The structure of the mouse mammary tumor virus: isolation and characterization of the core

Important role for the CA-NC spacer region in the assembly of bovine immunodeficiency virus Gag protein

Lentiviral Gag proteins contain a short spacer sequence that separates the capsid (CA) from the downstream nucleocapsid (NC) domain. This short spacer has been shown to play an important role in the assembly of human immunodeficiency virus type 1 (HIV-1). We have now extended this finding to the CA-NC spacer motif within the Gag protein of bovine immunodeficiency virus (BIV). Mutation of this latter spacer sequence led to dramatic reductions in virus production, which was mainly attributed to the severely disrupted association of the mutated Gag with the plasma membrane, as shown by the results of membrane flotation assays and confocal microscopy. Detailed mutagenesis analysis of the BIV CA-NC spacer region for virus assembly determinants led to the identification of two key residues, L368 and M372, which are separated by three amino acids, 369-VAA-371. Incidentally, the same two residues are present within the HIV-1 CA-NC spacer region at positions 364 and 368 and have also been shown to be crucial for HIV-1 assembly. Regardless of this conservation between these two viruses, the BIV CA-NC spacer could not be replaced by its HIV-1 counterpart without decreasing virus production, as opposed to its successful replacement by the CA-NC spacer sequences from the nonprimate lentiviruses such as feline immunodeficiency virus (FIV), equine infectious anemia virus and visna virus, with the sequence from FIV showing the highest effectiveness in this regard. Taken together, these data suggest a pivotal role for the CA-NC spacer region in the assembly of BIV Gag; however, the mechanism involved therein may differ from that for the HIV-1 CA-NC spacer.

The organization of mature Rous sarcoma virus as studied by cryoelectron microscopy

We have studied the organization of mature infectious Rous sarcoma virus (RSV), suspended in vitreous ice, using transmission electron microscopy. The enveloped virions are spherical in shape, have a mean diameter of 127 nm, and vary significantly in size. Image processing reveals the presence of the viral matrix protein underlying the lipid bilayer and the viral envelope proteins external to the lipid bilayer. In the interior of the virus, the characteristic mature retroviral core is clearly imaged. In contrast to lentiviruses, such as human immunodeficiency virus, the core of RSV is essentially isometric. The capsid, or external shell of the core, has a faceted, almost polygonal appearance in electron micrographs, but many capsids also exhibit continuous surface curvature. Cores are not uniform in size or shape. Serrations observed along the projected faces of the core suggest a repetitive molecular structure. Some isolated cores were observed in the sample, confirming that cores are at least transiently stable in the absence of the viral envelope. Using an approach grounded in geometric probability, we estimate the size of the viral core from the projection data. We show that the size of the core is not tightly controlled and that core size and virion size are positively correlated. From estimates of RNA packing density we conclude that either the RNA within the core is loosely packed or, more probably, that it does not fill the core.

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.

Biological and molecular studies on Syrian hamster intracisternal R-type particles

Retrovirus-like intracisternal R-type particles (IRP) are structures present in Syrian hamster (Mesocricetus auratus) cells cultured in vitro where they appear either spontaneously or under chemical induction conditions. We have tested several chemical inducers and ten different cell lines, looking for the best IRP induction conditions. BHK21 cl. 13 showed the highest inducibility one day after a 24 h treatment with 1 microgram/ml of 5-aza-2'-deoxycytidine. Using detergent treatments and sucrose gradients, we obtained semi-purified IRP cores. A 7.2 kb RNA associated with the core fraction was revealed by hybridization with total Syrian hamster genomic DNA, but not with Syrian hamster intracisternal A particle (IAP) specific probes. This suggests that the IRP genes are distinct from IAP ones.

Vpx of simian immunodeficiency virus is localized primarily outside the virus core in mature virions

Human immunodeficiency virus type 2 and the related simian immunodeficiency virus (SIV) contain a unique regulatory gene, vpx. The Vpx protein is packaged in mature virions and is required for efficient viral replication in peripheral blood lymphocytes and macrophages. To study the localization of Vpx in mature virions, conical and bar-shaped core structures of SIV from macaques (SIVmac) were purified. The SIVmac core has a density of approximately 1.25 g/cm3, compared with 1.16 g/cm3 for an intact virion. The relative proportions of major capsid protein (p27) and reverse transcriptase activity were similar for intact virions and core structures. The majority of matrix protein (p14) was removed from the purified core structure, suggesting its association with the viral membrane. Similarly, most of the Vpx protein was absent from the purified core structure. This result suggests that as with the matrix protein, the majority of Vpx proteins are localized outside the virus core. The localization of Vpx suggests that it may be involved in virus entry such as penetration or uncoating.

Mutational analysis of the major homology region of Mason-Pfizer monkey virus by use of saturation mutagenesis

The major capsid (CA) protein of retroviruses possesses a stretch of 20 amino acids, called the major homology region (MHR), which is evolutionarily conserved and invariant in location within the primary sequence of the protein. The function of this region was investigated by examining the effect of random single-amino-acid substitutions within the central 13 positions of the MHR on the life cycle of Mason-Pfizer monkey virus (M-PMV), an immunosuppressive D-type retrovirus. When these mutants were subcloned into an M-PMV proviral vector and expressed in COS cells, one of two major phenotypes was observed. The first group, containing three mutants bearing drastic amino acid substitutions, was unable to assemble capsids in the cytoplasm of the host cell. The second and more common group of mutants was able to assemble and release virions, but these either displayed greatly reduced levels of infectivity or were completely noninfectious. Included within this second group were two mutants with unusual phenotypes; mutant D158Y exhibited a novel cleavage site for the viral protease that resulted in cleavage of the major capsid protein, p27 (CA), within the MHR, whereas mutant F156L appeared to have lost a major site for antibody recognition within the mature CA protein. The results of this mutagenic analysis suggest that changes in the MHR sequence can interfere with the assembly of viral capsids and block an early stage of the infection cycle of M-PMV.

Purification of immature cores of mouse mammary tumor virus and immunolocalization of protein domains

The immature capsids of the mouse mammary tumor virus (MMTV), known as intracytoplasmic A particles, have been isolated from murine L1210 leukemia cells. The diameter of the isolated particles was 80 nm as determined by negative staining. Two polypeptides of 77 and 110 kDa were found to be their major polypeptide components, in agreement with the expected sizes of the Gag and Gag-Pro precursor polypeptides of the mature MMTV proteins. Both polypeptides were recognized by antibodies directed toward the matrix (p10) and capsid (p27) proteins of MMTV. Immunogold labeling of p10 on isolated A particles, visualized by negative staining, showed that this protein is located at the surface of the immature capsids, whereas p27 can be detected only in broken or disrupted particles, suggesting that it has an internal location. These observations were confirmed by immunolabeling of both proteins on thin sections of A particle-producing cells. In addition, the viral protease had a more internal position than p27. Since the sequential order of the viral proteins in the Gag precursor is p10-pp21-p27-p14 and that in Gag-Pro is p10-pp21-p27-p30-protease, our results demonstrate the radial organization of the polypeptide precursors forming the intracytoplasmic A particles.

Transforming growth factor beta suppresses human immunodeficiency virus expression and replication in infected cells of the monocyte/macrophage lineage

The pleiotropic immunoregulatory cytokine transforming growth factor beta (TGF-beta) potently suppresses production of the human immunodeficiency virus (HIV), the causative agent of the acquired immunodeficiency syndrome, in the chronically infected promonocytic cell line U1. TGF-beta significantly (50-90%) inhibited HIV reverse transcriptase production and synthesis of viral proteins in U1 cells stimulated with phorbol myristate acetate (PMA) or interleukin 6 (IL-6). Furthermore, TGF-beta suppressed PMA induction of HIV transcription in U1 cells. In contrast, TGF-beta did not significantly affect the expression of HIV induced by tumor necrosis factor alpha (TNF-alpha). These suppressive effects were not mediated via the induction of interferon alpha (IFN-alpha). TGF-beta also suppressed HIV replication in primary monocyte-derived macrophages infected in vitro, both in the absence of exogenous cytokines and in IL-6-stimulated cultures. In contrast, no significant effects of TGF-beta were observed in either a chronically infected T cell line (ACH-2) or in primary T cell blasts infected in vitro. Therefore, TGF-beta may play a potentially important role as a negative regulator of HIV expression in infected monocytes or tissue macrophages in infected individuals.

Characterisation of lymphoproliferative disease virus of turkeys. Structural polypeptides of the C-type particles

Lymphoproliferative disease virus of turkeys (LPDV), a C-type retrovirus, was shown to contain 3 major [32 kilodaltons (kd, p 32), 26 kd, 22/21 kd] and 2 minor (41 kd and 12 kd) polypeptides. Preliminary evidence suggests a glycoprotein of 76 kd (GP 76) and a major doublet polypeptide of 13.5/13 kd to be also of viral origin. Of these GP 76 was susceptible to bromelain action implying its surface location in the virion, while p 32, p 26 and p 13.5/13 were the main constituents of viral cores. p 13.5/13 bound an RNA probe, suggesting it to be the main constituent of viral ribonucleoprotein. p 22/21 was not cleaved by bromelain, and was absent in viral cores suggesting its intramembrane location between virion envelope and core. The polypeptide profile of LPDV is distinct from those of avian sarcoma-leukosis viruses and avian reticuloendotheliosis viruses.

Further evidence for a retrovirus as the aetiological agent of sheep pulmonary adenomatosis (jaagsiekte)

The infective agent of jaagsiekte was shown to be present in the fluid which accumulates in the respiratory tract of sheep during the terminal stages of the disease. The fluid also contained reverse transcriptase (RT) activity which showed a clear preference for a ribonucleic acid synthetic template over the corresponding deoxyribonucleic acid template and which utilised the RT specific template/primer poly (2'-0-methylcytidylate) oligodeoxyguanylate. This enzyme activity was associated with a particle which had typical retroviral buoyant densities in a range of gradient media.

Analysis of secondary modifications of mouse mammary tumor virus proteins by two-dimensional gel electrophoresis

The structural proteins of mouse mammary tumor virus (MMTV) were analyzed by two-dimensional electrophoresis on isoelectric focusing and sodium dodecyl sulfate gels. Many of the viral proteins displayed heterogeneity in charge due to variable contents of carbohydrates (in particular, sialic acid) and phosphate residues. Neuraminidase treatment of the virions influenced the isoelectric pattern of the envelope glycoproteins. The glycoproteins of an MMTV variant which was attenuated by replication in feline kidney cells had different isoelectric points. This suggested that the acquisition of an altered carbohydrate configuration had changed the host range of the virus. The major MMTV structural core protein, p27, consisted of two species, which had identical iodinated tryptic peptide compositions but differed in phosphate contents. Another MMTV phosphoprotein, p21, was separated into four different phosphorylated species. Phosphorylation of p21 could be performed in vitro by the MMTV virion-associated protein kinase. This enzyme also has a high affinity for MMTV p30 as a substrate. Possible functions of this enzyme are discussed.

Structure and processing of the mouse mammary tumor virus glycoprotein precursor pr73env

The polyprotein precursor to the envelope glycoproteins of mouse mammary tumor virus was investigated by using subcellular fractionation procedures, pactomycin mapping techniques, tunicamycin inhibition of glycosylation, and endo-beta-N-acetyl glucosaminidase H-catalyzed removal of glycosylated residues in order to characterize the biosynthesis and processing of the precursor. The results suggest that the precursor (Pr73env) is synthesized on the rough endoplasmic reticulum as a transmembrane protein, with the carboxyl terminus remaining on the cytoplasmic side. The apoprotein as an estimated molecular weight of 60,000 and acquires five core oligosaccharide units during synthesis. Cleavage of the precursor precedes the secondary glycosylation steps and therefore probably occurs before transport to the plasma membrane. However, a minor population of Pr73env containing complex oligosaccharides was also found in the plasma membrane. The order of the glycoproteins in the precursor, as determined by pactomycin mapping, in NH2-gp52-gp36-COOH.

Virus-like particles and macromolecules in human milk and breast tumors

Relevant data pertaining to present evidence for virus-like particles and virus-related macromolecules in human milk and breast tumors are presented. A critical review and discussion of reported observations concerning virus-related macromolecules will include RNA-directed DNA polymerase, viral antigens, and RNA related to murine mammary tumor virus and/or Mason-Pfizer monkey virus. From the standpoint of clinical applications, the finding of viral-related antigens in human breast tumors and evidence for specific host immune responses to one or more of these antigens may be especially pertinent. The latter data, therefore, will be discussed in depth as to possible employment of these parameters in diagnosis, prognosis and possible management of the human disease.