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Maturation of mouse mammary tumor virus envelope protein is blocked by a specific point mutation
R M Bedgood1, L D Snider, M R Stallcup
1Department of Pathology, University of Southern California Health Sciences Center, Los Angeles 90033.
Abstract:
Expression of a mouse mammary tumor virus (MMTV) envelope gene, ENV2, in COS-7 cells resulted in the synthesis of a precursor protein Pr74 that underwent complete proteolytic processing to the mature envelope species gp52 and gp33. In contrast, expression of a second MMTV envelope gene, ENV1, in COS-7 cells resulted in the synthesis of a precursor protein Pr74 that failed to undergo proteolytic processing to the mature products. Expression and sequencing of various chimeric ENV1/ENV2 genes determined that a single glycine-to-glutamic acid mutation at position 54 was responsible for the nonprocessable phenotype of ENV1-encoded Pr74. The significance of this point mutation in relation to the requirements for precursor protein folding and maturation are discussed.
Insights
A single mutation in the mouse mammary tumor virus (MMTV) envelope gene ENV1 prevents precursor protein processing, unlike the functional ENV2 gene. This finding highlights the critical role of specific amino acids in viral protein maturation.
Area of Science:
- Virology
- Molecular Biology
- Protein Biochemistry
Background:
- Mouse mammary tumor virus (MMTV) envelope proteins are synthesized as precursor proteins.
- Proteolytic processing is essential for generating mature, functional MMTV envelope glycoproteins.
Purpose of the Study:
- To investigate the molecular basis for differential processing of two MMTV envelope genes, ENV1 and ENV2.
- To identify the specific mutations responsible for the non-processing phenotype of ENV1.
Main Methods:
- Expression of MMTV envelope genes (ENV1, ENV2, and chimeric constructs) in COS-7 cells.
- Analysis of precursor protein synthesis and proteolytic processing using protein gel electrophoresis.
- Gene sequencing to identify mutations in the ENV1 gene.
Main Results:
- Expression of ENV2 resulted in a precursor protein (Pr74) that was fully processed into mature gp52 and gp33.
- Expression of ENV1 produced a Pr74 precursor that failed to undergo proteolytic cleavage.
- A single glycine-to-glutamic acid substitution at position 54 in ENV1 was identified as the cause of the non-processing defect.
Conclusions:
- A specific point mutation in the MMTV ENV1 gene significantly impacts precursor protein folding and maturation.
- This mutation disrupts the proteolytic processing pathway, preventing the formation of mature envelope glycoproteins.
- Understanding these molecular determinants is crucial for comprehending MMTV replication and pathogenesis.