THE BZIP TRANSCRIPTION FACTOR VITELLOGENIN-BINDING PROTEIN IS POST TRANSCRIPTIONAL DOWN-REGULATED IN CHICKEN LIVER

The vitellogenin-binding protein (VBP) is a member of the proline and acidic-region rich (PAR) family of bZip transcription factors. PAR is located N-terminally to the DNA-binding domain. VBP binds to specific sites within the 300-bp 5'-flanking region of the chicken-liver-specif ic estrogen-dependent very-low-density apolipoprotein gene (apoVLDL II ). One of these binding sites (site D) resembles the albumin site D an ti is positioned in close proximity of the major estrogen-responsive e lement. Previous studies showed th;lt VBP can bind simultaneously with the estrogen receptor to the putative complex regulatory element E1D. To investigate whether VBP is involved in apoVLDL II gene expression, we examined its capacity to enhance apoVLDL II transcription and its presence in liver. We show that VBP is capable of enhancing transcript ion in transfection experiments. However, VBP could not be detected in liver by Western-blots or immuno-electro mobility shift assays (EMSAs ) using antibodies against different moieties of the protein. We exami ned the possible reduction in translation efficienies due to a small u pstream open reading frame in the VBP leader sequence, but did not fin d any. Although VBP binds to the proximal apoVLDL II promoter region a nd enhances transcription in co-transfection experiments, the protein is unlikely to be involved in apoVLDL II gene transcription because of its undetectable low level in liver nuclei.