Distinct roles and actions of PDI family enzymes in catalysis of nascent-chain disulfide formation

The mammalian endoplasmic reticulum (ER) harbors more than 20 members of the protein disulfide isomerase (PDI) family that act to maintain proteostasis. Herein, we developed an in vitro system for directly monitoring PDI- or ERp46-catalyzed disulfide bond formation in ribosome-associated nascent chains (RNC) of human serum albumin. The results indicated that ERp46 more efficiently introduced disulfide bonds into nascent chains with short segments exposed outside the ribosome exit site than PDI. Single-molecule analysis by high-speed atomic force microscopy further revealed that PDI binds nascent chains persistently, forming a stable face-to-face homodimer, whereas ERp46 binds for a shorter time in monomeric form, indicating their different mechanisms for substrate recognition and disulfide bond introduction. Similarly to ERp46, a PDI mutant with an occluded substrate-binding pocket displayed shorter-time RNC binding and higher efficiency in disulfide introduction than wild-type PDI. Altogether, ERp46 serves as a more potent disulfide introducer especially during the early stages of translation, whereas PDI can catalyze disulfide formation in RNC when longer nascent chains emerge out from ribosome.