The Acidic C-terminal Domain Stabilizes the Chaperone Function of Protein Disulfide Isomerase

Protein disulfide isomerase (PDI, EC 5.3.4.1) is a chaperone and catalyzes the formation and rearrangement of disulfide bonds in proteins. Domain c-(463–491), containing 18 acidic residues, is an interesting and important C-terminal extension of PDI. In this study, the PDI mutant abba, in which domain c is truncated, was used to investigate the relationship between the C-terminal structure and chaperone function. Reactivation and light-scattering experiments show that both wild-type PDI and abba interact with lactate dehydrogenase (LDH, EC 1.1.1.27), which tends to self-aggregate during reactivation. The interaction enhances reactivation of LDH and reduces aggregation. According to these results, it seems as if domain c might be dispensable to the chaperone function of PDI. However, abba is prone to self-aggregation and causes increased aggregation of LDH during thermal denaturation. In contrast, wildtype PDI remains active as a chaperone under these conditions and prevents self-aggregation of LDH. Furthermore, measurements of intrinsic fluorescence and difference absorbance during denaturation show that abba is much more labile to heat or guanidine hydrochloride denaturation than wild-type PDI. This suggests that domain c is required for the stabilization and maintenance of the chaperone function of PDI under extreme conditions.