Coordination of cleavage of gag and env gene products of murine leukemia virus: implications regarding the mechanism of processing

Mutations affecting the cytoplasmic domain of the Moloney murine leukemia virus envelope protein: rapid reversion during replication

Five premature termination mutations and five missense mutations were introduced into the portion of cloned Moloney murine leukemia virus (M-MuLV) DNA encoding the Env cytoplasmic domain. All of the mutant DNAs gave rise to replication-competent virus after transfection of NIH/3T3 cells, but several of the mutant DNAs scored as replication-defective when introduced into Rat2 cells. Cell lines stably expressing the mutant DNAs all released virion particles, and in all but one case infectious virus were generated. These viable mutants were all found to have reverted to the wild-type sequence. To generate fully mutant virus stocks, the mutant DNAs were introduced transiently into COS cells, which are resistant to infection with MuLV, thus prohibiting reversion by error-prone mechanisms involving reverse transcription. Virions harvested from the COS cells were confirmed as mutant by analyzing both virion proteins and the viral DNA they generated, and were then tested for infectivity in NIH/3T3 cells. The mutant viruses were infectious, but still rapidly gave rise to revertants. We conclude that the mutations within the cytoplasmic domain do not provide an absolute block to virus replication, but that the mutants replicate more slowly than the wild-type and quickly give rise to revertants with selective advantage for replication.

Postassembly cleavage of a retroviral glycoprotein cytoplasmic domain removes a necessary incorporation signal and activates fusion activity

Viral protease-mediated cleavage within the cytoplasmic domain of the transmembrane (TM) glycoprotein of the type D retrovirus, Mason-Pfizer monkey virus, removes approximately 16 amino acids from the carboxy terminus of the protein. To determine the functional significance of this cleavage in the virus life cycle, we introduced premature stop codons into the TM coding domain, resulting in the production of truncated glycoproteins. Progressive truncated of the cytoplasmic domain identified the carboxy-terminal third as being required for efficient incorporation of the glycoprotein complex into budding virions and profoundly increased the fusogenic capability of the TM glycoprotein. These results, together with the ability of matrix protein mutations to suppress TM cleavage, imply that this portion of the glycoprotein interacts specifically with the capsid proteins during budding, suppressing glycoprotein fusion function until virus maturation has occurred.

A viral protease-mediated cleavage of the transmembrane glycoprotein of Mason-Pfizer monkey virus can be suppressed by mutations within the matrix protein

The envelope glycoprotein precursor of retroviruses undergoes proteolytic cleavage in the Golgi complex to yield the mature surface and transmembrane (TM) glycoproteins of the virus. We report here that the TM glycoprotein of Mason-Pfizer monkey virus undergoes a second proteolytic processing event during a late maturation step that can follow virus release and Gag polyprotein cleavage. Cleavage results in the conversion of the cell-associated TM glycoprotein (gp22) to a virus-associated gp20. Processing continues after virus release and yields virions that contain predominantly gp20. A mutation within the active site of the Mason-Pfizer monkey virus aspartyl protease was shown to block both TM glycoprotein cleavage and the processing of the Gag polyprotein precursor. The role of the viral protease in cleavage of the TM glycoprotein localizes the cleavage site to the cytoplasmic domain of this protein. Surprisingly, point mutations within the matrix (MA) coding region of the gag gene can affect the extent to which gp22 is processed to gp20 and in one case [p10(MA)-A79V] results in greater than 90% inhibition of gp22 cleavage. The data provide genetic evidence of a specific interaction between the capsid proteins and the cytoplasmic domain of the TM glycoprotein of a retrovirus. This interaction is required for cytoplasmic domain cleavage to occur and may play a critical role in virus assembly and viral infectivity.

Analysis of mutations in the envelope gene of Moloney murine leukemia virus: separation of infectivity from superinfection resistance

Six deletion mutations and an insertion were generated in the env gene of cloned copies of Moloney murine leukemia virus DNA. All seven mutants were replication-defective as tested by transformation of NIH/3T3 cells. The mutant DNAs were introduced into NIH/3T3 cells to generate stable producer lines; all released virion particles into the medium, suggesting that none of the mutations affected overall viral gene expression, gag and pol gene expression, gag and pol gene functions, or virion budding. Several of the mutations reduced the lifetime of the env protein or blocked its export to the cell surface. One mutation altering the membrane-spanning region and the cytoplasmic tail of the TM protein had no effect on export of the protein, proteolytic processing, or incorporation into virion particles, but still blocked the infectivity of the resulting virus. The results suggest that alterations in the transmembrane region can affect early steps of infection, such as the fusion of virion and host membranes. Cells expressing this mutant env protein were fully resistant to superinfection by wild-type virus. Thus, induction of virus resistance, presumably reflecting blocking the virus receptor, can be separated from virus infectivity.

Unmyristylated Moloney murine leukemia virus Pr65gag is excluded from virus assembly and maturation events

The gag precursor polyprotein of Moloney murine leukemia virus (MuLV) is normally modified by myristylation of the N-terminal glycine. Previous work showed that the Pr65gag lacking the myristylation site does not associate with cellular membranes or assemble into virus particles. We now report that it also is not cleaved to the mature gag cleavage products within the cell and that it sediments as a free 65-kilodalton monomer in detergent-free cell extracts containing 0.3 M NaCl. Even when the cells containing the mutant are productively infected with wild-type MuLV, the mutant Pr65gag is not processed into cleavage products and is not incorporated into the virions produced by these cells. Thus, the mutant gag molecules seem unable to participate in the normal processes of self-assembly and maturation. We propose that myristate-mediated membrane association is an essential first step in MuLV assembly. This association may also play a role in budding of MuLV.

Inhibition of cleavage of Moloney murine leukemia virus gag and env coded precursor polyproteins by cerulenin

Cerulenin, an inhibitor of de novo fatty acid (and cholesterol) biosynthesis, has been shown to significantly decrease (greater than 75%) the amount of Moloney murine leukemia virus (MMuLV) released into the culture medium of chronically infected mouse fibroblasts (I. Katoh, Y. Yoshinaka, and R.B. Luftig, 1986, Virus Res., in press). In order to clarify the mechanism by which this decrease in virus production occurs, we analyzed the kinetics of gag and env coded protein synthesis in M-MuLV infected, cerulenin-treated cells by immunoprecipitation with monospecific antisera to p30, p12, p10, gp70, and p15(E). We found that in pulse (15 min-2 hr)-chase (0-4 hr) experiments the cleavage of not only Pr65gag to p30 and other gag coded proteins but Pr80env to gp70 and Pr15(E) as well, was greatly reduced by cerulenin treatment. Further, since the total amount of label in the Pr65gag and Pr80env bands remained about the same or was slightly decreased in 2-hr pulsed, cerulenin-treated cells, this suggests that cerulenin decreases virus production, in part, by inhibiting the cleavage of both precursor gag and env coded polyproteins during virus assembly and budding at the cell membrane. We also observed that at longer chase periods (4 hr), the effect of cerulenin could be partially overriden in that minor amounts of cleaved gag and env coded polyproteins were produced and assembled into virion particles. However, these particles contained abnormally large amounts of the uncleaved precursor Pr65gag, suggesting that maturation was incomplete. The above results suggest two independent, but not exclusive, possible mechanisms of cerulenin action to block M-MuLV production, viz. cerulenin decreases the pool of fatty acids, thereby inhibiting fatty acid acylation of Pr65gag, as well as Pr80env, and thus preventing the interaction between gag (the p15 antigenic determinant on Pr65gag) and env [the p15(E) antigenic determinant of Pr15(E)] coded gene products at the cell membrane needed for efficient virus assembly (M. Satake and R. B. Luftig, 1983, Virology 124, 259-273), and cerulenin inhibits one or more proteolytic enzymes responsible for the cleavage of Pr65gag and Pr80env.

Maturation of murine leukemia virus env proteins in the absence of other viral proteins

A mutant of Akv which produces env but no detectable gag or pol products (A. Rein, D. R. Lowy, B. I. Gerwin, S. K. Ruscetti, and R. H. Bassin, J. Virol. 41, 626-634 (1982] was examined for maturation of env gene protein products. In comparison with wild-type Akv, the mutant AK 71 synthesizes gPr85env and produces gp70 and Prp15E in normal amounts and with normal kinetics. Cell surface gp70 was found alike in the mutant and wild type. However, cleavage of Pr15E to p15E did not occur in the mutant. This final cleavage step of AK 71 Prp15E could be made to occur by superinfecting cells containing the mutant with baboon endogenous virus. Thus, unlike earlier steps, this final step in maturation of the env gene product appears to require gag or pol gene products. It is proposed that the virus-encoded protease is required for this last step.

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.