Adenovirus E4 open reading frame 4 protein autoregulates E4 transcription by inhibiting E1A transactivation of the E4 promoter

Here we show that the adenovirus early region 4 (E4) open reading frame 4 (ORF4) protein autoregulates its own transcription by inhibiting adenovirus E1A-induced activation of E4 transcription both in transient transfection experiments and during lytic virus growth. The inhibitory activity of E4-ORF4 was selective for E1A-CR3-dependent transactivation and had no effect on CR1 transactivation. The inhibitory activity of E4-ORF4 was relieved by okadaic acid treatment, which inhibits the cellular protein phosphatase 2A (PP2A), suggesting that E4-ORF4 controls the phosphorylated status of transcription factors important for E4 promoter activity. This conclusion agrees with previous demonstrations that E4-ORF4 associates with PP2A and causes a partial dephosphorylation of certain transcription factors, including E1A (U. Müller, T. Kleinberger, and T. Shenk, J. Virol. 66:5869-5878, 1992; T. Kleinberger and T. Shenk, J. Virol. 67:7556-7560, 1993). However, our results indicate that dephosphorylation of E1A itself might not be the primary target for E4-ORF4. Instead, the E4-ORF4-PP2A complex appears to work by dephosphorylation of multiple cellular transcription factors that are involved in E1A transactivation of the E4 promoter.