Protein disulfide isomerase that catalyzes the formation, isomerization, and reduction or oxidation of disulfide bonds in client proteins and functions as a protein folding chaperone (PubMed:11825568, PubMed:16193070, PubMed:27897272, PubMed:36104323, PubMed:7487104). Core component of the major histocompatibility complex class I (MHC I) peptide loading complex where it functions as an essential folding chaperone for TAPBP. Through TAPBP, assists the dynamic assembly of the MHC I complex with high affinity antigens in the endoplasmic reticulum. Therefore, plays a crucial role in the presentation of antigens to cytotoxic T cells in adaptive immunity (PubMed:35948544, PubMed:36104323). {ECO:0000269|PubMed:11825568, ECO:0000269|PubMed:16193070, ECO:0000269|PubMed:27897272, ECO:0000269|PubMed:35948544, ECO:0000269|PubMed:36104323, ECO:0000269|PubMed:7487104}. This is the function of PDIA3 (protein disulfide isomerase family A member 3, ENSG00000167004).