Possesses thioredoxin activity (By similarity). Acts as a ligand for Drpr and is required for the phagocytosis of apoptotic cells (PubMed:19927123, PubMed:23337816). Binds to the extracellular region of Drpr and augments Drpr tyrosine phosphorylation (PubMed:19927123, PubMed:23337816). Relocates from the endoplasmic reticulum to the cell surface during apoptosis. Expressed in embryo, larva, pupa and adult (at protein level). Reduced level of apoptotic cell phagocytosis (PubMed:19927123). Loss of both Prtp and CaBP1 does not cause a further decrease in the reduced level of phagocytosis seen in either Prtp-lacking or CaBP1-lacking embryos (PubMed:22158613). Belongs to the protein disulfide isomerase family.