Summary auto-generated
This study demonstrates that human papillomavirus type 8 (HPV8) E2 protein activates the p21WAF1/CIP1 promoter through a non-conserved hinge region that directly interacts with the cellular transcription factor Sp1. Using transfection assays in keratinocyte cells, researchers found that HPV8 E2 lacking its N-terminal activation domain (E2ΔN) showed dramatically enhanced cooperation with Sp1 in activating the p21 promoter, generating 24-fold activation compared to weaker activation by full-length E2. Notably, the hinge region alone was sufficient for this cooperative activation. The interaction was specific to HPV8; HPV18 E2ΔN did not show similar Sp1 cooperation despite having structural similarities. Biochemical binding assays confirmed that Sp1 directly binds to HPV8 E2's hinge region and DNA-binding domain. In contrast, for activation of classical E2-responsive promoters containing E2 binding sites, the N-terminal activation domain remained essential. The findings reveal a novel 'super-activation' mechanism where HPV8 E2's variable hinge region mediates direct protein-protein interaction with Sp1, potentially enhancing p21 expression and contributing to viral pathogenesis in skin keratinocytes.
Key findings
- HPV8 E2 protein lacking its N-terminal activation domain shows enhanced cooperation with Sp1 transcription factor, generating 24-fold activation of the p21 promoter
- The non-conserved hinge region of HPV8 E2 is both necessary and sufficient for functional interaction and direct binding to Sp1
- Sp1 directly binds to HPV8 E2's hinge region and DNA-binding domain, unlike HPV18 E2 where Sp1 binds only the activation domain and DNA-binding domain
- HPV8 E2's ability to activate p21 promoter occurs through Sp1 binding sites in the absence of E2 recognition sequences, suggesting a novel transactivation mechanism distinct from classical E2 function
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Abstract
The E2 proteins regulate papillomavirus (PV) gene expression by sequence-specific DNA binding. However, E2 is also able to activate in the absence of E2 binding sites. We show here that the E2 protein of human PV type 8 (HPV8) can activate the expression of p21WAF1/CIP1 via promoter-proximal 200 nucleotides, which contain several Sp1 binding sites and no E2 binding sites. HPV8 E2 lacking the activation domain, which is rather conserved among E2 proteins, cooperated with co-expressed Sp1 in stimulation of the p21WAF1/CIP1 promoter, in contrast to HPV18 E2 lacking the activation domain. We can demonstrate that the internal non-conserved hinge region of HPV8 E2 is sufficient for this functional cooperativity with Sp1. In correlation, the hinge of HPV8 E2 directly binds to Sp1. These results suggest that HPV8 E2 might be able to super-activate Sp1-mediated transcription by a direct interaction via the non-conserved hinge region.