Quality control in the endoplasmic reticulum: PDI mediates the ER retention of unassembled procollagen C-propeptides

Quality control within the endoplasmic reticulum (ER) is thought to be medi ated by the interaction of a folding protein with one or several resident E R proteins [1]. Protein disulphide isomerase (PDI) is one such ER resident protein that has been previously shown to interact with proteins during the ir folding and assembly pathways [2, 3]. It has been assumed that, as a con sequence of this interaction, unassembled proteins are retained within the ER. Here, we experimentally show that this is indeed the case. We have take n advantage of our previous finding that PDI interacts with procollagen cha ins early on in their assembly pathway [2] to address the role of this prot ein in directly retaining unassembled chains within the ER. Our experimenta l approach involved expressing individual C-propeptide domains from differe nt procollagen chains in mammalian cells and determining the ability of the se domains to interact with PDI and to be secreted. The C-propeptide from t he pro alpha2(I) chain was retained within the cell, where it formed a comp lex with PDI. Conversely, the C-propeptide from the proal (III) chain did n ot form a complex with PDI and was secreted. Both domains were secreted, ho wever, from a stable cell line expressing a secreted form of PDI lacking it s ER retrieval signal. Hence, we have demonstrated directly that the intrac ellular retention of one substrate for ER quality control is due to an inte raction with PDI.