THE FOLDING CATALYST PROTEIN DISULFIDE-ISOMERASE IS CONSTRUCTED OF ACTIVE AND INACTIVE THIOREDOXIN MODULES

Background: Protein disulfide isomerase (PDI), a multifunctional prote in of the endoplasmic reticulum, catalyzes the formation, breakage and rearrangement of disulfide bonds during protein folding. Dissection o f this protein into its individual domains has confirmed the presence of the a and a' domains, which are homologous to thioredoxin, having r elated structures and activities. The a and a' domains both contain a -Cys-Gly-His-Cys- active-site sequence motif. The remainder of the mol ecule consists primarily of two further domains, designated b and b', which are thought to be sequence repeats on the basis of a limited seq uence similarity. The functions of the b and b' domains are unknown an d, until now, the structure of neither domain was known. Results: Hete ronuclear nuclear magnetic resonance (NMR) methods have been used to d etermine the global fold of the PDI b domain. The protein has an alpha /beta fold with the order of the elements of secondary structure being beta 1-alpha 1-beta 2-alpha 2-beta 3-alpha 3-beta 4-beta-5-alpha 4. T he strands are all in a parallel arrangement with respect to each othe r, except for beta 4 which is antiparallel. The arrangement of the sec ondary structure elements of the b domain is identical to that found i n the a domain of PDI and in the ubiquitous redox protein thioredoxin; the three-dimensional folding topology of the b domain is also very s imilar to that of these proteins. Conclusions: Our determination of th e global fold of the b domain of PDI by NMR reveals that, like the a d omain, the b domain contains the thioredoxin motif, even though the b domain has no significant amino-acid sequence similarities to any memb ers of the thioredoxin family. This observation, together with indicat ions that the b' domain adopts a similar fold, suggests that PDI consi sts of active? and inactive thioredoxin modules. These modules may hav e been adapted during evolution to provide PDI with its complete spect rum of enzymatic activities. (C) Current Biology Ltd.