Promotes efficient viral genome replication by accelerating both G1 and S phase progression of the cell cycle. Inhibits host PP2A by binding to the A subunit, thereby displacing lower affinity regulatory B subunit. Inactivation of PP2A in turn results in the transactivation of cyclin A and cyclin D1 promoters. Late during the infection cycle, ST may induce dephosphorylation of host MTOR, leading to the inhibition of cap-dependent translation. May establish and maintain high levels of viral genomes during persistent infection in cell culture. Interacts with host PPP2R1A; the interaction inhibits PP2A activity. The common region of ST and LT proteins comprises the J domain. This domain is essential for multiple viral activities, including virion assembly, viral DNA replication, transformation and transcriptional activation. This domain is also required for cyclin A-transactivating activity of ST. The strain Dunlop sequence is shown.