Promotes efficient viral genome replication by modulating several host signaling pathways including transport network, interferon production or cell cycle progression (PubMed:20485545, PubMed:33092197, PubMed:33849980). Inhibits host PP2A phosphatase activity and thereby prevents agnoprotein dephosphorylation (PubMed:20485545, PubMed:18353419). Inactivation of PP2A also results in the transactivation of cyclin A and cyclin D1 promoters (By similarity). In addition, antagonizes the RIG-I/DDX58-mediated IFN response through interaction with E3 ligase TRIM25 leading to the inhibition of 'Lys-63'-linked ubiquitination of DDX58. Inhibits nucleotide excision repair (NER) pathway which leads to DNA strand breaks during DNA replication and micronuclei formation (PubMed:25744060). Interacts with host PPP2R1A; the interaction inhibits PP2A activity (PubMed:20485545). Interacts with agnoprotein; this interaction prevents agnoprotein dephosphorylation by host PP2A (PubMed:18353419). Interacts with host RBL1 and RBL2 (PubMed:20485545). Interacts with SMARCA5 (PubMed:33092197). Interacts with SDHB (PubMed:33092197). 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.