Annular Arrangement and Collaborative Actions of Four Domains of Protein-disulfide Isomerase

We presented for the first time a small angle x-ray scattering study of intact protein-disulfide isomerase (PDI) in solution. The restored model revealed that PDI is a short and roughly elliptical cylinder with a molecular mass of 69 kDa and dimensions of 105 65 40 Å, and the four thioredoxin-fold domains in the order a-b-b-a are arranged in an annular fashion. Atomic force microscope imaging also supported the finding that PDI appears as an approx-imately flat elliptical cylinder. A PDI species with apparent molec-ular mass of 116 kDameasured by using size-exclusion chromatog-raphy, previously assumed to be a dimer, was determined to exist mainly as a monomer by using analytical ultracentrifugation. The C-terminal fragment 441–491 contributed to the anomalous molecularmass determination of PDI by size-exclusion chromatog-raphy. The annular model of PDI accounted for the cooperative properties of the four domains in both the isomerase and chaperon