Comparative morphology and structural classification of retroviruses

Oncolytic Viro-Immunotherapy: An Emerging Option in the Treatment of Gliomas

The prognosis of malignant gliomas remains poor, with median survival fewer than 20 months and a 5-year survival rate merely 5%. Their primary location in the central nervous system (CNS) and its immunosuppressive environment with little T cell infiltration has rendered cancer therapies mostly ineffective, and breakthrough therapies such as immune checkpoint inhibitors (ICIs) have shown limited benefit. However, tumor immunotherapy is developing rapidly and can help overcome these obstacles. But for now, malignant gliomas remain fatal with short survival and limited therapeutic options. Oncolytic virotherapy (OVT) is a unique antitumor immunotherapy wherein viruses selectively or preferentially kill tumor cells, replicate and spread through tumors while inducing antitumor immune responses. OVTs can also recondition the tumor microenvironment and improve the efficacy of other immunotherapies by escalating the infiltration of immune cells into tumors. Some OVTs can penetrate the blood-brain barrier (BBB) and possess tropism for the CNS, enabling intravenous delivery. Despite the therapeutic potential displayed by oncolytic viruses (OVs), optimizing OVT has proved challenging in clinical development, and marketing approvals for OVTs have been rare. In June 2021 however, as a genetically engineered OV based on herpes simplex virus-1 (G47Δ), teserpaturev got conditional and time-limited approval for the treatment of malignant gliomas in Japan. In this review, we summarize the current state of OVT, the synergistic effect of OVT in combination with other immunotherapies as well as the hurdles to successful clinical use. We also provide some suggestions to overcome the challenges in treating of gliomas.

Morphologic differentiation of viruses beyond the family level

Electron microscopy has been instrumental in the identification of viruses by being able to characterize a virus to the family level. There are a few cases where morphologic or morphogenesis factors can be used to differentiate further, to the genus level. These include viruses in the families Poxviridae, Reoviridae, Retroviridae, Herpesviridae, Filoviridae, and Bunyaviridae.

CMV-promoter driven codon-optimized expression alters the assembly type and morphology of a reconstituted HERV-K(HML-2)

The HERV-K(HML-2) family contains the most recently integrated and best preserved endogenized proviral sequences in the human genome. All known elements have nevertheless been subjected to mutations or deletions that render expressed particles non-infectious. Moreover, these post-insertional mutations hamper the analysis of the general biological properties of this ancient virus family. The expression of consensus sequences and sequences of elements with reverted post-insertional mutations has therefore been very instrumental in overcoming this limitation. We investigated the particle morphology of a recently reconstituted HERV-K113 element termed oriHERV-K113 using thin-section electron microscopy (EM) and could demonstrate that strong overexpression by substitution of the 5'LTR for a CMV promoter and partial codon optimization altered the virus assembly type and morphology. This included a conversion from the regular C-type to an A-type morphology with a mass of cytoplasmic immature cores tethered to the cell membrane and the membranes of vesicles. Overexpression permitted the release and maturation of virions but reduced the envelope content. A weaker boost of virus expression by Staufen-1 was not sufficient to induce these morphological alterations.

The GLN family of murine endogenous retroviruses contains an element competent for infectious viral particle formation

Several families of endogenous retroviruses (ERVs) have been identified in the mouse genome, in several instances by in silico searches, but for many of them it remains to be determined whether there are elements that can still encode functional retroviral particles. Here, we identify, within the GLN family of highly reiterated ERVs, one, and only one, copy that encodes retroviral particles prone to infection of mouse cells. We show that its envelope protein confers an ecotropic host range and recognizes a receptor different from mCAT1 and mSMIT1, the two previously identified receptors for other ecotropic mouse retroviruses. Electron microscopy disclosed viral particle assembly and budding at the cell membrane, as well as release of mature particles into the extracellular space. These particles are closely related to murine leukemia virus (MLV) particles, with which they have most probably been confused in the past. This study, therefore, identifies a new class of infectious mouse ERVs belonging to the family Gammaretroviridae, with one family member still functional today. This family is in addition to the two MLV and mouse mammary tumor virus families of active mouse ERVs with an extracellular life cycle.

An infectious progenitor for the murine IAP retrotransposon: emergence of an intracellular genetic parasite from an ancient retrovirus

Mammalian genomes contain a high load of mobile elements among which long terminal repeat (LTR)- retrotransposons may represent up to 10% of the genomic DNA. The murine intracisternal A-type particle (IAP) sequences, the prototype of these mammalian "genetic parasites," have an intracellular replicative life cycle and are responsible for a very large fraction of insertional mutagenesis in mice. Yet, phylogenetic analyses strongly suggest that they derive from an ancestral retrovirus that has reached the germline of a remote rodent ancestor and has been "endogenized." A genome-wide screening of the mouse genome now has led us to identify the likely progenitor of the intracellular IAP retrotransposons. This identified "living fossil"-that we found to be present only as a single fully active copy-discloses all the characteristics of a bona fide retrovirus, with evidence for particle formation at the cell membrane, and release of virions with a mature morphology that are infectious. We show, by generating appropriate chimeras, that IAPs derive from this element via passive loss of its env gene, and gain of an endoplasmic reticulum targeting signal, resulting in its "intracellularization" and in the gain of transpositional activity. The identification within the mouse genome of the still active retroviral progenitor of the IAP endogenous mobile elements and the experimental dissection of the molecular events responsible for the shift in its life cycle provide a conclusive illustration of the process that has led, during evolution, to the generation of very successful intracellular retrotransposons from ancient retroviruses.

Murine endogenous retrovirus MuERV-L is the progenitor of the "orphan" epsilon viruslike particles of the early mouse embryo

Viruslike particles which displayed a peculiar wheellike appearance that distinguished them from A-, B- or C-type particles had previously been described in the early mouse embryo. The maximum expression of these so-called epsilon particles was observed in two-cell-stage embryos, followed by their rapid decline at later stages of development and no particles detected at the zygote one-cell stage. Here, we show that these particles are in fact produced by a newly discovered murine endogenous retrovirus (ERV) belonging to the widespread family of mammalian ERV-L elements and named MuERV-L. Using antibodies that we raised against the Gag protein of these elements, Western blot analysis and in toto immunofluorescence studies of the embryos at various stages disclosed the same developmental expression profile as that observed for epsilon particles. Using expression vectors for cloned, full-length, entirely coding MuERV-L copies and cell transfection, direct identification of the epsilon particles was finally achieved by high-resolution electron microscopy.

Host ABCE1 is at plasma membrane HIV assembly sites and its dissociation from Gag is linked to subsequent events of virus production

In primate cells, assembly of a single HIV-1 capsid involves multimerization of thousands of Gag polypeptides, typically at the plasma membrane. Although studies support a model in which HIV-1 assembly proceeds through complexes containing Gag and the cellular adenosine triphosphatase ABCE1 (also termed HP68 or ribonuclease L inhibitor), whether these complexes constitute true assembly intermediates remains controversial. Here we demonstrate by pulse labeling in primate cells that a population of Gag associates with endogenous ABCE1 within minutes of translation. In the next ∼2 h, Gag–ABCE1 complexes increase in size to approximately that of immature capsids. Dissociation of ABCE1 from Gag correlates closely with Gag processing during virion maturation and occurs much less efficiently when the HIV-1 protease is inactivated. Finally, quantitative double-label immunogold electron microscopy reveals that ABCE1 is recruited to sites of assembling wild-type Gag at the plasma membrane but not to sites of an assembly-defective Gag mutant at the plasma membrane. Together these findings demonstrate that a population of Gag present at plasma membrane sites of assembly associates with ABCE1 throughout capsid formation until the onset of virus maturation, which is then followed by virus release. Moreover, the data suggest a linkage between Gag–ABCE1 dissociation and subsequent events of virion production.

Consideration of the three-dimensional structure of core shells (capsids) in spherical retroviruses

The problem of three-dimensional organization of retroviral cores has been a matter of interest for the past 30 years. The general opinion in favor of icosahedral symmetry based on electron microscopy observations was questioned when cryo-electron microscopy failed to provide convincing evidence in its favor. More recent studies by cryo-electron microscopy, X-ray crystallography and in vitro assembly of the CA domain of Human immuno deficiency virus (HIV), Murine leukemia virus (MuLV) and Rous sarcoma virus (RSV) threw new light on the organization of retroviral cores. In this communication we report how we produced a three-dimensional (3D) model of MuLV core using data from CA assembly on a lipid film [Ganser, B.K., Cheng, A., Sundquist, W.I., Yeager, M., 2003. Three-dimensional structure of the M-MuLV CA protein on a lipid monolayer: a general model for retroviral capsid assembly. EMBO J. 22, 2886-2892]. The resulting structure revealed that the molecular organization of the core shell is specific and the presence of a 5,3,2 rotational symmetry of the 3D model provides support for icosahedral shape of MuLV cores. The model made it possible to determine the diameter of the cores and calculate the number of CA copies as well as the molecular mass of a core of specific diameter. Thus MuLV cores 68 (or 81.6) nm in diameter consist of 1500 (or 2160) copies of CA. About 12% of molecules from fullerene-like Gag shells versus 71% of molecules of closely packed (core-like). Gag shells were not incorporated into the core shells (capsids). Our 3D models received support from X-ray data of MuLV CA NTD domain published by Mortuza et al. [Mortuza, G., Haire, L.F., Stevens, A., Smerdon, S.J., Stoye, J.P., Taylor, I.A., 2004. High resolution structure of a retroviral capsid hexameric amino-terminal domain. Nature 431, 481-485].

Formation of disulfide-linked complexes between the three minor envelope glycoproteins (GP2b, GP3, and GP4) of equine arteritis virus

Equine arteritis virus (EAV) is an enveloped, positive-stranded RNA virus belonging to the family Arteriviridae of the order NIDOVIRALES: Six transmembrane proteins have been identified in EAV particles: the nonglycosylated membrane protein M and the glycoprotein GP(5) (previously named G(L)), which occur as disulfide-bonded heterodimers and are the major viral envelope proteins; the unglycosylated small envelope protein E; and the minor glycoproteins GP(2b) (formerly designated G(S)), GP(3), and GP(4). Analysis of the appearance of the GP(2b), GP(3), and GP(4) proteins in viral particles by gel electrophoresis under reducing and nonreducing conditions revealed the occurrence of two different covalently linked oligomeric complexes between these proteins, i.e., heterodimers of GP(2b) and GP(4) and heterotrimers of GP(2b), GP(3), and GP(4). Shortly after their release from infected cells, virions contained mainly cystine-linked GP(2b)/GP(4) heterodimers, which were subsequently converted into disulfide-bonded GP(2b)/GP(3)/GP(4) trimers through the covalent recruitment of GP(3). This process occurred faster at a higher pH but was arrested at 4 degrees C. Furthermore, the conversion was almost instantaneous in the presence of the thiol oxidant diamide. In contrast, the sulfhydryl-modifying agent N-ethylmaleimide inhibited the formation of disulfide-bonded GP(2b)/GP(3)/GP(4) trimers. Using sucrose density gradients, we could not demonstrate a noncovalent association of GP(3) with the cystine-linked GP(2b)/GP(4) dimer in freshly released virions, nor did we observe higher-order structures of the GP(2b)/GP(4) or GP(2b)/GP(3)/GP(4) complexes. Nevertheless, the instantaneous diamide-induced formation of disulfide-bonded GP(2b)/GP(3)/GP(4) heterotrimers at 4 degrees C suggests that the three minor glycoproteins of EAV are assembled as trimeric complexes. The existence of a noncovalent interaction between the cystine-linked GP(2b)/GP(4) dimer and GP(3) was also inferred from coexpression experiments showing that the presence of GP(3) increased the electrophoretic mobility of the disulfide-bonded GP(2b)/GP(4) dimers. Our study reveals that the minor envelope proteins of arteriviruses enter into both covalent and noncovalent interactions, the function of which has yet to be established.

Murine leukemia virus nucleocapsid mutant particles lacking viral RNA encapsidate ribosomes

A single retroviral protein, termed Gag, is sufficient for assembly of retrovirus-like particles in mammalian cells. Gag normally selects the genomic RNA of the virus with high specificity; the nucleocapsid (NC) domain of Gag plays a crucial role in this selection process. However, encapsidation of the viral RNA is completely unnecessary for particle assembly. We previously showed that mutant murine leukemia virus (MuLV) particles that lack viral RNA because of a deletion in the cis-acting packaging signal ("Psi") in the genomic RNA compensate for the loss of the viral RNA by incorporating cellular mRNA. The RNA in wild-type and Psi- particles was also found to be necessary for virion core structure. In the present work, we explored the role of RNA in MuLV particles that lack genomic RNA because of mutations in the NC domain of Gag. Using a fluorescent dye assay, we observed that NC mutant particles contain the same amount of RNA that wild-type virions do. Surprisingly enough, these particles contained large amounts of rRNAs. Furthermore, ribosomal proteins were detected by immunoblotting, and ribosomes were observed inside the particles by electron microscopy. The biological significance of the presence of ribosomes in NC mutant particles lacking genomic RNA is discussed.

Kinetic analysis of the role of intersubunit interactions in human immunodeficiency virus type 1 capsid protein assembly in vitro

The human immunodeficiency virus type 1 (HIV-1) capsid protein (CA) plays a crucial role in both assembly and maturation of the virion. Numerous recent studies have focused on either the soluble form of CA or the polymer end product of in vitro CA assembly. The CA polymer, in particular, has been used to study CA-CA interactions because it is a good model for the CA interactions within the virion core. However, analysis of the process of in vitro CA assembly can yield valuable insights into CA-CA interactions and the mechanism of core assembly. We describe here a method for the analysis of CA assembly kinetics wherein the progress of assembly is monitored by using turbidity. At pH 7.0 the addition of either of the isolated CA domains (i.e., the N or the C domain) to an assembly reaction caused a decrease in the assembly rate by competing for binding to the full-length CA protein. At pH 8.0 the addition of the isolated C domain had a similar inhibitory affect on CA assembly. However, at pH 8.0 the isolated N domain had no affect on the rate of CA assembly but, when mixed with the C domain, it alleviated the C-domain inhibition. These data provide biochemical evidence for a pH-sensitive homotypic N-domain interaction, as well as for an N- and C-domain interaction.

Molecular organization of Mason-Pfizer monkey virus capsids assembled from Gag polyprotein in Escherichia coli

We describe the results of a study by electron microscopy and image processing of Gag protein shells-immature capsids--of Mason-Pfizer monkey virus assembled in Escherichia coli from two truncated forms of the Gag precursor: Deltap4Gag, in which the C-terminal p4Gag was deleted, and Pro(-)CA.NC, in which the N-terminal peptides and proline 1 of the CA domain were deleted. Negative staining of capsids revealed small patches of holes forming a trigonal or hexagonal pattern most clearly visible on occasional tubular forms. The center-to-center spacing of holes in the network was 7.1 nm in Deltap4Gag capsids and 7.4 nm in Pro(-)CA.NC capsids. Image processing of Deltap4Gag tubes revealed a hexagonal network of holes formed by six subunits with a single subunit shared between rings. This organization suggests that the six subunits are contributed by three trimers of the truncated Gag precursor. Similar molecular organization was observed in negatively stained Pro(-)CA.NC capsids. Shadowed replicas of freeze-etched capsids produced by either construct confirmed the presence of a hexagonal network of holes with a similar center-to-center spacing. We conclude that the basic building block of the cage-like network is a trimer of the Deltap4Gag or Pro(-)CA.NC domains. In addition, our results point to a key role of structurally constrained CA domain in the trimeric interaction of the Gag polyprotein.

Analysis of Mason-Pfizer monkey virus Gag particles by scanning transmission electron microscopy

Mason-Pfizer monkey virus immature capsids selected from the cytoplasm of baculovirus-infected cells were imaged by scanning transmission electron microscopy. The masses of individual selected Gag particles were measured, and the average mass corresponded to 1,900 to 2,100 Gag polyproteins per particle. A large variation in Gag particle mass was observed within each population measured.

Specific interaction of a novel foamy virus Env leader protein with the N-terminal Gag domain

Cryoelectron micrographs of purified human foamy virus (HFV) and feline foamy virus (FFV) particles revealed distinct radial arrangements of Gag proteins. The capsids were surrounded by an internal Gag layer that in turn was surrounded by, and separated from, the viral membrane. The width of this layer was about 8 nm for HFV and 3.8 nm for FFV. This difference in width is assumed to reflect the different sizes of the HFV and FFV MA domains: the HFV MA domain is about 130 residues longer than that of FFV. The distances between the MA layer and the edge of the capsid were identical in different particle classes. In contrast, only particles with a distended envelope displayed an invariant, close spacing between the MA layer and the Env membrane which was absent in the majority of particles. This indicates a specific interaction between MA and Env at an unknown step of morphogenesis. This observation was supported by surface plasmon resonance studies. The purified N-terminal domain of FFV Gag specifically interacted with synthetic peptides and a defined protein domain derived from the N-terminal Env leader protein. The specificity of this interaction was demonstrated by using peptides varying in the conserved Trp residues that are known to be required for HFV budding. The interaction with Gag required residues within the novel virion-associated FFV Env leader protein of about 16.5 kDa.

Molecular modelling study of HIV p17gag (MA) protein shell utilising data from electron microscopy and X-ray crystallography

The matrix protein p17gag (MA) is a product of proteolytic cleavage of the gag gene encoded polyprotein (pr55gag) and is formed when HIV particles undergo the process of maturation. The MA protein is associated with the inner surface of the viral membrane and determines the overall shape of the virion. Previous studies have shown the existence of trimers of MA in solution and in the crystalline state. Here, we used molecular modelling methods to identify feasible interactions between pairs of MA trimers and have related this to structural data from electron microscopy. A systematic search docking procedure was able to identify many energetically favourable conformations for a pair of trimers, including some which have been previously reported. These conformations were used to generate several networks of MA trimers, which were then evaluated against structural observations of the MA network. The model suggested here provides a good match with experimental data such as the spacing between gag protein rings, the number and disposition of glycoprotein (gp41-gp120) knobs and the number of copies of MA in a virus particle. It also rationalizes the observed distribution of sizes of virus particles and is consistent with the presence of icosahedral organisation in mature HIV. Energy minimisation performed with explicit water and counter ions, was used to identify residues participating in inter-trimer interactions. The nature of these interactions is discussed in relation to the conservation of these residues in reported variants of the HIV and SIV MA protein sequences.

The intact retroviral Env glycoprotein of human foamy virus is a trimer

Electron microscopy of negatively stained human foamy virus particles provides direct evidence for the trimeric nature of intact Env surface glycoproteins. Three-dimensional image reconstruction reveals that the Env trimer is a tapering spike 14 nm in length. The spikes were often arranged in hexagonal rings which shared adjacent Env trimers.

Biochemical and structural analysis of isolated mature cores of human immunodeficiency virus type 1

Mature human immunodeficiency virus type 1 (HIV-1) particles contain a cone-shaped core structure consisting of the internal ribonucleoprotein complex encased in a proteinaceous shell derived from the viral capsid protein. Because of their very low stability after membrane removal, HIV-1 cores have not been purified in quantities sufficient for structural and biochemical analysis. Based on our in vitro assembly experiments, we have developed a novel method for isolation of intact mature HIV-1 cores. Concentrated virus suspensions were briefly treated with nonionic detergent and immediately centrifuged in a microcentrifuge for short periods of time. The resuspended pellet was subsequently analyzed by negative-stain and thin-section electron microscopy and by immunoelectron microscopy. Abundant cone-shaped cores as well as tubular and aberrant structures were observed. Stereo images showed that core structures preserved their three-dimensional architecture and exhibited a regular substructure. Detailed analysis of 155 cores revealed an average length of ca. 103 nm, an average diameter at the base of ca. 52 nm, and an average angle of 21.3 degrees. There was significant variability in all parameters, indicating that HIV cores are not homogeneous. Immunoblot analysis of core preparations allowed semiquantitative estimation of the relative amounts of viral and cellular proteins inside the HIV-1 core, yielding a model for the topology of various proteins inside the virion.

An intact TAR element and cytoplasmic localization are necessary for efficient packaging of human immunodeficiency virus type 1 genomic RNA

Although most reports defining the human immunodeficiency virus type 1 (HIV-1) genomic RNA packaging signal have focused on the region downstream of the major 5' splice site, others have suggested that sequences upstream of the splice site may also play an important role. In this study we have directly examined the role played by the HIV-1 TAR region in RNA packaging. For these experiments we used a proviral expression system that is largely independent of Tat for transcriptional activation. This allowed us to create constructs that efficiently expressed RNAs carrying mutations in TAR and to determine the ability of these RNAs to be packaged. Our results indicate that loss of sequences in TAR significantly reduce the ability of a viral RNA to be packaged. The requirement for TAR sequences in RNA packaging was further examined by using a series of missense mutations positioned throughout the entire TAR structure. TAR mutations previously shown to influence Tat transactivation, such as G31U in the upper loop region or UCU to AAG in the bulge (nucleotides [nt] 22 to 24), failed to have any effect on RNA packaging. Mutations which disrupted the portion of the TAR stem immediately below the bulge also had little effect. In contrast, dramatic effects on RNA packaging were observed with constructs containing mutations in the lower portion of the TAR stem. Point mutations which altered nt 5 to 9, 10 to 15, 44 to 49, or 50 to 54 all reduced RNA packaging 11- to 25-fold. However, compensatory double mutations which restored the stem structure were able to restore packaging. These results indicate that an intact lower stem structure, rather than a specific sequence, is required for RNA packaging. Our results also showed that RNA molecules retained within the nucleus cannot be packaged, unless they are transported to the cytoplasm by either Rev/Rev response element or the Mason-Pfizer monkey virus constitutive transport element.

Actin associates with the nucleocapsid domain of the human immunodeficiency virus Gag polyprotein

Recently, it was shown that actin molecules are present in human immunodeficiency virus type 1 (HIV-1) particles. We have examined the basis for incorporation and the location of actin molecules within HIV-1 and murine retrovirus particles. Our results show that the retroviral Gag polyprotein is sufficient for actin uptake. Immunolabeling studies demonstrate that actin molecules localize to a specific radial position within the immature particle, clearly displaced from the matrix domain underneath the viral membrane but in proximity to the nucleocapsid (NC) domain of the Gag polyprotein. When virus or subviral Gag particles were disrupted with nonionic detergent, actin molecules remained associated with the disrupted particles. Actin molecules remained in a stable complex with the NC cleavage product (or an NC-RNA complex) after treatment of the disrupted HIV-1 particles with recombinant HIV-1 protease. In contrast, matrix and capsid molecules were released. The same result was obtained when mature HIV-1 particles were disrupted with detergent. Taken together, these results indicate that actin molecules are associated with the NC domain of the viral polyprotein.

Molecular events in the assembly of retrovirus particles

Retrovirus assembly results from the ability of a single gene product, the gag polyprotein precursor, to coalesce into a spherical particle capable of release from the cell. In conjunction with this primary process of capsid formation additional viral gene products such as the replicative enzymes and envelope glycoproteins as well as the genomic RNA are incorporated to form an infectious virus.

Morphopoietic determinants of HIV-1 Gag particles assembled in baculovirus-infected cells

The determinants for HIV-1 particle morphology were investigated using various deletion and insertion mutants of the Gap precursor protein (Gag) expressed in baculovirus-infected cells and ultrastructural analysis of membrane-enveloped Gag particles under the electron microscope. Five discrete regions were found to influence the size, the variability in dimension, and the sphericity of the particles: (i) the matrix (MA) N-terminal domain, within residues 10-21, the junctions of (ii) MA-CA (capsid), (iii) CA-spacer peptide SP1 and (iv) nucleocapsid (NC)-SP2, and (v) the p6gag C-terminus. Internal regions (ii), (iii), and (iv) contained HIV-1 protease cleavage sites separating major structural domains. No particle assembly was observed for am276, a MA-CA polyprotein mutant lacking the C-terminal third of the CA domain. However, MA-CA domains including the MHR (residues 277-306), or downstream sequence to CA residue 357, resulted in the assembly into tubular or filamentous structures, suggesting a helical symmetry of Gag packing. Mutant amb374, derived from amb 357 by further addition of the heptadecapeptide motif HKARVLAEAMSQVTNSA, overlapping the CA-SP1 junction and the SP1 domain, showed a drastic change in the pattern of Gag assembly, compared to amb357, with formation of spherical particles. These data suggested a novel function for the spacer domain SP1, acting as a spherical shape determinant of the Gag particle which would negatively affect the helical symmetry of assembly of the Gag precursor molecules conferred by the MHR and the downstream CA sequence, within residues 307-357.

Supramolecular organization of immature and mature murine leukemia virus revealed by electron cryo-microscopy: implications for retroviral assembly mechanisms

We have used electron cryo-microscopy and image analysis to examine the native structure of immature, protease-deficient (PR-) and mature, wild-type (WT) Moloney murine leukemia virus (MuLV). Maturational cleavage of the Gag polyprotein by the viral protease is associated with striking morphological changes. The PR- MuLV particles exhibit a rounded central core, which has a characteristic track-like shell on its surface, whereas the WT MuLV cores display a polygonal surface with loss of the track-like feature. The pleomorphic shape and inability to refine unique orientation angles suggest that neither the PR- nor the WT MuLV adheres to strict icosahedral symmetry. Nevertheless, the PR- MuLV particles do exhibit paracrystalline order with a spacing between Gag molecules of approximately 45 A and a length of approximately 200 A. Because of the pleomorphic shape and paracrystalline packing of the Gag-RNA complexes, we raise the possibility that assembly of MuLV is driven by protein-RNA, as well as protein-protein, interactions. The maturation process involves a dramatic reorganization of the packing arrangements within the ribonucleoprotein core with disordering and loosening of the individual protein components.

Particle size determinants in the human immunodeficiency virus type 1 Gag protein

The retroviral Gag protein plays the central role in the assembly process and can form membrane-enclosed, virus-like particles in the absence of any other viral products. These particles are similar to authentic virions in density and size. Three small domains of the human immunodeficiency virus type 1 (HIV-1) Gag protein have been previously identified as being important for budding. Regions that lie outside these domains can be deleted without any effect on particle release or density. However, the regions of Gag that control the size of HIV-1 particles are less well understood. In the case of Rous sarcoma virus (RSV), the size determinant maps to the CA (capsid) and adjacent spacer sequences within Gag, but systematic mapping of the HIV Gag protein has not been reported. To locate the size determinants of HIV-1, we analyzed a large collection of Gag mutants. To our surprise, all mutants with defects in the MA (matrix), CA, and the N-terminal part of NC (nucleocapsid) sequences produced dense particles of normal size, suggesting that oncoviruses (RSV) and lentiviruses (HIV-1) have different size-controlling elements. The most important region found to be critical for determining HIV-1 particle size is the p6 sequence. Particles lacking all or small parts of p6 were uniform in size distribution but very large as measured by rate zonal gradients. Further evidence for this novel function of p6 was obtained by placing this sequence at the C terminus of RSV CA mutants that produce heterogeneously sized particles. We found that the RSV-p6 chimeras produced normally sized particles. Thus, we present evidence that the entire p6 sequence plays a role in determining the size of a retroviral particle.

The life-cycle of human immunodeficiency virus type 1

The life-cycle of human immunodeficiency virus type 1 (HIV-1) has been studied using several techniques including immunoelectron microscopy and cryomicroscopy. The HIV-1 particle consists of an envelope, a core and the region between the core and the envelope (matrix). Virus particles in the extracellular space are observed as having various profiles: a central or an eccentric round electron-dense core, a bar-shaped electron-dense core, and immature doughnut-shaped particle. HIV-1 particles in the hydrated state were observed by high-resolution electron cryomicroscopy to be spherical and the lipid membrane was clearly resolved as a bilayer. Projections around the circumference were seen to be knob-like. The shapes and sizes of the projections, especially the head parts, were found to vary with each projection. HIV-1 cores were isolated with a mixture of Nonidet P40 and glutaraldehyde, and were confirmed to consist of HIV-1 Gag p24 protein by immunogold labelling. On infection, the HIV-1 virus was found to enter the cell in two ways: membrane fusion and endocytosis. After viral entry, no structures resembling virus particles could be seen in the cytoplasm. In the infected cells, positive reactions by immunolabelling suggest that HIV-1 Gag is produced in membrane-bound structures and transported to the cell surface by the cytoskeletons. A crescent electron-dense layer is then formed underneath the cell membrane. Finally, the virus particle is released from the cell surface and found extracellularly to be a complete virus particle with an electron-dense core. However, several cell clones producing defective mature, doughnut-shaped (immature) or teardrop-shaped particles were found to be produced in the extracellular space. In the doughnut-shaped particles, Gag p17 and p24 proteins exist facing each other against an inner electron-dense ring, suggesting that the inner ring consists of a precursor Gag protein showing a defect at the viral proteinase.

In vitro assembly properties of purified bacterially expressed capsid proteins of human immunodeficiency virus

The Gag polyprotein of retroviruses is sufficient for assembly and budding of virus-like particles from the host cell. In the case of human immunodeficiency virus (HIV), Gag contains the domains matrix, capsid (CA), nucleocapsid (NC) and p6 which are separated by the viral proteinase inside the nascent virion, leading to morphological maturation to yield an infectious virus. In the mature virus, CA forms a capsid shell surrounding the ribonucleoprotein core consisting of NC and the genomic RNA. To define requirements for particle assembly and functional contributions of individual domains, we expressed domains of HIV Gag in Escherichia coli and purified the products to near homogeneity. In vitro assembly of CA, with or without the C-terminally adjacent spacer peptide, yielded tubular structures with a diameter of approximately 55 nm and heterogeneous length. Efficient particle formation required high protein concentration, high salt and neutral to alkaline pH. In contrast, in vitro assembly of CA-NC occurred at a 20-fold lower protein concentration and in low salt, but required addition of RNA. These results suggest that hydrophobic interactions of capsid proteins are sufficient for particle formation while the RNA-binding nucleocapsid domain may concentrate and align structural proteins on the viral genome.