Probable disulfide isomerase, which participates in the folding of proteins containing disulfide bonds. May act as a dithiol oxidase (PubMed:15623505). Acts as a regulator of endoplasmic reticulum-mitochondria contact sites via its ability to regulate redox signals (PubMed:31304984). {ECO:0000269|PubMed:15623505, ECO:0000269|PubMed:31304984}. This is the function of TMX3 (thioredoxin related transmembrane protein 3, ENSG00000166479).