Isoform large T antigen is a key early protein essential for both driving viral replication and inducing cellular transformation. Plays a role in viral genome replication by driving entry of quiescent cells into the cell cycle and by autoregulating the synthesis of viral early mRNA. Displays highly oncogenic activities by corrupting the host cellular checkpoint mechanisms that guard cell division and the transcription, replication, and repair of DNA. Participates in the modulation of cellular gene expression preceeding viral DNA replication. This step involves binding to host key cell cycle regulators retinoblastoma protein RB1/pRb and TP53. Induces the disassembly of host E2F1 transcription factors from RB1, thus promoting transcriptional activation of E2F1-regulated S-phase genes. Inhibits host TP53 binding to DNA, abrogating the ability of TP53 to stimulate gene expression. Plays the role of a TFIID-associated factor (TAF) in transcription initiation for all three RNA polymerases, by stabilizing the TBP-TFIIA complex on promoters. Initiates viral DNA replication and unwinding via interactions with the viral origin of replication. Binds two adjacent sites in the SV40 origin. The replication fork movement is facilitated by Large T antigen helicase activity. Activates the transcription of viral late mRNA, through host TBP and TFIIA stabilization. Interferes with histone deacetylation mediated by HDAC1, leading to activation of transcription (By similarity). Forms homohexamers in the presence of ATP. Interacts with host HDAC1. Interacts (via LXCXE domain) with host RB1; the interaction induces the aberrant dissociation of RB1-E2F1 complex thereby disrupting RB1's activity. Interacts (via LXCXE domain) with host pRB-related proteins RBL1 and RBL2. Interacts (via C-terminus) with host TOP1 and POLA1 allowing DNA replication. Interacts with host preinitiation complex components TBP, TFIIA and TFIID to regulate transcription initiation (By similarity). The J domain is essential for multiple viral activities, including virion assembly, viral DNA replication, transformation and transcriptional activation. The LXCXE motif specifically binds to host pRB, RBL1, and RBL2. The zinc finger region contributes to protein-protein interactions essential for the assembly of stable T-antigen hexamers at the origin of replication. The hexamers are required for subsequent alterations in the structure of origin DNA (By similarity). The ATP binding/ATPase domain is required for proper hexamer assembly and helicase activity. Phosphorylated on both serine and threonine residues. Small t antigen inhibits the dephosphorylation by the AC form of PP2A (By similarity). O-Glycosylated near the C-terminal region. Acetylated by CBP in a TP53-dependent manner.