In vitro characterisation of the interaction between newly synthesised proteins and a pancreatic isoform of protein disulphide isomerase

The lumen of the endoplasmic reticulum (ER) contains an array of molecular chaperones and folding factors that modulate the folding and assembly of newly synthesised proteins entering the secretory pathway. One of these components, protein disulphide isomerase (PDI), facilitates the formation of the correct disulphide bonds within newly synthesised polypeptides, and is the archetype for a family of sequence related PDI‐like proteins. We have investigated the interaction between a recently identified, pancreas‐specific PDI‐like protein (PDIp), and in vitro synthesised secretory and membrane proteins produced in the presence of ER‐derived canine pancreatic microsomes. We have previously established that a second PDI‐like protein, ERp57, interacts specifically with N‐glycosylated proteins. In contrast, we find that the interaction of PDIp with newly synthesised proteins occurs independently of any requirement for N‐linked glycosylation. In this respect, the properties of PDIp mirror those of archetypal PDI. When the carbohydrate‐dependent interactions between glycoproteins and ERp57 are blocked by drug treatment, the association of these precursors with both PDIp and PDI is enhanced. We propose that PDI‐like proteins have overlapping specificity and may exhibit some degree of functional redundancy.