Kaposi's sarcoma-associated herpesvirus K-bZIP is a coregulator of K-Rta: physical association and promoter-dependent transcriptional repression.
Kaposi's sarcoma-associated herpesvirus (KSHV) is a human gammaherpesvirus that has been implicated in the pathogenesis of Kaposi's sarcoma and B-cell neoplasms. The genomic organization of KSHV is similar to that of Epstein-Barr virus (EBV). EBV encodes two transcriptional factors, Rta and Zta, which functionally interact to transactivate EBV genes during replication and reactivation from latency. KSHV encodes a basic leucine zipper protein (K-bZIP), a homologue of EBV Zta, and K-Rta, the homologue of EBV Rta. EBV Rta and Zta are strong transcriptional transactivators. Although there is ample evidence that K-Rta is a potent transactivator, the role of K-bZIP as a transcriptional factor is much less clear. In this study, we report that K-bZIP modulates K-Rta function. We show that K-bZIP directly interacts with K-Rta in vivo and in vitro. This association is specific, requiring the basic domain (amino acids 122 to 189) of K-bZIP and a specific region (amino acids 499 to 550) of K-Rta, and can be detected with K-bZIP and K-Rta endogenously expressed in BCBL-1 cells treated with tetradecanoyl phorbol acetate. The functional relevance of this association was revealed by the observation that K-bZIP represses the transactivation of the ORF57 promoter by K-Rta in a dose-dependent manner. K-bZIP lacking the interaction domain fails to repress K-Rta-mediated transactivation; this finding attests to the specificity of the repression. Interestingly, this repression is not observed for the promoter of polyadenylated nuclear (PAN) RNA, another target of K-Rta; thus, repression is promoter dependent. Finally, we provide evidence that the modulation of K-Rta by K-bZIP also occurs in vivo during reactivation of the viral genome in BCBL-1 cells. When K-bZIP is overexpressed in BCBL-1 cells, the level of expression of ORF57 but not PAN RNA is repressed. These data support the model that one function of K-bZIP is to modulate the activity of the transcriptional transactivator K-Rta.
Izumiya
Yoshihiro
Y
Department of Biological Chemistry, School of Medicine, University of California, Davis, UC Davis Cancer Center, Sacramento 95817, USA.
Lin
Su-Fang
SF
Ellison
Thomas
T
Chen
Ling-Yu
LY
Izumiya
Chie
C
Luciw
Paul
P
Kung
Hsing-Jien
HJ
D000595:Amino Acid Sequence; D001483:Base Sequence; D050976:Basic-Leucine Zipper Transcription Factors; D002460:Cell Line; D017931:DNA Primers; D004261:DNA Replication; D004279:DNA, Viral / Q000096:biosynthesis; D004268:DNA-Binding Proteins / Q000378:metabolism / Q000502:physiology*; D051790:G-Box Binding Factors; D019288:Herpesvirus 8, Human / Q000254:growth & development / Q000378:metabolism* / Q000502:physiology; D017874:Immediate-Early Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D015534:Trans-Activators / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D014157:Transcription Factors / Q000378:metabolism / Q000502:physiology*; D014158:Transcription, Genetic / Q000502:physiology*; D014764:Viral Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D014775:Virus Activation; D014779:Virus Replication
D050976:Basic-Leucine Zipper Transcription Factors; D017931:DNA Primers; D004279:DNA, Viral; D004268:DNA-Binding Proteins; D051790:G-Box Binding Factors; D017874:Immediate-Early Proteins; C417604:Rta protein, Human herpesvirus 8; D015534:Trans-Activators; D014157:Transcription Factors; D014764:Viral Proteins