Phosphorylation of polyoma T antigens

Area of Science:

• Virology
• Molecular Biology
• Biochemistry

Background:

• Polyoma virus T antigens play crucial roles in viral replication and cellular transformation.
• Understanding the post-translational modifications, such as phosphorylation, of T antigens is essential for elucidating their functions.
• Previous studies have indicated potential kinase activity associated with polyoma virus T antigens.

Purpose of the Study:

• To investigate the phosphorylation status of polyoma virus T antigens in vivo.
• To characterize the associated protein kinase activities of T antigens in vitro.
• To determine the role of hr-t mutations in T antigen phosphorylation and kinase activity.

Main Methods:

• Labeling of polyoma virus-infected cells with 32P-orthophosphate to assess in vivo phosphorylation.
• In vitro kinase assays using T antigen immunoprecipitates and gamma-32P-ATP to measure associated kinase activity.
• Photoaffinity labeling with 8-azido cyclic AMP to detect cyclic AMP-dependent protein kinase subunits.
• Analysis of T antigen phosphorylation and kinase activity in wild-type and hr-t mutant viruses.

Main Results:

• The 100K large T antigen is the major phosphoprotein in vivo, while the 56K middle T antigen is phosphorylated to a lesser extent.
• Hr-t mutants exhibit normal in vivo phosphorylation of their T antigens but show defective in vitro protein kinase activity.
• Wild-type middle T antigen is the major phosphate acceptor in vitro, whereas hr-t mutants show reduced or absent labeling of altered middle T antigens.
• Cellular kinases, including cyclic AMP-dependent protein kinases, are present in T antigen immunoprecipitates.

Conclusions:

• Polyoma virus hr-t mutants are defective in an associated protein kinase activity, suggesting that middle T antigen may be involved in kinase function or regulation.
• While T antigens are phosphorylated in vivo, their in vitro kinase activity appears distinct and is affected by hr-t mutations.
• Further studies are needed to definitively determine if middle T antigen is a protein kinase or a substrate for cellular kinases.