Protease bypass of temperature-sensitive murine leukemia virus maturation mutants

Moloney murine leukemia virus decay mediated by retroviral reverse transcriptase degradation of genomic RNA

Retroviral vectors are powerful tools for the introduction of transgenes into mammalian cells and for long-term gene expression. However, their application is often limited by a rapid loss of bioactivity: retroviruses spontaneously loose activity at 37 degrees C, with a half-life of 4 to 9 h depending on the retrovirus type. We sought to determine which components of the retrovirus are responsible for this loss in bioactivity and to obtain a quantitative characterization of their stability. To this end, we focused on RNA and viral proteins, two major components that we hypothesized may undergo degradation and negatively influence viral infectivity. Reverse transcription PCR (RT-PCR) targeting RNA encoding portions of the viral genome clearly demonstrated time-dependent degradation of RNA which correlated with the loss in viral bioactivity. Circular dichroism spectroscopy, SDS-PAGE and two-dimensional SDS-PAGE analyses of viral proteins did not show any change in secondary structure or evidence of proteolysis. The mechanism underlying the degradation of viral RNA was investigated by site-directed mutagenesis of proteins encoded by the viral genome. Reverse transcriptase and protease mutants exhibited enhanced RNA stability in comparison to wild type recombinant virus, suggesting that the degradation of RNA, and the corresponding virus loss of activity, is mediated by the reverse transcriptase enzyme.

Protease inhibitors as potential therapeutic agents for AIDS

A decade since the epidemic of the acquired immunodeficiency syndrome (AIDS) was first recognized, a wealth of information has accumulated on the molecular biology of the causative agents, the human immunodeficiency viruses (HIV). Of particular interest is knowledge of the viral enzymes involved in the formation of new virus particles. Such enzymes constitute attractive targets for efforts aimed at selecting agents that interfere with virus multiplication and subsequent spread and pathogenesis. Already, several agents that inhibit the viral reverse transcriptase (e.g., nucleoside analogs such as Zidovudine) have proved to have a beneficial effect on the course off the disease, but their prolonged use has been associated with significant toxicity and the emergence of resistant mutants. A second enzyme that has recently attracted attention is the virus-coded protease. This enzyme is involved in the cleavage of viral precursor polyproteins into the final products that constitute the mature virus particle. Protease inhibitors interfere with the process of virus maturation which is required for the formation of infective virus particles. Several custom-made inhibitors with a high selective action against HIV protease have been produced recently. They are nonhydrolyzable peptide analogs that mimic the cleavage sequences of the natural substrate of the enzyme during the transition state of the cleavage reaction. It is hoped that a similar selectivity in vivo may make protease inhibitors a promising new category of AIDS therapeutics.

Import of rat ornithine transcarbamylase precursor into mitochondria: two-step processing of the leader peptide

The mitochondrial matrix enzyme ornithine transcarbamylase (OTC) is synthesized on cytoplasmic polyribosomes as a precursor (pOTC) with an NH2-terminal extension of 32 amino acids. We report here that rat pOTC synthesized in vitro is internalized and cleaved by isolated rat liver mitochondria in two, temporally separate steps. In the first step, which is dependent upon an intact mitochondrial membrane potential, pOTC is translocated into mitochondria and cleaved by a matrix protease to a product designated iOTC, intermediate in size between pOTC and mature OTC. This product is in a trypsin-protected mitochondrial location. The same intermediate-sized OTC is produced in vivo in frog oocytes injected with in vitro-synthesized pOTC. The proteolytic processing of pOTC to iOTC involves the removal of 24 amino acids from the NH2 terminus of the precursor and utilizes a cleavage site two residues away from a critical arginine residue at position 23. In a second cleavage step, also catalyzed by a matrix protease, iOTC is converted to mature OTC by removal of the remaining eight residues of leader sequence. To define the critical regions in the OTC leader peptide required for these events, we have synthesized OTC precursors with alterations in the leader. Substitution of either an acidic (aspartate) or a "helix-breaking" (glycine) amino acid residue for arginine 23 of the leader inhibits formation of both iOTC and OTC, without affecting translocation. These mutant precursors are cleaved at an otherwise cryptic cleavage site between residues 16 and 17 of the leader. Interestingly, this cleavage occurs at a site two residues away from an arginine at position 15. The data indicate that conversion of pOTC to mature OTC proceeds via the formation of a third discrete species: an intermediate-sized OTC. The data suggest further that, in the rat pOTC leader, the essential elements required for translocation differ from those necessary for correct cleavage to either iOTC or mature OTC.

Molecular genetic analysis of a vaccinia virus gene with an essential role in DNA replication

We have identified a gene encoded by vaccinia virus which is essential for DNA replication. The gene, located in the HindIII D fragment of the viral genome, is transcribed early after infection into two transcripts of 3.0 and 3.7 kilobases which share a 3' terminus. The lesions of three temperature-sensitive DNA replication mutants with defects in this gene have been localized by marker rescue with progressively smaller DNA fragments. We have determined by hybrid selection that the gene encodes an 82-kilodalton protein. An antibody has been prepared against this polypeptide and used to quantitate expression of the protein after infection with wild-type virus or with a viral mutant whose lesion maps within this gene. The temporal pattern of expression in the mutant is unaffected, but the product encoded by the mutant is significantly more thermolabile than the wild-type protein.

Mutants of murine leukemia viruses and retroviral replication

The analysis of retroviral mutants has played a critical role in the development of our understanding of the complex viral life cycle. The most fundamental result of that analysis has been the definition of the replication functions encoded by the viruses. From a biochemical examination of a particular step in the life cycle it is difficult to determine, for example, whether that step is catalyzed by a viral or a host enzyme; but the isolation of a viral mutant defective in that step can firmly establish that a viral function is involved. In this way many facts about the viruses have been established. We know that reverse transcriptase is encoded by the virus; that RNAase H and DNA polymerase activities reside on the same gene product; that processing of many precursor proteins is mediated by a viral proteinase; and that establishment of the integrated provirus requires a viral protein. The list of functions mediated by viral enzymes has largely been defined by the mutants isolated and studied in various laboratories. The second significant result of the studies of viral mutants has been the assignation of the replication functions to particular viral genes, and then more specifically to particular domains of these genes. Mutants and viral variants have been essential in the determination, for example, that the gag protein is the critical gene product for the assembly of a virion particle; that the env protein is the determinant of species specificity of infection; or that the LTR is a major determinant of tissue tropism and leukemogenicity. The subdivisions of functions within a given gene have similarly hinged on mutants. Genetic mapping was needed to establish that P30 is the most important region for assembly; that the proteinase and integrase functions reside, respectively, in the 5' and 3' portions of the pol gene; and that the glycosylated gag protein is dispensable for replication. A third important area of knowledge has depended heavily on viral mutants: the determination of host functions and proteins that interact with viral proteins. Variant viruses with altered or restricted host ranges serve to define differences between pairs of different host cells, and the mapping of the viral mutations serves to define the viral protein important in that interaction with the host. These studies are only in their infancy, but it is clear that substantial efforts will be made to further analyze these host functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Sequences near the 5' long terminal repeat of avian leukosis viruses determine the ability to induce osteopetrosis

Avian leukosis virus (ALV)-induced osteopetrosis is associated with the accumulation of unintegrated viral DNA in osteoblasts. Viruses constructed from the DNAs of an osteopetrosis-inducing ALV (Br21) and a non-osteopetrosis-inducing ALV (RAV-0) have been used to test for the role of viral genes in the induction of osteopetrosis. Our results map osteopetrotic potential to a 1,400-base-pair region near the 5' long terminal repeat. This region contains signals for the splicing, translation, and packaging of viral RNAs and coding sequences for the gag proteins p19 and p10 and the N terminus of p27.

Regulation of Moloney murine leukemia virus replication in chronically infected cells arrested at the G0/G1 phase

The replication of Moloney murine leukemia virus (MMuLV) in chronically infected mouse cells arrested at the G0/G1 phase of the cell cycle by different procedures was investigated. MMuLV production was inhibited in glutamine- and isoleucine (Gln-Ile)-deprived G0/G1 cells. In contrast, butyric acid treatment, which efficiently arrested the cells at the G0/G1 phase of the cell cycle, did not inhibit MMuLV production. Furthermore, the inhibition of MMuLV production caused by either Gln-Ile deprivation or by interferon (IFN) treatment was overcome by butyric acid treatment. Thus, the replication of MMuLV could be dissociated from cell proliferation. The inhibition of MMuLV production in Gln-Ile-deprived cell cultures was compared to the inhibitory effect of IFN, which is known to affect budding and release of the virus. Rates of MMuLV protein synthesis were not affected in both the IFN-treated and Gln-Ile-deprived cells. However, processing of the viral polyprotein Pre65gag into p30 was blocked in the Gln-Ile-deprived cells. Furthermore, whereas in IFN-treated cells, MMuLV accumulated on the cell surface and could be released upon treatment with trypsin, in Gln-Ile-deprived cells, no virions were released by such treatment. These results indicate that in cells arrested by Gln-Ile deprivation, MMuLV is inhibited at a posttranslation step. This step appears to precede the anti-MMuLV block induced by IFN.

A deletion mutation in the 5' part of the pol gene of Moloney murine leukemia virus blocks proteolytic processing of the gag and pol polyproteins

Deletion mutations in the 5' part of the pol gene of Moloney murine leukemia virus were generated by restriction enzyme site-directed mutagenesis of cloned proviral DNA. DNA sequence analysis indicated that one such deletion was localized entirely within the 5' part of the pol gene, did not affect the region encoding reverse transcriptase, and preserved the translational reading frame downstream of the mutation. The major viral precursor polyproteins (Pr65gag, Pr200gag-pol, and gPr80env) were synthesized at wild-type levels in cell lines carrying the mutant genome. These cell lines assembled and released wild-type levels of virion particles into the medium. Cleavage of both Pr65gag and Pr200gag-pol precursors to the mature proteins was completely blocked in the mutant virions. Surprisingly, these virions contained high levels of active reverse transcriptase; examination of the endogenous reverse transcription products synthesized by the mutant virions revealed normal amounts of minus-strand strong-stop DNA, indicating that the RNA genome was packaged and that reverse transcription in detergent-permeabilized virions was not significantly impaired. Processing of gPr80env to gP70env and P15E was not affected by the mutation, but cleavage of P15E to P12E was not observed. The mutant particles were poorly infectious; analysis indicated that infection was blocked at an early stage. The data are consistent with the idea that the 5' part of the pol gene encodes a protease directly responsible for processing Pr65gag, and possibly Pr200gag-pol, to the structural virion proteins. It appears that cleavage of the gag gene product is not required for budding and release of virions and that complete processing of the pol gene product to the mature form of reverse transcriptase is not required for its functional activation.

Mutations in the gag gene of Moloney murine leukemia virus: effects on production of virions and reverse transcriptase

We have constructed a series of deletion mutations in the p30 and p10 domains of the gag gene of Moloney murine leukemia virus. Mutants with deletions in P30 were completely defective in virion particle production even though an altered gag precursor protein is synthesized. This domain is apparently critical for particle formation. A mutant in P10 was able to release virion particles into the medium, and low levels of reverse transcriptase activity could be detected in these virions. To explore the effects of these mutations on the utilization of the gag-pol precursor, we have introduced these mutants into cells already releasing defective particles from an endogenous provirus which directs the synthesis of gag gene products and not pol gene products. The P10 mutant was capable of providing pol function as judged by the incorporation of high levels of reverse transcriptase into the particles and complete complementation for XC plaque formation. In contrast, the mutants in P30 were negative in this complementation test. Thus, those gag mutants which were unable on their own to assemble virion particles were also unable to contribute the gag-pol precursor to these particles. These mutations are the first to be mapped to the gag region which affect pol function, suggesting that the gag-pol precursor must be assembled before pol is functionally separated from the gag domain. The concordance of the effects of different mutations on both particle formation and gag-pol utilization suggests that similar domains of gag (namely, domains in the P30 region) are needed for these two processes.

Isolation and characterization of a Mo-MuSV-transformed TB cell line that produces noninfectious MuSV particles with uncleaved gag protein which is processed in the presence of Mo-MuLV

A cell line, TBSV7, that produces noninfectious murine sarcoma virus (MuSV) in the absence of helper MuLV was isolated from TB cells infected with the supernatant of MuSV349 cells. These noninfectious MuSV particles with "immature" C-type virus morphology contain a 2.2 X 10(6)-Da genomic RNA and an uncleaved 62,000-Da gag precursor protein (Pr62). Neither viral envelope proteins (gp70, p15E, p12E) nor reverse transcriptase were detected in these virus particles. Pr62 was found to be phosphorylated in vivo and it could be phosphorylated in vitro with [gamma-32P]ATP, indicating that protein kinase was packaged in these noninfectious virions. In vitro processing of Pr62 to smaller molecular weight proteins could be achieved by the addition of Mo-MuLV and Nonidet P-40. The initial cleavage products were proteins with molecular weights of 38K (Pr38) and 27K (Pr27). Under optimum conditions Pr38 was cleaved to p30 and a protein band migrating with MuLV-p10, while Pr27 was cleaved to a 17,000-Da protein that migrated slower than MuLV-p15 and a protein band migrating with MuLV-p12. Pulse-chase experiments performed on TBSV7 cells superinfected with Mo-MuLV indicated that intracellular processing of Pr62 was much slower than that of Pr65. Cleavage protein products of Pr62 similar in size to the in vitro protein products were also detected in TBSV7 cells superinfected with MuLV.

Deletion mutants of Moloney murine leukemia virus which lack glycosylated gag protein are replication competent

A series of deletion mutations localized near the 5' end of the Moloney murine leukemia virus genome was generated by site-specific mutagenesis of cloned viral DNA. The mutants recovered from such deleted DNAs failed to synthesize the normal glycosylated gag protein gPr80gag. Two of the mutants made no detectable protein, and a third mutant, containing a 66-base pair deletion, synthesized an altered gag protein which was not glycosylated. All the mutants made normal amounts of the internal Pr65gag protein. The viruses were XC positive and replicated normally in NIH/3T3 cells as well as in lymphoid cell lines. These results indicate that the additional peptides of the glycosylated gag protein are encoded near the 5' end, that the glycosylated and internal gag proteins are synthesized independently, and that the glycosylated gag protein is not required during the normal replication cycle. In addition, the region deleted in these mutants apparently encodes no cis-acting function needed for replication. Thus, all essential sequences, including those for packaging viral RNA, must lie outside this area.