Promoter of the gene for the multifunctional protein disulfide isomerase polypeptide. Functional significance of the six CCAAT boxes and other promoter elements.
Protein disulfide isomerase (PDI) is a highly unusual multifunctional polypeptide that is identical to the beta-subunit of prolyl 4-hydroxylase, a cellular thyroid hormone-binding protein and a subunit of the microsomal triglyceride transfer protein complex, and very similar to a polypeptide functioning in vitro as a glycosylation site binding protein of oligosaccharyl transferase. The human PDI gene possesses several putative transcriptional control elements, including the highly unusual presence of six CCAAT boxes between -108 and -378 of the 5'-flanking region. We report here on a promoter analysis of this gene. Eleven PDI promoter elements recognized by DNA-binding proteins present in HeLa cell and HT-1080 cell nuclear extracts were identified by DNase I footprinting analysis within the first 630 nucleotides of the 5'-flanking region. Interestingly, these included all six CCAAT elements. Functional 5' deletion analysis suggested that only two or three of the CCAAT elements may contribute significantly to the promoter activity in HeLa cells. Mutations introduced into each of the CCAAT boxes separately indicated, however, that all six appear to contribute to the promoter strength, the largest decreases (approximately 50%) being seen with mutations in the second or fifth CCAAT box. These data thus suggested that efficient expression of the multifunctional PDI polypeptide is secured by multiple CCAAT elements, some of which appear to be functionally redundant. The 5' deletion analysis further suggested that a region between -623 and -518 may contain additional positively and negatively acting elements.