Interaction of transcription factor Sp1 with the promoter of the gene for the multifunctional protein disulphide isomerase polypeptide.

Protein disulphide isomerase (PDI) is a unique polypeptide which resides in the lumen of the endoplasmic reticulum and also functions as the beta-subunit of prolyl 4-hydroxylase, as a cellular thyroid hormone-binding protein, as the smaller subunit of the microsomal triacylglycerol transfer protein complex, as a dehydroascorbate reductase and as a protein that binds various peptides in a specific manner. We have recently demonstrated that the promoter of the PDI gene contains six CCAAT boxes and other elements which are needed for efficient transcription. We now demonstrate that purified human recombinant transcription factor Sp1 interacts with two perfect GGGCGG sequences and three other GC-rich elements of the PDI promoter. Sp1 also appears to participate in the regulation of PDI gene expression, since overexpression of Sp1 stimulated PDI promoter activity in HeLa cells and mutations introduced into each of these Sp1-binding sites separately reduced the promoter strength, although even the largest decrease was only about 50%. These results support our view that expression of the gene for this polypeptide with multiple functions is secured by several regulatory elements, some of which are functionally redundant.