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Protein disulfide isomerase is the most abundant and best studied of the disulfide isomerases that catalyze disulfide bond formation in the endoplas-mic reticulum, yet the specifics of how it binds substrate have been elusive. Protein disulfide isomerase is composed of four thioredoxin-like domains (abb¢a¢). | Solution structure of the bb domains of human protein disulfide isomerase Alexey Y. Denisov Pekka Maattanen Christian Dabrowski Guennadi Kozlov David Y. Thomas and Kalle Gehring Department of Biochemistry McGillUniversity and Groupe de Recherche Axe sur la Structure des Proteines GRASP Montreal Canada Keywords chaperone endoplasmic reticulum NMR solution structure protein disulfide isomerase family Correspondence K. Gehring Department of Biochemistry McGillUniversity 3655 Promenade Sir William Osler Montreal QC H3G 1Y6 Canada Fax 1 514 398 7384 Tel 1 514 398 7287 E-mail kalle.gehring@mcgill.ca These authors contributed equally to this work Received 16 November 2008 revised 30 December 2008 accepted 30 December 2008 Protein disulfide isomerase is the most abundant and best studied of the disulfide isomerases that catalyze disulfide bond formation in the endoplasmic reticulum yet the specifics of how it binds substrate have been elusive. Protein disulfide isomerase is composed of four thioredoxin-like domains abb a . Cross-linking studies with radiolabeled peptides and unfolded proteins have shown that it binds incompletely folded proteins primarily via its third domain b . Here we determined the solution structure of the second and third domains of human protein disulfide isomerase b and b respectively by triple-resonance NMR spectroscopy and molecular modeling. NMR titrations identified a large hydrophobic surface within the b domain that binds unfolded ribonuclease A and the peptides mastoparan and somatostatin. Protein disulfide isomerase-catalyzed refolding of reduced ribonuclease A in vitro was inhibited by these peptides at concentrations equal to their affinity to the bb fragment. Our findings provide a structural basis for previous kinetic and cross-linking studies which have shown that protein disulfide isomerase exhibits a saturable substratebinding site. doi 10.1111 j.1742-4658.2009.06884.x The endoplasmic reticulum ER is the cell compartment where .
