Expression Characterization of AtPDI11 and Functional Analysis of AtPDI11 D Domain in Oxidative Protein Folding.

The formation and isomerization of disulfide bonds mediated by protein disulfide isomerase (PDI) in the endoplasmic reticulum (ER) is of fundamental importance in eukaryotes. Canonical PDI structure comprises four domains with the order of a-b-b'-a'. In Arabidopsis thaliana, the PDI-S subgroup contains only one member, AtPDI11, with an a-a'-D organization, which has no orthologs in mammals or yeast. However, the expression pattern of AtPDI11 and the functioning mechanism of AtPDI11 D domain are currently unclear. In this work, we found that PDI-S is evolutionarily conserved between land plants and algal organisms. AtPDI11 is expressed in various tissues and its induction by ER stress is disrupted in bzip28/60 and ire1a/b mutants that are null mutants of key components in the unfolded protein response (UPR) signal transduction pathway, suggesting that the induction of AtPDI11 by ER stress is mediated by the UPR signaling pathway. Furthermore, enzymatic activity assays and genetic evidence showed that the D domain is crucially important for the activities of AtPDI11. Overall, this work will help to further understand the working mechanism of AtPDI11 in catalyzing disulfide formation in plants.

Arabidopsis PDI11 interacts with lectin molecular chaperons calreticulin 1 and 2 through its D domain.

The endoplasmic reticulum (ER) is equipped with protein disulfide isomerases (PDIs), molecular chaperons, and other folding enzymes to ensure that newly synthesized proteins in the ER are properly folded. Molecular chaperons and PDIs can form complex to promote protein folding in the ER of mammalian cells. In plants, many PDIs associate with each other and function cooperatively in oxidative protein folding. As a plant unique protein disulfide isomerase, Arabidopsis thaliana PDI11 (AtPDI11) demonstrates oxidative protein folding activities and works synergistically with AtPDI2/5. However, whether AtPDI11 associates with molecular chaperons or AtPDIs in catalyzing disulfide formation remained unknown. Here, we find that AtPDI11 interacts with ER resident lectin chaperones calreticulin 1 (CRT1) and CRT2. Furthermore, the D domain, but not the a or a' domain of AtPDI11 provides the biding sites for its interaction with CRT1/2. Moreover, the P domain of CRT1 is responsible for its interaction with AtPDI11. Our work implies that Arabidopsis CRT1/2 may specifically recruit AtPDI11 to assist the folding of glycoproteins in the ER.

Characterization of the oxidative protein folding activity of a unique plant oxidoreductase, Arabidopsis protein disulfide isomerase-11.

Protein disulfide isomerases (PDIs) can catalyze disulfide bond formation in nascent secretory proteins and membrane proteins and can introduce correct disulfide bonds into substrate proteins containing mispaired disulfides. The functions of mammalian PDIs have been extensively studied; however, relative to mammalian PDIs, the systematic characterization of PDIs for their oxidoreductase activity in plants is still lacking. Arabidopsis protein disulfide isomerases-11 (AtPDI11), with the structure of a-a'-D, has no ortholog in animals or yeast. In this study, we demonstrated that AtPDI11 has oxidoreductase activity in vitro using a GSSG/GSH-mediated oxidative protein folding system. Moreover, the active site in the a' domain of AtPDI11 is critical for its oxidative folding activity. AtPDI11 is present in four redox forms in vivo, which are determined by the active site cysteines (Cys52 and Cys55 in the a domain, and Cys171 and Cys174 in the a' domain). Genetic evidence suggests that AtPDI11 is required for plant growth under reducing conditions. Our work provides an example for studying the oxidoreductase function of other plant PDIs.