A unique adhesive motif of protein disulfide isomerase P5 supports its function via dimerization

SUMMARY P5, also known as PDIA6, is a PDI-family member that plays an important role in the ER quality control. Herein, we revealed that P5 dimerizes via a unique adhesive motif contained in the N-terminal thioredoxin-like domain. This motif is apparently similar to, but radically different from conventional leucine-zipper motifs, in that the former includes a periodic repeat of leucine or valine residues at the third or fourth position spanning five helical turns on 15-residue anti-parallel α-helices, unlike the latter of which the leucine residues appear every two helical turns on ∼30-residue parallel α-helices at dimer interfaces. A monomeric P5 mutant with the impaired adhesive motif showed structural instability and local unfolding, and behaved as an aberrant protein that induces the ER stress response. Disassembly of P5 to monomers compromised its ability to inactivate IRE1α via reduction of intermolecular disulfide bonds and its Ca 2+ -dependent regulation of chaperone function in vitro . Thus, the leucine-valine adhesive motif supports structure and physiological function of P5.