Binding of Kaposi's sarcoma-associated herpesvirus K-bZIP to interferon-responsive factor 3 elements modulates antiviral gene expression.
Kaposi's sarcoma-associated herpesvirus encodes numerous regulatory proteins capable of modulating viral and cellular gene expression and affecting host cell functions. K-bZIP, a leucine zipper-containing transcription factor encoded by ORFK8, is one such protein. During infection, transcription of the ORFK8 early gene is turned on by the immediate-early replication and transcription factor activator (RTA). One described function of the K-bZIP nuclear protein is to interact with and repress RTA-mediated transactivation of viral promoters, including that of the K8 gene. In the present work, we provide evidence that the expression of K-bZIP results in the activation of the ifn-beta gene. Of interest, ifn-beta gene activation by K-bZIP is independent of interferon (IFN)-responsive factor 3 (IRF-3) and nuclear factor kappaB (NF-kappaB) activation. Using a DNA binding affinity assay and electromobility shift assay, we report that K-bZIP binds efficiently to the PRDIII-I region of the beta IFN (IFN-beta) promoter, and, in doing so, it prevents the attachment of activated IRF-3 but not that of NF-kappaB or ATF2/c-Jun to the IFN-beta promoter sequence. As a consequence, ifn-beta gene activation in response to IFN inducers such as Sendai virus infection or expression of retinoic acid-inducible gene I, mitochondrial antiviral signaling protein, or TANK-binding kinase 1 (TBK-1) is severely impaired (>90%) by the presence of K-bZIP. K-bZIP also prevents the activation of RANTES and CXCL11, whose promoters are also regulated by IRF-3. Lysine 158 (target for SUMO conjugation), threonine 111, and serine 167 (targets for phosphorylation) mutants of K-bZIP were equally effective as wild-type K-bZIP in mediating the repression of TBK-1-activated ifn-beta gene expression. Lastly, the overexpression of CREB binding protein could not reverse the K-bZIP repression of TBK-1-activated ifn-beta gene expression. In all, our results indicate that K-bZIP binds directly to the PRDIII-I region of the IFN-beta promoter and, as a consequence, causes a low level of ifn-beta gene transcription. In doing so, K-bZIP prevents IRF-3 from binding to the IFN-beta promoter and precludes the formation of the enhanceosome, which is required for maximal ifn-beta gene transcription. A new role for K-bZIP as a protein involved in immune evasion is therefore uncovered.
Lefort
Sylvain
S
Rheumatology and Immunology Research Center, Room T1-49, 2705 Laurier Blvd., Quebec, Quebec G1V 4G2, Canada.
Soucy-Faulkner
Anton
A
Grandvaux
Nathalie
N
Flamand
Louis
L
D050976:Basic-Leucine Zipper Transcription Factors / Q000378:metabolism / Q000502:physiology*; D001665:Binding Sites; D002460:Cell Line; D005786:Gene Expression Regulation / Q000276:immunology*; D019288:Herpesvirus 8, Human / Q000276:immunology / Q000472:pathogenicity*; D006801:Humans; D050838:Interferon Regulatory Factor-3 / Q000378:metabolism*; D016899:Interferon-beta / Q000235:genetics*; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D015533:Transcriptional Activation; D014764:Viral Proteins / Q000502:physiology
D050976:Basic-Leucine Zipper Transcription Factors; D050838:Interferon Regulatory Factor-3; D014764:Viral Proteins; D016899:Interferon-beta