The genetics of murine leukemia viruses

Characterization of human immunodeficiency virus type 1 Pr55gag membrane association in a cell-free system: requirement for a C-terminal domain

Association of the human immunodeficiency virus type 1 (HIV-1) gag polyprotein precursor with cellular membranes is necessary for assembly of virions. We used in vitro synthesized HIV-1 gag to study its association with isolated cellular membranes. Rabbit reticulocyte lysates programmed with HIV-1 gag mRNA incorporated [35S]methionine and [3H]myristate into two predominant species of 55 kDa and 40 kDa. Radioimmunoprecipitation with HIV-1-specific antibodies suggested that the 55-kDa protein represented the polyprotein precursor (Pr55gag), while the 40-kDa protein was a mixture of N- or C-terminal truncations of the gag precursor. The Pr55gag protein bound to cellular membranes, while the 40-kDa mixed protein species did not. Membrane binding studies with C terminus-truncated and point mutants revealed that the seven-amino acid sequence located between the two Cys-His arrays in the nucleocapsid region was necessary for stable association to occur. Therefore, we propose that signals in addition to myristate are required for the membrane association of HIV-1 gag proteins and that these signals include a domain in the nucleocapsid protein.

Tumorigenesis mediated by an antigen receptor

The heavy chain variable region of the immunoglobulin receptors on cells of the B lymphoma NYC are almost identical to that of other independent B-cell tumors of B/W mice. NYC IgM binds a viral antigen produced by the tumor cells; and despite extensive screening, immunoglobulin-negative variants were never found in the NYC cells, suggesting that NYC loses the capacity to grow in culture when it does not synthesize surface immunoglobulin. These findings indicate that the interaction of endogenous antigen with surface IgM continuously stimulates growth and, thus, that the tumorigenesis of B lymphomas in B/W mice is mediated by antigen receptors.

Characterization of virus-like particles produced by a recombinant baculovirus containing the gag gene of the bovine immunodeficiency-like virus

The entire gag gene of the bovine immunodeficiency-like virus (BIV) was inserted behind the strong polyhedron promoter of Autographa californica nuclear polyhedrosis virus (AcNPV). The resultant recombinant baculovirus (AcNPV-BIVgag) was used to infect insect cells in order to overexpress and characterize BIV gag gene products. The infection resulted in the high-level expression of a protein similar in size to the predicted BIV gag precursor (Pr53gag). BIV Pr53gag was detected in AcNPV-BIVgag-infected insect cells and in culture supernatants. Electron microscopy of these cells revealed an abundance of virus-like particles (VLPs) in the cytoplasm, budding from the cell membrane, and free in the culture medium. The size and morphology of the VLPs were similar to those of the immature forms of BIV observed in infected mammalian cells. The VLPs sedimented at a density of 1.16 g of sucrose per milliliter in linear gradients and were shown to contain the majority of the supernatant Pr53gag. Antigenic determinants on Pr53gag from VLPs were recognized by BIV and HIV-1 antiserum, and serum from rats immunized with VLPs reacted with recombinant and viral BIV Pr53gag and processed products. The protease (PR) activity in BIV virions was capable of processing recombinant Pr53gag; this activity was blocked by pepstatin A, a potent aspartyl PR inhibitor. Baculovirus-expressed BIV Pr53gag appears to be an excellent source of gag precursor; it may prove useful for structural studies and enable the development of assays to detect retroviral PR inhibitors. The data further suggest that unprocessed BIV Pr53gag plays a major role in the assembly of BIV particles. The expression of other BIV structural genes in insect cells may prove instructive in the study of molecular events involved in the assembly and processing of these BIV proteins.

Human immunodeficiency virus-like particles produced by a vaccinia virus expression vector

Infectious retrovirus particles consist of a core structure containing RNA and gag-pol polypeptides surrounded by a lipid membrane studded with env proteins. A recombinant vaccinia virus was designed to express the entire gag-pol precursor protein of the human immunodeficiency virus type 1. Synthesis and processing of gag proteins occurred in mammalian cells infected with this live recombinant virus, and reverse transcriptase was detected largely in the medium. Electron micrographs revealed immature retrovirus-like particles budding from the plasma membrane and extracellular particles with morphological characteristics of immature and mature human immunodeficiency virus. The latter contained functional reverse transcriptase as well as processed p24 and p17 gag polypeptides. Thus, assembly and maturation of human immunodeficiency virus-like particles can occur in the absence of either infectious RNA molecules or env proteins. The production of noninfectious virus-like particles by expression vectors should be useful for biochemical studies and could provide a safe source of material for the development of vaccines.

Charge configurations in viral proteins

The spatial distribution of the charged residues of a protein is of interest with respect to potential electrostatic interactions. We have examined the proteins of a large number of representative eukaryotic and prokaryotic viruses for the occurrence of significant clusters, runs, and periodic patterns of charge. Clusters and runs of positive charge are prominent in many capsid and core proteins, whereas surface (glyco)proteins frequently contain a negative charge cluster. Significant charge configurations are abundant in regulatory proteins implicated in transcriptional transactivation and cellular transformation. Proteins with charge structures are much more predominant in animal DNA viruses as compared to animal RNA viruses and prokaryotic viruses. This contrast might reflect the role of protein charge structures in facilitating competitive virus-host interactions involving the cellular transcription, translation, protein sorting, and transport apparatus.

Characterization of human immunodeficiency virus gag/pol gene products expressed by recombinant vaccinia viruses

Recombinant vaccinia viruses containing either the entire gag/pol gene or the reverse transcriptase (RT) domain of the human immunodeficiency virus (HIV) were constructed. In mammalian cells infected with the recombinant vaccinia virus containing the gag/pol gene, major and minor polypeptides of 55 and 41 kDa were made, but processed gag products (p24/p17/p15) were not detected. In addition, none of the products of the pol open-reading frame were seen. Both the 55- and 41-kDa gag proteins were post-translationally modified by addition of myristic acid residues in recombinant vaccinia-infected cells, and were immunoprecipitated by antiserum to p24 gag, as well as by antisera from HIV-infected patients. These results indicate that neither proteolytic processing nor other HIV proteins are required for myristilation, and suggest that the 55- and 41-kDa gag precursors share the same amino terminus as p17. Cells infected with a separate vaccinia recombinant containing a truncated piece of the gag/pol gene with added start and stop codons at the 5' and 3' ends of the RT reading frame synthesized a major 61-kDa and a minor 51-kDa protein product which reacted immunologically with both a monoclonal antibody to native HIV p66/51 and antisera from HIV-infected patients. These proteins were purified from recombinant vaccinia-infected mammalian cells, and their enzyme activity was found to be similar to that of authentic HIV RT. Cells infected with the vaccinia/RT vector contained approximately 200-fold more RT per milligram of protein than cells infected with HIV. Recombinant RT was inhibited by dideoxynucleoside triphosphates and should be useful in screening for specific inhibitors of this enzyme. Mice inoculated intradermally with 10(8) plaque-forming units of the vaccinia/RT vector developed specific antibodies to the p66/51 proteins of HIV, but anti-HIV antibodies were not detected in mice inoculated with the vaccinia/gag vector.

Expression of the gag-pol fusion protein of Moloney murine leukemia virus without gag protein does not induce virion formation or proteolytic processing

A mutant of Moloney murine leukemia virus was generated in which the UAG termination codon at the 3' end of the gag gene was changed to a CAG codon encoding glutamine. Cells carrying the mutant provirus constitutively express the gag-pol fusion protein and no detectable gag protein. The precursor is stable, is not processed by the protease domain within the precursor, and does not induce assembly and release of virion particles.

Mutational analysis of the human immunodeficiency virus: the orf-B region down-regulates virus replication

Mutations were made by recombinant DNA techniques in an infectious molecular clone of the human immunodeficiency virus San Francisco isolate 2 (HIVSF2) [formerly the prototype isolate of the acquired immunodeficiency syndrome-associated retrovirus (ARV-2)]. The effect of these changes on the replicative and cytopathologic properties of the virus was studied by transfecting modified virus clones into cultured human cells. Mutations in the gag, pol, env, and tat regions precluded virus replication and cytopathology in lymphoid cells. A mutation in orf-A dramatically reduced but did not abolish virus replication. Mutant viruses with deletions in the orf-B region were highly cytopathic and replicated to approximately 5-fold higher levels than wild-type virus. They also produced approximately 5-fold more viral DNA in infected lymphoid cells than did wild-type virus. Thus, the orf-B region may function to down-regulate virus replication. This mutational analysis of the HIVSF2 genome is a means of assessing genes regulating viral replication and cytopathology.

Hepadnaviruses and retroviruses share genome homology and features of replication

The hepadnavirus family includes hepatitis B virus (HBV), woodchuck hepatitis virus (WHV), ground squirrel hepatitis virus (GSHV) and duck hepatitis B virus (DHBV). These viruses share unique ultrastructural, molecular and biological features. HBV has great medical importance in many parts of the world. More important numerically than acute hepatitis B in high prevalence geographic regions is the liver disease associated with chronic infection. There appear to be more than 200 million chronically infected humans in the world, and these HBV infections appear to be the single most common cause of chronic liver disease and liver cancer in man. All hepadnaviruses share the propensity for silent infection in early life leading to persistence of the virus, and hepatocellular carcinoma (HCC) is clearly associated with long-standing persistent infection in man, woodchucks and ground squirrels. Although the viral DNA has been found to be integrated in cellular DNA of many HCC in man, woodchucks and ground squirrels, the precise role of the virus in tumor formation has not been defined. Hepadna viruses have an interesting molecular structure and mechanisms of replication, and they appear to share certain important features with retroviruses as reviewed here. It is of interest to define similarities and differences between hepadnaviruses and retroviruses in order to understand their evolutionary relationship and to determine whether they share a common oncogenic mechanism, since infection with members of both virus families is associated with neoplastic disease.

Common evolutionary origin of hepatitis B virus and retroviruses

Hepatitis B virus (HBV), although classified as a double-stranded DNA virus, has been shown recently to replicate by reverse transcription of an RNA intermediate. Also, the putative viral polymerase has been found to share amino acid homology with reverse transcriptase of retroviruses. Using computer-assisted DNA and protein sequence analyses, we examined the genomes of 13 hepadnavirus isolates (nine human, two duck, one woodchuck, and one ground squirrel) and found that other conserved regions of the hepadnavirus genome share homology to corresponding regions of the genomes of type C retroviruses and retrovirus-like endogenous human DNA elements. Specifically, the most highly conserved sequence of the HBV genome, positioned at or near the initiation site for first-strand HBV DNA synthesis, is homologous over 67 nucleotides to the U5 region, a comparable region in retrovirus long terminal repeats. Within a highly conserved (i.e., 90%) 16-nucleotide sequence a heptanucleotide sequence CCTTGGG is 97% homologous between 27 virus isolates. Also, we found that the highly conserved HBV core, or nucleocapsid, protein shares 41% homology over 98 amino acids with the carboxyl-terminal region of the p30 gag nucleocapsid protein of type C retroviruses. In both cases, as with the previously reported polymerase homology, HBV is most homologous to the murine leukemia/sarcoma retroviruses. Further analysis revealed additional similarities between hepadnavirus and retroviral genomes. Taken together, our results suggest that HBV and retroviruses have a common evolutionary origin, with HBV arising through a process of deletion from a retrovirus, or retrovirus-like, progenitor.