T antigens encoded by replication-defective simian virus 40 mutants dl1135 and 5080

Area of Science:

• Virology
• Molecular Biology
• Biochemistry

Background:

• Simian virus 40 (SV40) T antigen is crucial for viral DNA replication.
• Mutants dl1135 and 5080 exhibit defects in in vivo viral DNA replication.
• Previous studies linked these mutants to cell transformation and tumor induction.

Purpose of the Study:

• To biochemically characterize SV40 T antigen mutants dl1135 and 5080.
• To elucidate the molecular basis for their in vivo replication defects.
• To assess the impact of mutations on T antigen's replication-associated functions.

Main Methods:

• Expression and purification of mutant T antigen proteins using the baculovirus system and immunoaffinity chromatography.
• Biochemical assays to evaluate ATPase, DNA binding, helicase, and unwinding activities.
• In vitro replication assays to assess viral DNA synthesis.

Main Results:

• The 5080 mutant protein showed defects in ATPase, helicase, ori-specific unwinding, and ATP-induced hexamerization, but retained some DNA binding.
• The dl1135 mutant protein demonstrated significant activity in all tested biochemical assays, including in vitro SV40 DNA replication.
• dl1135 can replicate DNA in vitro but not in vivo, unlike 5080.

Conclusions:

• The 5080 mutation directly affects T antigen enzymatic functions essential for viral DNA synthesis.
• The dl1135 mutation may impact an activity required for cellular preparation, rather than direct DNA synthesis.
• These findings differentiate the molecular mechanisms underlying the in vivo replication defects of SV40 mutants.