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Phosphorylation of adenovirus E1A proteins by the p34cdc2 protein kinase
1Department of Pathology, McMaster University, Hamilton, Ontario, Canada.
Abstract:
Adenovirus early region 1A (E1A) products are phosphorylated nuclear oncoproteins which appear to derive transforming activity largely through interactions with cellular proteins including the tumor suppressor p105/Rb-1 and cyclin A (p60cycA), a regulatory subunit associated with p34cdc2 and the related protein kinase p33cdk2. We have identified several sites of phosphorylation on E1A proteins previously and showed that phosphorylation at Ser-89 alters electrophoretic mobility significantly and affects E1A-mediated transforming activity to some extent. We now report that both Ser-89 and Ser-219, the major E1A phosphorylation site, were phosphorylated in vitro by p34cdc2 purified from HeLa cells. We also found that E1A proteins seemed to be phosphorylated at the highest levels in vivo in mitotic cells which express maximal levels of p34cdc2 kinase activity. Thus, in addition to forming complexes with p60cycA, a regulator of p34cdc2 and related kinases, and p105/Rb-1 which exhibits cell cycle-dependent phosphorylation, E1A proteins seem to be substrates for p34cdc2. These data suggested that a link could exist between phosphorylation, cell cycle progression, and the regulation of transforming activity of E1A proteins.
Insights
Adenovirus E1A oncoproteins are phosphorylated by p34cdc2, a key cell cycle kinase. This phosphorylation, particularly at Ser-219, links E1A activity to cell cycle progression and its transforming potential.
Area of Science:
- Molecular Biology
- Cell Biology
- Oncology
Background:
- Adenovirus early region 1A (E1A) proteins are nuclear oncoproteins.
- E1A proteins interact with cellular proteins like p105/Rb-1 and cyclin A (p60cycA).
- Phosphorylation of E1A, notably at Ser-89, affects its transforming activity.
Purpose of the Study:
- To investigate the role of p34cdc2 in E1A phosphorylation.
- To determine if E1A proteins are direct substrates of p34cdc2.
- To explore the link between E1A phosphorylation, cell cycle, and transforming activity.
Main Methods:
- In vitro phosphorylation assays using purified p34cdc2 from HeLa cells.
- Analysis of E1A phosphorylation levels in vivo during different cell cycle stages.
- Correlation of E1A phosphorylation with p34cdc2 kinase activity.
Main Results:
- Both Ser-89 and the major site Ser-219 of E1A were phosphorylated in vitro by p34cdc2.
- E1A proteins showed highest phosphorylation levels in mitotic cells, coinciding with maximal p34cdc2 activity.
- E1A proteins appear to be substrates for p34cdc2, in addition to interacting with cell cycle regulators.
Conclusions:
- E1A proteins are phosphorylated by the cell cycle-dependent kinase p34cdc2.
- This phosphorylation suggests a direct link between E1A activity, cell cycle progression, and its oncogenic functions.
- Further research into E1A-p34cdc2 interactions could reveal novel therapeutic targets for adenovirus-associated cancers.