THE B' DOMAIN PROVIDES THE PRINCIPAL PEPTIDE-BINDING SITE OF PROTEIN DISULFIDE-ISOMERASE BUT ALL DOMAINS CONTRIBUTE TO BINDING OF MISFOLDEDPROTEINS

Protein disulfide isomerase (PDI) is a very efficient catalyst of fold ing of many disulfide-bonded proteins, A great deal is known about the catalytic functions of PDI, while little is known about its substrate binding, We recently demonstrated by cross-linking that PDI binds pep tides and misfolded proteins, with high affinity but broad specificity . To characterize the substrate-binding site of PDI, we investigated t he interactions of various recombinant fragments of human PDI, express ed in Escherichia coil, with different radiolabelled model peptides, W e observed that the b' domain of human PDI is essential and sufficient for the binding of small peptides, In the case of larger peptides, sp ecifically a 28 amino acid fragment derived from bovine pancreatic try psin inhibitor, or misfolded proteins, the b' domain is essential but not sufficient for efficient binding, indicating that contributions fr om additional domains are required, Hence we propose that the differen t domains of PDI all contribute to the binding site, with the b' domai n forming the essential core.