Early protein essential for viral replication. Strong and global transcriptional activator of both viral and cellular genes. Activates transcription by blocking host retinoblastoma protein (RB) and inhibiting the SUMO pathway. Inhibition of host RB leads to the activation of E2F1-dependent transcription and, in particular, of E2F1-regulated S-phase genes. Stimulation of progression from G1 to S phase allows the virus to efficiently use the cellular DNA replicating machinery to achieve viral genome replication. Blocks the SUMO pathway by targeting the E1 enzyme (SAE1/UBA2 heterodimer) to the ubiquitin-proteasome machinery. Mediates SAE1 degradation possibly through the formation of complexes with either CUL2-elongin BC complex-RBX1 or CUL5-elongin BC complex-RBX1. The degradation of UBA2 is probably a consequent effect of SAE1 disappearance. Inhibits HDAC1 sumoylation, thereby interfering with histone deacetylation mediated by HDAC1, leading to activation of transcription. Mediates induction of heat-shock response. Seems to have an antiapoptotic function. Interacts with host HDAC1. Interacts with host E1-activating enzyme (SAE1/UBA2 heterodimer). Interacts with host retinoblastoma protein. Seems to form a complex with host E4F1 and HDAC1. Seems to form complexes with either CUL2-elongin BC complex-RBX1 or CUL5-elongin BC complex-RBX1. Interacts with TCEB1/Elongin-C, CUL2 and CUL5 in vitro. The BC-box-like motif could mediate the interaction with host cullin RING ubiquitin ligases.