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Polyomavirus large T mutants affected in retinoblastoma protein binding are defective in immortalization
A Larose1, N Dyson, M Sullivan
1Department of Microbiology, University of Sherbrooke, Quebec, Canada.
Abstract:
To clarify the relationship between the various activities of the polyomavirus large T antigen and the contribution of this oncogene to neoplastic transformation, we constructed a series of mutants with small deletions or single-amino-acid substitutions in two separate regions of the protein. These sequences were targeted because they showed considerable similarity to conserved regions 1 and 2 of adenovirus E1A which are thought to be binding sites for the retinoblastoma gene product (pRB). The pRB-binding properties of the large T mutants were assessed with an in vitro coimmunoprecipitation assay. pRB binding was readily detected with wild-type large T, but coprecipitation was completely abolished by as little as a single amino acid substitution (Asp-141----Glu or Glu-146----Asp) in region 2 of the polyomavirus large T antigen. Mutants defective in pRB binding were unable to immortalize primary rat embryo fibroblasts, suggesting that association with pRB is an important component of immortalization mediated by polyomavirus large T. The mutations in region 1 affected pRB binding only marginally, yet some of them severely impaired immortalization, indicating that pRB binding may be essential but not sufficient for immortalization.
Insights
Polyomavirus large T antigen
Area of Science:
- Oncogenic viral proteins
- Cellular transformation mechanisms
- Tumor suppressor interactions
Background:
- Polyomavirus large T antigen is an oncogene implicated in neoplastic transformation.
- Conserved regions 1 and 2 of the protein show similarity to adenovirus E1A.
- These regions are hypothesized to interact with the retinoblastoma gene product (pRB).
Purpose of the Study:
- To elucidate the link between polyomavirus large T antigen activities and neoplastic transformation.
- To investigate the role of pRB binding in the oncogenic functions of large T antigen.
Main Methods:
- Construction of polyomavirus large T antigen mutants with deletions or substitutions.
- Assessment of pRB-binding properties using in vitro coimmunoprecipitation assays.
- Evaluation of immortalization capacity in primary rat embryo fibroblasts.
Main Results:
- Single amino acid substitutions in region 2 abolished pRB binding.
- Mutants lacking pRB binding were unable to immortalize fibroblasts.
- Mutations in region 1 had minor effects on pRB binding but impaired immortalization.
Conclusions:
- pRB binding is crucial for polyomavirus large T antigen-mediated immortalization.
- pRB binding is necessary but not sufficient for the immortalization process.
- Understanding these interactions is key to deciphering oncogenesis by polyomavirus.