The High-Risk HPV16 E7 Oncoprotein Mediates Interaction between the Transcriptional Coactivator CBP and the Retinoblastoma Protein pRb

Abstract

The oncoprotein E7 from human papillomavirus (HPV) strains that confer high cancer risk mediates cell transformation by deregulating host cellular processes and activating viral gene expression through recruitment of cellular proteins such as the retinoblastoma protein (pRb) and the CREB-binding protein (CBP) and its paralog p300. Here we show that the intrinsically disordered N-terminal region of E7 from high risk HPV16 binds the TAZ2 domain of CBP with greater affinity than E7 from low risk HPV6b. HPV E7 and the tumor suppressor p53 compete for binding to TAZ2. The TAZ2 binding site in E7 overlaps the LxCxE motif that is crucial for interaction with pRb. While TAZ2 and pRb compete for binding to a monomeric E7 polypeptide, the full-length E7 dimer mediates an interaction between TAZ2 and pRb by promoting formation of a ternary complex. Cell-based assays show that expression of full-length HPV16 E7 promotes increased pRb acetylation and that this response depends both on the presence of CBP/p300 and the ability of E7 to form a dimer. These observations suggest a model for the oncogenic effect of high risk HPV16-E7. The disordered region of one E7 molecule in the homodimer interacts with the pocket domain of pRb, while the same region of the other E7 molecule binds the TAZ2 domain of CBP/p300. Through its ability to dimerize, E7 recruits CBP/p300 and pRb into a ternary complex, bringing the histone acetyltransferase domain of CBP/p300 into proximity to pRb and promoting acetylation, leading to disruption of cell cycle control.

Recognition of the disordered p53 transactivation domain by the transcriptional adapter zinc finger domains of CREB-binding protein

“…Structures have been determined for TAZ2 bound to the activation domains of STAT1 (49), and C/EBP (51), both of which bind through helical structures to the same hydrophobic surface as the p53 AD2 motif. The adenovirus oncoprotein E1A and oncoprotein E7 from high-risk strains of human papillomavirus also bind to the same surface on TAZ2 as the AD2 motif of p53, but with higher affinity (E1A, 3 nM; phosphorylated E7, 7 nM; p53, 26 nM) (25,52,72). By binding with higher affinity to the same region of TAZ2, the viral proteins can out-compete p53 for binding to CBP/p300 and thereby inhibit activation of p53-mediated stress response or apoptotic pathways.…”

Role of Intrinsic Protein Disorder in the Function and Interactions of the Transcriptional Coactivators CREB-binding Protein (CBP) and p300

“…The CR1 and CR2 regions of E7 are intrinsically disordered (91,92), whereas CR3 is a zinc binding domain that mediates formation of a homodimer (93,94). The disordered CR1 and CR2 regions of E7 from high risk HPV16 bind with high affinity to the TAZ2 domain of CBP (95). Although the TAZ2 binding site overlaps the binding site for pRb, the full-length E7 dimer functions as a scaffold to recruit TAZ2 and the pRb pocket domains into a ternary complex.…”

“…Although the TAZ2 binding site overlaps the binding site for pRb, the full-length E7 dimer functions as a scaffold to recruit TAZ2 and the pRb pocket domains into a ternary complex. By bringing the CBP/p300 HAT domain into proximity to pRb, the E7 dimer stimulates acetylation and subsequent degradation of pRb, and drives S phase entry and deregulation of the host cell cycle (95). In contrast, E7 from the low risk strain HPV6b binds TAZ2 with much lower affinity than high risk HPV16 E7.…”

“…In contrast, E7 from the low risk strain HPV6b binds TAZ2 with much lower affinity than high risk HPV16 E7. This would impair the ability of HPV6b to suppress p53-mediated transcriptional programs and disfavor formation of a ternary complex with CBP/p300 and pRb (95).…”

Convergent evolution and mimicry of protein linear motifs in host–pathogen interactions

“…This ternary complex formed between the host CBP and retinoblastoma proteins and the adenovirus E1A protein shows either positive or negative cooperativity, depending on the available E1A interaction sites [37 ]. On the other hand, the dimeric E7 protein displaces the host targets of Convergent evolution of pathogen linear motif mimics Chemes, de Prat-Gay and Sá nchez 93 [33 ]. However, while displacement of the E2F transcription factors by both viral proteins induces S-phase reentry, pathogen-specific effects include E1A-induced acetylation and E7 mediated degradation of the retinoblastoma protein [38].…”