Activation of the HPV16 late promoter by viral E2 and cellular BRD4 and ZC3H4 proteins.

High-risk human papillomaviruses (HPV), particularly HPV16, are major causes of anogenital and oropharyngeal cancers. The HPV late promoter, P670 in the case of HPV16, is activated upon host cell differentiation and drives the expression of viral capsid proteins. While differentiation-specific host transcription factors have been implicated in regulating this promoter, the mechanism remains incompletely understood. HPV E2 proteins activate transcription by interacting with the host protein BRD4 (Bromodomain-containing protein 4). A biotin proximity ligation screen identified several novel E2 interactors, of which many overlap with the BRD4 interactome, suggesting BRD4 mediates a large fraction of these interactions. One such interactor, ZC3H4 (Zinc finger CCCH domain-containing protein 4), is known to restrict the expression of long non-coding RNAs, including enhancer and promoter upstream antisense RNAs (uaRNAs). E2 recruits ZC3H4 in a BRD4-dependent manner to specifically activate the P670 promoter in reporter assays. Supporting this, E2 and ZC3H4 co-localize in cells with high P670 activity. ZC3H4 is upregulated during differentiation, and its knockdown in differentiated HPV16- or HPV31-positive cells reduces late viral transcripts in an E2-BRD4-dependent manner. Interestingly, knockdown of ZC3H4 does not increase viral uaRNAs, suggesting that ZC3H4 does not enhance HPV late transcription by regulating viral antisense transcription. IMPORTANCE: High-risk human papillomaviruses (HPVs), particularly HPV16, can cause anogenital and oropharyngeal cancers. HPV16 relies on the differentiation-dependent activation of its late promoter, P670, to produce capsid proteins. While host transcription factors contribute to this regulation, the mechanisms remain incompletely defined. Our findings reveal that the viral E2 protein collaborates with the host protein BRD4-a critical transcriptional regulator-to recruit other cellular partners, such as ZC3H4. Normally, ZC3H4 suppresses non-coding RNAs in cells, but HPV16 repurposes it via BRD4 to activate P670. This interaction intensifies in differentiated cells, where ZC3H4 levels rise, and disrupting ZC3H4 specifically blocks late viral gene expression without affecting antisense viral transcription. This highlights a unique, differentiation-dependent strategy HPV16 uses to hijack host machinery for its replication.