Mechanism of interaction of the hepatitis B viral X protein (pX) with the CREB/bZIP family of transcription factors

Sangeeta John Barnabas, Purdue University

Abstract

The 16.5 KDa X protein, pX, expressed by the hepatitis B virus, is a multifunctional protein exhibiting different functions affecting transcription, cell growth and apoptosis. This study has investigated one aspect of pX-mediated transactivation involving the functional interaction of pX with the bZip class of transcription factors. Direct protein-protein interactions are demonstrated both in vitro and in vivo between pX and the bZip transcription factors CREB, ATF3, NF-IL6 and ICER IIγ, all of which play significant roles in liver physiology. This direct interaction of pX with the bZip transcription factors effects increased DNA-binding for their cognate binding site in vitro, resulting in increased transcriptional efficacy of the studied bZip factors in vivo. The mechanism of the bZip/pX interaction employs a common region of pX, determined by deletional analyses. Mutants pX49–140 and pX49–115 bind CREB, ATF3, ICER IIγ and NF-IL6 in vitro, comparable to wt pX, importantly this minimal region enhances the cAMP response element binding potential of CREB in vitro. Accordingly, the region of pX spanning amino acids 49 to 115 is delineated as the minimal region of pX required for direct and functional CREB/bZip interaction in vitro. Transfections in AML12 hepatocytes demonstrate that only pX49–140 increases the transcriptional efficacy of CREB; by contrast, both pX deletions increase equally the repression efficacy of repressors ATF3 and ICER IIγ in vivo, suggesting that amino acids 115 to 140 are crucial in bridging the activator CREB/CRE complex to the basal transcription apparatus. We conclude, firstly, that the promiscuity of pX in interacting with CREB/bZip family members involves a common mechanism of pX/bZip interactions, via the minimal pX49–115 region; secondly, the increased transcriptional efficacy of bZip transactivators effected by pX in vivo requires two pX-dependent events, i.e., the pX-dependent increase of their DNA-binding potential, coupled with their pX-dependent integration to the basal transcriptional apparatus.

Degree

Ph.D.

Advisors

Andrisani, Purdue University.

Subject Area

Molecular biology|Biochemistry|Microbiology

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