Apolipoprotein B Gene Regulatory Factor-2 (BRF-2) Is Structurally and Immunologically Highly Related to Hepatitis B Virus X Associated Protein-1 (XAP-1)†

Hepatic cell-specific expression of the human apolipoprotein B (apoB) gene is controlled by at least four cis-acting elements located between positions −128 and +122 [Chuang, S. S., & Das, H. K. (1996) Biochem. Biophys. Res. Commun. 220, 553−562]. The distal element (−128 to −85) appears to be liver specific because it shows positive activity in HepG2 cells and negative activity in HeLa cells. ApoB gene regulatory factor-2 (BRF-2) interacts with the sequence (−104 to −85). BRF-2 has been purified from rat liver nuclear extract, and its molecular weight has been determined to be ∼120 kDa [Zhuang et al. (1992) Mol. Cell. Biol. 12, 3183−3191]. In this paper we report the isolation of two isoforms of BRF-2 by further purification using high-performance liquid chromatography. Both isoforms produced a single ∼120-kDa band in sodium dodecyl sulfate polyacrylamide gel electrophoresis detected by silver stain. The amino acid sequences of two tryptic peptides derived from HPLC-purified heavier BRF-2 isoform were de...