Adenovirus E4 open reading frame 4-induced dephosphorylation inhibits E1A activation of the E2 promoter and E2F-1-mediated transactivation independently of the retinoblastoma tumor suppressor protein.
Previous studies have shown that the cell cycle-regulated E2F transcription factor is subjected to both positive and negative control by phosphorylation. Here we show that in transient transfection experiments, adenovirus E1A activation of the viral E2 promoter is abrogated by coexpression of the viral E4 open reading frame 4 (E4-ORF4) protein. This effect does not to require the retinoblastoma protein that previously has been shown to regulate E2F activity. The inhibitory activity of E4-ORF4 appears to be specific because E4-ORF4 had little effect on, for example, E4-ORF6/7 transactivation of the E2 promoter. We further show that the repressive effect of E4-ORF4 on E2 transcription works mainly through the E2F DNA-binding sites in the E2 promoter. In agreement with this, we find that E4-ORF4 inhibits E2F-1/DP-1-mediated transactivation. We also show that E4-ORF4 inhibits E2 mRNA expression during virus growth. E4-ORF4 has previously been shown to bind to and activate the cellular protein phosphatase 2A. The inhibitory effect of E4-ORF4 was relieved by okadaic acid, which inhibits protein phosphatase 2A activity, suggesting that E4-ORF4 represses E2 transcription by inducing transcription factor dephosphorylation. Interestingly, E4-ORF4 did not inhibit the transactivation capacity of a Gal4-E2F hybrid protein. Instead, E4-ORF4 expression appears to result in reduced stability of E2F/DNA complexes.
Mannervik
M
M
BMC, Uppsala University, Box 582, Uppsala, 751 23, Sweden.
Fan
S
S
Ström
A C
AC
Helin
K
K
Akusjärvi
G
G
D017500:Adenovirus E1A Proteins / Q000235:genetics*; D017502:Adenovirus E2 Proteins / Q000235:genetics*; D017504:Adenovirus E4 Proteins / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins / Q000235:genetics; D019281:Dimerization; D029721:Drosophila Proteins; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; D015967:Gene Expression Regulation, Viral; D006367:HeLa Cells; D006801:Humans; D016366:Open Reading Frames; D010766:Phosphorylation; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger; D016160:Retinoblastoma Protein / Q000378:metabolism*; D056465:Retinoblastoma-Binding Protein 1; D015534:Trans-Activators / Q000235:genetics; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000378:metabolism*; D015533:Transcriptional Activation; D014407:Tumor Cells, Cultured
D017500:Adenovirus E1A Proteins; D017502:Adenovirus E2 Proteins; D017504:Adenovirus E4 Proteins; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; C088053:Dp transcription factor, Drosophila; D029721:Drosophila Proteins; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; C493753:E2F1 protein, human; D012333:RNA, Messenger; D016160:Retinoblastoma Protein; D056465:Retinoblastoma-Binding Protein 1; D015534:Trans-Activators; D050680:Transcription Factor DP1; D014157:Transcription Factors