THE MULTIDOMAIN STRUCTURE OF PROTEIN DISULFIDE-ISOMERASE IS ESSENTIALFOR HIGH CATALYTIC EFFICIENCY

Protein disulfide isomerase (PDI) catalyzes protein folding Linked to disulfide bond formation in secreted proteins. It consists of four maj or domains, denoted a, b, b' and a'. The a and a' domains each contain an active site motif, -CGHC-, which is directly involved in thiol-dis ulfide exchange reactions during catalysis. The roles of the b and b' domains and the functional necessity for the multi-domain structure of PDI are unknown. We now demonstrate that full catalytic activity requ ires the involvement of multiple PDI domains and that the b' domain ha s a particularly important role in catalysis. Reconstruction of the PD I molecule from the isolated a and a' domains results in a progressive increase in catalytic efficiency as further domains are added. These effects are especially significant in the catalysis of disulfide bond rearrangements in folded substrates, for which all the domains of the protein are required for maximum catalytic efficiency. It is likely th at all of the domains of PDI participate in substrate binding interact ions and that PDI has evolved its multidomain structure as an adaptati on that allows it to catalyze transformations involving difficult conf ormational changes. (C) 1998 Academic Press Limited.