**6.2 SV40 large-T antigen**

172 DNA Repair

The Transcriptional Regulator Interacting with the PHD zinc finger and/or the Bromodomain-1 (TRIP-Br1) protein (also known as p34) is a transcriptional modulator that directly interacts with DP-1, as well as with the co-activators p300/CBP and KRIP1(Hsu et al., 2001). As such, TRIP-Br co-activates E2F responsive genes, such as B-myb, in U2OS osteosarcoma cells, an ability potentiated by KRIP1. This effect is impaired by pRb. TRIP-Br1 also interferes with deactivation of Cyclin D/Cdk4 complex by p16INK4, effectively

Cut homeobox 1 (CUX1) proteins are transcription factors that can either activate or repress transcription. In particular, the CUX1 isoform p110 can stably interact with DNA and promote entry into the S-phase of the cell cycle (Truscott et al., 2008). P110 CUX1 interacts with E2F-1 or E2F-2, stimulating their recruitment to the DNA polymerase α gene promoter, in a manner that requires ability of E2F to bind DNA. Further, common targets for E2F and p110 CUX1 include genes involved in cell cycle progression, DNA repair and

The transcriptional repressor YY1 can bind to target sites adjacent to E2F binding elements in the promoters of genes such as *Cdc6* (Schlisio et al., 2002). In addition, the YY1 accessory protein Ring1 and YY1 binding protein (RYBP) can interact with E2F-2, -3 and -4 to synergistically enhance binding of E2F-2 and -3 (but not of E2F-1). In this manner, YY1 and RYBP not only enhance the binding and transcription of E2F to certain promoters, but also

Studies of the p68 promoter have shown that transcription factor E3 (TFE3) operates in a similar manner to YY1. Thus, the E Box bound by TFE3 and the E2F consensus sequence occur in close proximity in the *p68* promoter. TFE3 and E2F-3 bind to those sites cooperatively (Giangrande et al., 2003; Giangrande et al., 2004). This interaction requires E2F-3, but not TFE3 binding to DNA. Although a direct interaction between these two proteins was not be demonstrated, these two factors likely work together in a larger protein

Viruses work by hijacking the cellular machinery of their host cell, to facilitate their replication. Hence, it is not surprising that constitutive activation of E2F, which induces cell transition into a state of DNA replication (S-phase), is a critical step in the viral modification

The human papillomaviruses (HPV) are commonly known oncoviruses. This notoriety is due to their ability to activate E2F proteins, causing rapid and unregulated progression through the cell cycle (Lee et al., 1998). HPV couples this action with deactivation of

complex, or interact temporarily to recruit one another to the *p68* promoter.

**6. Regulation of E2F activity by viral oncoproteins** 

**5.5 TRIP-BR1** 

**5.6 p110 CUX1** 

**5.7 YY1** 

add specificity.

of infected cell functions.

**6.1 Human papillomavirus protein E7** 

**5.8 TFE3** 

replication (Truscott et al., 2008).

activating E2F by inhibiting pRb (Sim et al., 2004).

The simian virus 40 (SV40) genome encodes a protein that shares some characteristics with HPV E7, termed large T-antigen. Similar to HPV E7, large-T antigen has a LXCXE domain, which can bind all three pRb family proteins, leading to release of free E2F and expression of its target genes (DeCaprio, 2009). In addition, large-T antigen binds preferentially to the hypophosphorylated form of pRb, present during the G1-phase of the cell cycle (Ludlow et al., 1989). The characterization of the interactions between large-T antigen and complexes containing p130 or p107 and E2F-4 has been central to understanding the mechanisms involved in deactivation of pRb family proteins by this viral factor (Sullivan et al., 2000). Dissociation of p107 or p130 from E2F also requires Large-T antigen interactions with the J type of chaperone protein Hsc70 and ATP.

Similar to HPV E7, large-T antigen binds to p300/CBP through its C-terminus (Eckner et al., 1996). This interaction is likely involved in histone acetylation and transcriptional activation of E2F target genes. Significantly, mutations in the C-termimus of large-T antigen impair its ability to bind p300/CBP, but are without effect on its capacity to disrupt pRb binding to E2F (Nemethova et al., 2004).

#### **6.3 Adenovirus E1A**

Adenovirus protein E1A functions in a similar manner to HPV E7 and SV40 large-T antigen. E1A interacts with multiple cellular proteins, including the pRb family and p300/CBP (Raychaudhuri, 1991; Liu, 2007). X-ray crystallographic characterization of E1A has revealed that its N-terminal domain competes with the transactivation domain of E2F for binding to pRb. This induces a decrease in E2F binding to pRb by competition (Liu, 2007). Similar to other viral oncoproteins, E1A also has an LXCXE domain that binds to pRb, p107 and p130 (Dyson, 1992). E1A also binds the 400-kDa protein p400, which mediates further interactions with TRRAP/PAF400, along with the DNA helicase TAP54α/β). Together, these proteins form a chromatin remodeling complex, which contributes to cell transformation and activation of E2F target genes that mediate viral DNA replication (Liu, 2007).
