of retroviral matrix proteins.

Structural analysis of human immunodeficiency virus type 1 Gag protein interactions, using cysteine-specific reagents

We have examined structural interactions of Gag proteins in human immunodeficiency virus type 1 (HIV-1) particles by utilizing cysteine mutagenesis and cysteine-specific modifying reagents. In immature protease-minus but otherwise wild-type (wt) particles, precursor Pr55Gag proteins did not form intermolecular cystines naturally but could be cross-linked at cysteines, and cross-linking appeared to occur across nucleocapsid (NC) domains. Capsid (CA) proteins in wt mature viruses possess cysteines near their carboxy termini at gag codons 330 and 350, but these residues are not involved in natural covalent intermolecular bonds, nor can they be intermolecularly cross-linked by using the membrane-permeable cross-linker bis-maleimido hexane. The cysteine at gag codon 350 (C-350) is highly reactive to thiol-specific modifying reagents, while the one at codon 330 (C-330) appears considerably less reactive, even in the presence of ionic detergent. These results suggest that the HIV-1 CA C terminus forms an unusually stable conformation. Mutagenesis of C-350 to a serine residue in the mutant C350S (C-350 changed to serine) virtually eliminated particle assembly, attesting to the importance of this region. We also examined a C330S mutant, as well as mutants in which cysteines were created midway through the capsid domain or in the C-terminal section of the major homology region. All such mutants appeared wt on the basis of biochemical assays but showed greatly reduced infectivities, indicative of a postassembly, postprocessing replicative block. Interestingly, capsid proteins of mature major homology region mutant particles could be cysteine cross-linked, implying either that these mutations permit cross-linking of the native C-terminal CA cysteines or that major homology regions on neighbor capsid proteins are in close proximity in mature virions.

Synthesis and assembly of retrovirus Gag precursors into immature capsids in vitro

The assembly of retroviral particles is mediated by the product of the gag gene; no other retroviral gene products are necessary for this process. While most retroviruses assemble their capsids at the plasma membrane, viruses of the type D class preassemble immature capsids within the cytoplasm of infected cells. This has allowed us to determine whether immature capsids of the prototypical type D retrovirus, Mason-Pfizer monkey virus (M-PMV), can assemble in a cell-free protein synthesis system. We report here that assembly of M-PMV Gag precursor proteins can occur in this in vitro system. Synthesized particles sediment in isopycnic gradients to the appropriate density and in thin-section electron micrographs have a size and appearance consistent with those of immature retrovirus capsids. The in vitro system described in this report appears to faithfully mimic the process of assembly which occurs in the host cell cytoplasm, since M-PMV gag mutants defective in in vivo assembly also fail to assemble in vitro. Likewise, the Gag precursor proteins of retroviruses that undergo type C morphogenesis, Rous sarcoma virus and human immunodeficiency virus, which do not preassemble capsids in vivo, fail to assemble particles in this system. Additionally, we demonstrate, with the use of anti-Gag antibodies, that this cell-free system can be utilized for analysis in vitro of potential inhibitors of retrovirus assembly.

Mutations in the Ty3 major homology region affect multiple steps in Ty3 retrotransposition

The Saccharomyces cerevisiae retroviruslike element Ty3 encodes the major structural proteins capsid (CA) and nucleocapsid in the GAG3 open reading frame. The Ty3 CA protein contains a sequence (QGX2EX5FX3LX3H, where H is a hydrophobic residue) which has not been observed in other retrotransposons but which is similar to the major homology region (MHR) described for retrovirus CA. In this study the effects of mutations in the Ty3 MHR on particle formation, processing, DNA synthesis, and transposition were examined. Each of the mutations tested resulted in severe defects in transposition, with disruption occurring prior to or at particle formation, subsequent to particle formation and prior to completion of DNA synthesis, and subsequent to DNA synthesis. Changing the Q in the motif to R had relatively little effect on particle formation but decreased transposition to about 13% of that of a wild-type element. Changing G to A or V almost completely eliminated the formation of intracellular particles, possibly by disruption of CA-CA interactions. Changes introduced at the position of E resulted in blocked processing, blocked DNA synthesis, or a block at some post-reverse transcription step, depending on the nature of the mutation introduced. These results showed that the integrity of the Ty3 MHR is required for multiple aspects of Ty3 replication involving CA. These functions are independent of extracellular budding and of infection, aspects of the retroviral life cycle which are not recapitulated in replication of the Ty3 retrotransposon.

A large region within the Rous sarcoma virus matrix protein is dispensable for budding and infectivity

All retroviruses have a layer of matrix protein (MA) situated directly beneath the lipid of their envelope. This protein is initially expressed as the amino-terminal sequence of the Gag polyprotein, where it plays an important role in binding Gag to the plasma membrane during the early steps of the budding process. Others have suggested that MA may provide additional functions during virion assembly, including the selective incorporation of viral glycoproteins and the RNA genome into the emerging virion. To further study the role of the Rous sarcoma virus MA sequence in the viral replication cycle, we have pursued an extensive deletion analysis. Surprisingly, the entire second half of MA (residues 87 to 155) and part of the neighboring p2 sequence were found to be dispensable not only for budding but also for infectivity in avian cells. Thus, all of the functions associated with the Rous sarcoma virus MA sequence must be contained within its first half.

The membrane-binding domain of the Rous sarcoma virus Gag protein

The Gag protein of Rous sarcoma virus (RSV) can direct particle assembly and budding at the plasma membrane independently of the other virus-encoded products. A previous deletion analysis has suggested that the first 86 amino acids of RSV Gag constitute a large membrane-binding domain that is absolutely required for these processes. To test this hypothesis, we inserted these residues in place of the N-terminal membrane-binding domain of the pp60v-src, a transforming protein whose biological activity requires plasma membrane localization. The ability of the Src chimera to induce cellular transformation suggests that the RSV sequence indeed contains an independent, functional domain.

Regulation of HIV-1 gag protein subcellular targeting by protein kinase C

The human immunodeficiency virus type 1 internal structural protein precursor, p55, and its corresponding matrix proteolytic fragment, p17, are phosphorylated at Ser111 by protein kinase C. COS-7 cells transfected with plasmids encoding either the wild-type or Ser111-->Ala mutated human immunodeficiency virus type 1 gag gene matrix domain proteins were treated with phorbol 12-myristate 13-acetate (PMA), and the phosphorylation of the expressed p17 proteins was examined by radioimmunoprecipitation, SDS-polyacrylamide gel electrophoresis, and autoradiography. PMA treatment of transfected cells resulted in a 4-5-fold increase in wild-type p17 (but not mutated p17) phosphorylation; however, mutated p17 exhibited a low basal level of phosphorylation that was not affected by PMA, suggesting that additional sites were phosphorylated. PMA treatment of cells expressing wild-type p17 produced a dramatic shift in the localization of p17 from the cytosol to the membrane fraction within 8-15 min, followed by a slow quantitative dissociation of p17 back into the cytosol by 90 min. The cytosol-to-membrane translocation was dependent on N-myristoylated p17 since cells expressing p17 with a Gly2-->Ala mutation did not localize to the membrane. PMA also failed to induce the translocation of fully N-myristoylated Ser111-->Ala p17, suggesting that p17 phosphorylation at Ser111 was responsible for membrane association. This conclusion was confirmed by the finding of phosphorylated wild-type p17 in the membrane fraction only after PMA treatment. These results suggest that a "myristoyl-protein switch" regulates the reversible membrane targeting of p17 by protein kinase C-mediated phosphorylation. This signal may provide a mechanism for the cellular regulation of virus development through modulation of gag protein-related developmental steps such as capsid targeting, assembly, encapsidation, budding, and maturation.

Human immunodeficiency virus type 1 MA deletion mutants expressed in baculovirus-infected cells: cis and trans effects on the Gag precursor assembly pathway

The role of the matrix protein (MA) of human immunodeficiency virus type 1 in intracellular transport, assembly, and extracellular release of Gag polyprotein precursor (Pr55gag) was investigated by deletion mutagenesis of the MA domain of recombinant Gag precursor expressed in baculovirus-infected cells. In addition, three carboxy-terminally truncated forms of the Gag precursor, representing mainly the MA, were constructed. One corresponded to an MA with a deletion of its last 12 residues (amb120), while the others corresponded to the entire MA with an additional sequence from the N-terminal portion of the CA (amb143 and och180). Deletions within the MA central region (residues 41 to 78) appeared to be detrimental to Gag particle assembly and budding from the plasma membrane. A slightly narrower domain, between amino acids 41 and 68, was found to be critical for soluble Gag secretion. Mutations which totally or partially deleted one or the other of the two polybasic signals altered the transport of N-myristylated Gag precursor to the plasma membrane. In coexpression with wild-type Gag precursor, a discrete trans-dominant negative effect on wild-type Gag particle assembly and release was observed with deletion mutants located in the central MA region (residues 41 to 78). A more significant negative effect was obtained with the two recombinant proteins of amb120 and och180, which redirected the Gag particle assembly pathway from the plasma membrane compartment to intracellular vesicles (amb120) and to the nuclear compartment (och180).

Amino acid sequence analysis of the proteolytic cleavage products of the bovine immunodeficiency virus Gag precursor polypeptide

Bovine immunodeficiency virus Gag proteins were purified from virions, and their amino acid sequences and molecular masses were determined. The matrix, capsid, and nucleocapsid (MA, CA, and NC, respectively) and three smaller proteins (p2L, p3, and p2) were found to have molecular masses of 14.6, 24.6, and 7.3 and 2.5, 2.7, and 1.9 kDa, respectively. The order of these six proteins in the Gag precursor, Pr53gag, is NH2-MA-p2L-CA-p3-NC-p2-COOH. In contrast to other retroviral MA proteins, the bovine immunodeficiency virus MA retains its N-terminal methionine and is not modified by fatty acids. In addition, the bovine immunodeficiency virus NC migrates as a 13-kDa protein in denaturing gel electrophoresis; however, its molecular mass was determined to be 7.3 kDa.

Efficient particle formation can occur if the matrix domain of human immunodeficiency virus type 1 Gag is substituted by a myristylation signal

Lentiviruses, such as human immunodeficiency virus type 1 (HIV-1), assemble at and bud through the cytoplasmic membrane. Both the matrix (MA) domain of Gag and its amino-terminal myristylation have been implicated in these processes. We have created HIV-1 proviruses lacking the entire matrix domain of gag which either lack or contain an amino-terminal myristate addition sequence at the beginning of the capsid domain. Myristate- and matrix-deficient [myr(-)MA(-)] viruses produced after transient transfection are still able to assemble into particles, although the majority do not form at the plasma membrane or bud efficiently. Myristylation of the amino terminus of the truncated Gag precursor permits a much more efficient release of the mutant virions. While myr(-)MA(-) particles were inefficient in proteolytic processing of the Gag precursor, myristylation enabled efficient proteolysis of the mutant Gag. All matrix-deficient viruses are noninfectious. Particles produced by matrix-deficient mutants contain low levels of glycoproteins, indicating the importance of matrix in either incorporation or stable retention of Env. Since matrix-deficient viruses contain a normal complement of viral genomic RNA, a role for MA in genomic incorporation can be excluded. Contrary to previous reports, the HIV-1 genome does not require sequences between the 5' splice donor site and the gag start codon for efficient packaging.

Viral gene products and replication of the human immunodeficiency type 1 virus

The acquired immunodeficiency syndrome (AIDS) epidemic represents a modern-day plague that has not only resulted in a tragic loss of people from a wide spectrum of society but has reshaped our viewpoints regarding health care, the treatment of infectious diseases, and social issues regarding sexual behavior. There is little doubt now that the cause of the disease AIDS is a virus known as the human immunodeficiency virus (HIV). The HIV virus is a member of a large family of viruses termed retroviruses, which have as a hallmark the capacity to convert their RNA genome into a DNA form that then undergoes a process of integration into the host cell chromosome, followed by the expression of the viral genome and translation of viral proteins in the infected cell. This review describes the organization of the HIV-1 viral genome, the expression of viral proteins, as well as the functions of the accessory viral proteins in HIV replication. The replication of the viral genome is divided into two phases, the early phase and the late phase. The early phase consists of the interaction of the virus with the cell surface receptor (CD4 molecule in most cases), the uncoating and conversion of the viral RNA genome into a DNA form, and the integration into the host cell chromosome. The late phase consists of the expression of the viral proteins from the integrated viral genome, the translation of viral proteins, and the assembly and release of the virus. Points in the HIV-1 life cycle that are targets for therapeutic intervention are also discussed.

Role of the matrix protein in the virion association of the human immunodeficiency virus type 1 envelope glycoprotein

The matrix (MA) protein of human immunodeficiency virus type 1 (HIV-1) forms an inner coat directly underneath the lipid envelope of the virion. The outer surface of the lipid envelope surrounding the capsid is coated by the viral Env glycoproteins. We report here that the HIV-1 capsid-Env glycoprotein association is very sensitive to minor alterations in the MA protein. The results indicate that most of the MA domain of the Gag precursor, except for its carboxy terminus, is essential for this association. Viral particles produced by proviruses with small missense or deletion mutations in the region coding for the amino-terminal 100 amino acids of the MA protein lacked both the surface glycoprotein gp120 and the transmembrane glycoprotein gp41, indicating a defect at the level of Env glycoprotein incorporation. Alterations at the carboxy terminus of the MA domain had no significant effect on the levels of particle-associated Env glycoprotein or on virus replication. The presence of HIV-1 MA protein sequences was sufficient for the stable association of HIV-1 Env glycoprotein with hybrid particles that contain the capsid (CA) and nucleocapsid (NC) proteins of visna virus. The association of HIV-1 Env glycoprotein with the hybrid particles was dependent upon the presence of the HIV-1 MA protein domain, as HIV-1 Env glycoprotein was not efficiently recruited into virus particles when coexpressed with authentic visna virus Gag proteins.

Conditional infectivity of a human immunodeficiency virus matrix domain deletion mutant

We constructed a human immunodeficiency virus (HIV) matrix (MA) deletion mutant by deletion of about 80% of the HIV type 1 Gag MA domain but retaining myristylation and proteolytic processing signals. The effects of this deletion matrix (dl.MA) mutant on HIV particle assembly, processing, and infectivity were analyzed. Surprisingly, the dl.MA mutant still could assemble and process virus particles, had a wild-type (wt) retrovirus particle density, and possessed wt reverse transcriptase activity. RNase protection experiments showed that dl.MA mutant particles preferentially packaged viral genomic RNA. When both mutant and wt particles were pseudotyped with an amphotropic murine leukemia virus envelope protein, mutant infectivity was about 10% of wt level. In contrast, infectivity of the dl.MA mutant was 1,000-fold less than that of wild-type when mutant and wt particles were pseudotyped with the HIV envelope protein. Protein analyses of pseudotyped virions indicated that there were no major differences between mutant and wt viruses in the efficiency of amphotropic murine leukemia virus envelope protein incorporation. In contrast, there was a reduction in the amount of mutant particle-associated HIV envelope protein gp120. Our results suggest that an intact HIV matrix domain is not absolutely required for reverse transcription, nuclear localization, or integration but is necessary for appropriate HIV envelope protein function.

Incorporation of human immunodeficiency virus type 1 Gag proteins into murine leukemia virus virions

The retroviral Gag polyprotein is necessary and sufficient for assembly and budding of viral particles. However, the exact inter- and intramolecular interactions of the Gag polyproteins during this process are not known. To locate functional domains within Gag, we generated chimeric proviruses between human immunodeficiency virus type 1 (HIV-1) and murine leukemia virus (MuLV). In these chimeric proviruses, the matrix or capsid proteins of MuLV were precisely replaced with the matrix or capsid proteins of HIV-1. Although the chimeric proviruses were unable to efficiently assemble into mature viral particles by themselves, coexpression of wild-type MuLV Gag rescued the HIV proteins into virions. The specificity of the rescue of HIV proteins into MuLV virions shows that specific interactions involving homologous matrix or capsid regions of Gag are necessary for retroviral particle formation.

Mutations in the N-terminal region of human immunodeficiency virus type 1 matrix protein block intracellular transport of the Gag precursor

The matrix domain of human immunodeficiency virus type 1 Gag polyprotein was studied for its role in virus assembly. Deletion and substitution mutations caused a dramatic reduction in virus production. Mutant Gag polyproteins were myristoylated and had a high affinity for membrane association. Immunofluorescence staining revealed a large accumulation of mutant Gag precursors in the cytoplasm, while wild-type Gag proteins were primarily associated with the cell surface membrane. These results suggest a defect in intracellular transport of the mutant Gag precursors. Thus, in addition to myristoylation, the N-terminal region of the matrix domain is involved in determining Gag protein transport to the plasma membrane. Wild-type Gag polyproteins interacted with and efficiently packaged mutant Gag into virions. This finding is consistent with the hypothesis that intermolecular interaction of Gag polyproteins might occur in the cytoplasm prior to being transported to the assembly site on the plasma membrane.

Assembly and composition of intracellular particles formed by Moloney murine leukemia virus

Assembly of type C retroviruses such as Moloney murine leukemia virus (M-MuLV) ordinarily occurs at the plasma membranes of infected cells and absolutely requires the particle core precursor protein, Pr65gag. Previously we have shown that Pr65gag is membrane associated and that at least a portion of intracellular Pr65gag protein appears to be routed to the plasma membrane by a vesicular transport pathway. Here we show that intracellular particle formation can occur in M-MuLV-infected cells. M-MuLV immature particles were observed by electron microscopy budding into and within rough endoplasmic reticulum, Golgi, and vacuolar compartments. Biochemical fractionation studies indicated that intracellular Pr65gag was present in nonionic detergent-resistant complexes of greater than 150S. Additionally, viral RNA and polymerase functions appeared to be associated with intracellular particles, as were Gag-beta-galactosidase fusion proteins which have the capacity to be incorporated into virions. Immature intracellular particles in postnuclear lysates could be proteolytically processed in vitro to mature forms, while extracellular immature M-MuLV particles remained immature as long as 10 h during incubations. The occurrence of M-MuLV-derived intracellular particles demonstrates that Pr65gag can associate with intracellular membranes and indicates that if a plasma membrane Pr65gag receptor exists, it also can be found in other membrane compartments. These results support the hypothesis that intracellular particles may serve as a virus reservoir during in vivo infections.

A large deletion in the matrix domain of the human immunodeficiency virus gag gene redirects virus particle assembly from the plasma membrane to the endoplasmic reticulum

Morphogenesis of retroviruses involves assembly of the structural Gag and Gag-Pol polyproteins with subsequent budding of the virus particle from the plasma membrane and proteolytic cleavage by the viral proteinase. The matrix (MA) domain, representing the N-terminal segment of Gag, plays a critical role in this process. We constructed an in-frame deletion in the MA coding region (lacking codons 16 to 99) of the human immunodeficiency virus (HIV) type 1 gag gene. Following transient transfection of the complete proviral DNA carrying the deletion, the mutant polyprotein was synthesized and proteolytically processed like the wild-type polyprotein. However, release of virus particles was reduced approximately 10-fold. The extracellular particles that were released did not contain viral glycoproteins and were noninfectious. Electron micrographs revealed budding of virus particles into the endoplasmic reticulum (ER) of transfected cells and large numbers of particles within the ER. These particles were all immature and morphologically indistinguishable from intracisternal A-type particles, a class of murine endogenous retrovirus elements. Budding structures at the plasma membrane were rarely seen and only a few extracellular particles were observed, but in contrast to those in the ER, these particles had the morphology of mature particles, similar to that of wild-type HIV, except for the lack of surface projections.

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.

Bacterial expression, purification, and in vitro N-myristoylation of HIV-1 p17gag

The coding region of the N-terminal 17-kDa portion of HIV-1 Pr55gag (p17gag) was cloned into the pET-3c expression vector and was used to overexpress HIV-1 p17gag in Escherichia coli. Induction of the transformed bacteria caused the accumulation of a 17-kDa polypeptide in the soluble cell fraction which was released by sonication in hypotonic nondetergent buffer. The 17-kDa polypeptide was purified by ammonium sulfate precipitation and successive chromatography on G-75 Sephadex, DEAE-Sephacel, and S-Sephadex. The final product was purified 12-fold with about a 16% recovery from the original soluble cell lysate and was judged to be 97+% pure by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Western blotting with two different antibodies confirmed the identify of the purified 17-kDa polypeptide as authentic p17gag. In the presence of myristoyl-CoA and bovine brain N-myristoyl-transferase, p17gag was quantitatively N-myristoylated in vitro with a pseudo-first-order rate constant of 4.7 +/- 1.0 x 10(-3) min-1, but with only about 3% of the catalytic efficiency of N-myristoylation of a 16-residue peptide homologous to the N-terminus of p17gag. The myristate group in the N-myristoylated p17gag was stable to treatment with detergent and hydroxylamine consistent with a covalent N-acyl-amide linkage. The N-myristoylglycyl linkage was confirmed by partial acid hydrolysis and identification of the p-nitrobenzylazlactone derivative of the resulting N-myristoylglycine by high-performance liquid chromatography.

The C terminus of human immunodeficiency virus type 1 matrix protein is involved in early steps of the virus life cycle

Deletion mutations at the C terminus of the matrix (MA) protein of human immunodeficiency virus type 1 (HIV-1) were generated by site-directed mutagenesis. The resultant mutant viruses had a severe defect in virus infectivity. This defect did not involve late steps of the virus life cycle, as the synthesis and processing of the Gag polyprotein and the assembly and release of mutant virions were not greatly affected. The incorporation of viral proteins and the viral RNA genome was similar for mutant and wild-type virions. In contrast, the early steps of the virus life cycle were severely affected, as the synthesis of viral DNA postinfection was dramatically reduced in mutant-virus-infected cells. One stretch of amino acids that was deleted in one of the mutants has significant homology with a region in VP1 of the picornavirus family. This region of VP1 is presumably involved in poliovirus penetration into cells. These results suggest that in addition to its functional role in virus assembly, the MA protein of HIV-1, and possibly of other retroviruses, plays an important role in virus entry.

The matrix protein of human immunodeficiency virus type 1 is required for incorporation of viral envelope protein into mature virions

Accumulating evidence suggests that the matrix (MA) protein of retroviruses plays a key role in virus assembly by directing the intracellular transport and membrane association of the Gag polyprotein. In this report, we show that the MA protein of human immunodeficiency virus type 1 is also critical for the incorporation of viral Env proteins into mature virions. Several deletions introduced in the MA domain (p17) of human immunodeficiency virus type 1 Gag polyprotein did not greatly affect the synthesis and processing of the Gag polyprotein or the formation of virions. Analysis of the viral proteins revealed normal levels of Gag and Pol proteins in these mutant virions, but the Env proteins, gp120 and gp41, were hardly detectable in the mutant virions. Our data suggest that an interaction between the viral Env protein and the MA domain of the Gag polyprotein is required for the selective incorporation of Env proteins during virus assembly. Such an interaction appears to be very sensitive to conformational changes in the MA domain, as five small deletions in two separate regions of p17 equally inhibited viral Env protein incorporation. Mutant viruses were not infectious in T cells. When mutant and wild-type DNAs were cotransfected into T cells, the replication of wild-type virus was also hindered. These results suggest that the incorporation of viral Env protein is a critical step for replication of retroviruses and can be a target for the design of antiviral strategies.

Matrix protein of Akv murine leukemia virus: genetic mapping of regions essential for particle formation

Type C retroviruses assemble at the plasma membrane of the infected cell. Attachment of myristic acid to the N terminus of the Gag precursor polyprotein has been shown to be essential for membrane localization and virus morphogenesis. Here, we report that the matrix (MA) protein contains regions that in conjunction with myristylation are important for Gag protein stability and the assembly of murine leukemia viruses. We identified these domains by generating a series of Akv murine leukemia virus mutants carrying small in-frame deletions within the coding region of the MA protein encompassing 129 amino acids. Studies show that mutants with deletions within the segment encoding the first 102 amino acids were all replication defective, whereas the C-terminal residues 103 to 124 seem not to have any critical function in virus maturation. Cells expressing the replication-defective genomes did not release any detectable Gag proteins. In one mutant, deletion of 3 amino acids in the N terminus resulted in an inefficiently myristylated, stable Gag polyprotein. The remaining defect genomes encoded unstable Gag proteins, although they were modified with myristic acid. The results suggest that the matrix domain plays an important role in stabilizing the Gag polyprotein.