Precursor polyproteins of Moloney murine leukemia virus

Intracellular production of virus particles and viral components in NIH/3T3 cells chronically infected with Moloney murine leukemia virus: effect of interferon

The effect of interferon on the biochemical properties and the maturation process of intracellular viral particles isolated from the cytoplasmic fraction of NIH/3T3 cells chronically infected with Moloney murine leukemia virus was investigated. By labeling these virions with either [35S]methionine or [3H]glucosamine, we demonstrated that they contain the same viral proteins and glycoproteins found in extracellular virions. Interferon treatment was found to reduce the rate of intracellular virus assembly. This effect was not a consequence of an interferon inhibition of viral RNA synthesis or its translation or a consequence of an interference with the posttranslational cleavage processing of viral precursor proteins, since all of these steps were not affected by interferon. However, the reduced rate of virus assembly could be attributed to the inhibition of viral protein glycosylation observed in interferon-treated cells. Nevertheless, despite this reduced rate, virus particles accumulated in interferon-treated cells. This accumulation was probably due to the strong inhibition of their final release from such cells.

Characterization of viral polyproteins in cells transformed and producing Moloney murine sarcoma virus-124

The viral proteins of Moloney murine sarcoma virus-transformed mouse cells (G8-124), that overproduce the sarcoma virus relative to helper leukemia virus, were examined by immunoprecipitation, sizing by gel electrophoresis and peptide mapping. Using antisera to the viral core proteins, a p10-deficient core polyprotein of 63 000 daltons (P63gag) was detected which was found to be a product of the MuSV-124 genome. Helper virus proteins Pr67gag, gPr85gag, Pr200gag-pol, and gPr83env were also detected and characterized. An unusual helper virus precursor polyprotein of 93 000 daltons was also detected and characterized. It was labeled with [3H]fucose, suggesting that it was an intermediate precursor containing gp70 on which further modification of the core oligosaccharide had occurred relative to gPr83env. The usual viral proteins were also found in sarcoma virus particles and they were undistinguishable in size from those of Moloney murine leukemia virus (cl-1 strain). These experiments together with pulse-labeling experiments performed in the presence of the arginine analog canavanine, which prevents proteolytic cleavage, suggest that the product derived from the Mo-MuSV-specific sequences must be expressed as a separate gene product since we were unable to detect in transformed cells a polyprotein containing both viral (e.g., 'gag', 'pol' or 'env') and non-viral components. Cell-free protein synthesis studies performed with Mo-MuSV-124 genomic RNA have confirmed this interpretation and have identified three size classes of polypeptides as translation products of the sarcoma virus genome. They are P63gag, P42-P38 and P23. Intracellular p63gag and cell-free synthesized P63gag, appear to have identical sizes, antigenic determinants and tryptic peptides. The P42-P38 proteins contain core protein determinants. P23 is not related to the products of the replication genes of MuLV. Thus, P23 is a candidate for the 'src' gene product of Mo-MuSV-124.

Suppression of murine retrovirus polypeptide termination: effect of amber suppressor tRNA on the cell-free translation of Rauscher murine leukemia virus, Moloney murine leukemia virus, and Moloney murine sarcoma virus 124 RNA

The effect of suppressor tRNA's on the cell-free translation of several leukemia and sarcoma virus RNAs was examined. Yeast amber suppressor tRNA (amber tRNA) enhanced the synthesis of the Rauscher murine leukemia virus and clone 1 Moloney murine leukemia virus Pr200(gag-pol) polypeptides by 10- to 45-fold, but at the same time depressed the synthesis of Rauscher murine leukemia virus Pr65(gag) and Moloney murine leukemia virus Pr63(gag). Under suppressor-minus conditions, Moloney murine leukemia virus Pr70(gag) was present as a closely spaced doublet. Amber tRNA stimulated the synthesis of the "upper" Moloney murine leukemia virus Pr70(gag) polypeptide. Yeast ochre suppressor tRNA appeared to be ineffective. Quantitative analyses of the kinetics of viral precursor polypeptide accumulation in the presence of amber tRNA showed that during linear protein synthesis, the increase in accumulated Moloney murine leukemia virus Pr200(gag-pol) coincided closely with the molar loss of Pr63(gag). Enhancement of Pr200(gag-pol) and Pr70(gag) by amber tRNA persisted in the presence of pactamycin, a drug which blocks the initiation of protein synthesis, thus arguing for the addition of amino acids to the C terminus of Pr63(gag) as the mechanism behind the amber tRNA effect. Moloney murine sarcoma virus 124 30S RNA was translated into four major polypeptides, Pr63(gag), P42, P38, and P23. In the presence of amber tRNA, a new polypeptide, Pr67(gag), appeared, whereas Pr63(gag) synthesis was decreased. Quantitative estimates indicated that for every 1 mol of Pr67(gag) which appeared, 1 mol of Pr63(gag) was lost.