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The endogenous mouse mammary tumour virus locus Mtv-8 contains a defective envelope gene
Abstract:
The Mtv-8 associated provirus, GR40, is not expressed in vivo. However, upon transfection into rat XC cells, the two MMTV specific mRNAs of 35S and 24S are transcribed. Further, the level of these transcripts is augmented when the transfected cells are grown in the presence of dexamethasone (Ponta et al., 1983). No virus can be detected in the medium of the transfected cells. Intracellular protein analysis of these transfected cells shows that although apparently authentic gag proteins are synthesized, a novel protein of 68 kDa is the only env related protein detectable. The sequence of the env gene of GR40 was determined and the predicted amino acid sequence of the env protein obtained. A premature termination codon is present 68 amino acids before the COOH-terminus of the known MMTV env precursor Pr73env. This would result in an env protein of about 68 kDa, in agreement with the size of the env protein found in GR40 transfected cells. The lack of the carboxy terminus may be responsible for the non-processing of the aberrant env precursor protein, p68.
Insights
The GR40 provirus, associated with MMTV-8, is not expressed in vivo. However, GR40 transfection into rat XC cells produces MMTV mRNAs and a truncated 68 kDa env protein, suggesting a premature termination codon.
Area of Science:
- Virology
- Molecular Biology
- Genetics
Background:
- Mouse mammary tumor virus (MMTV) is a retrovirus implicated in mammary carcinogenesis.
- The GR40 provirus, derived from MMTV-8, has been studied for its expression and protein products.
Purpose of the Study:
- To investigate the expression of the GR40 provirus in transfected cells.
- To characterize the MMTV-specific mRNAs and proteins produced by GR40.
- To determine the molecular basis for the truncated env protein observed.
Main Methods:
- Transfection of rat XC cells with the GR40 provirus.
- Analysis of MMTV-specific mRNA transcripts using Northern blotting.
- Intracellular protein analysis via Western blotting.
- DNA sequencing of the GR40 env gene.
Main Results:
- GR40 provirus transfection into rat XC cells resulted in transcription of 35S and 24S MMTV mRNAs.
- Dexamethasone treatment augmented MMTV transcript levels.
- Authentic gag proteins and a novel 68 kDa env-related protein were detected intracellularly.
- Sequencing revealed a premature termination codon in the GR40 env gene, predicting a 68 kDa env protein.
Conclusions:
- The GR40 provirus expresses MMTV-specific mRNAs and a truncated env protein in transfected cells.
- The truncated env protein (p68) results from a premature termination codon.
- The absence of the carboxy terminus likely prevents proper processing of the aberrant env precursor protein.