Protein disulphide isomerase PDI

In vivo, formation of the disulphide bridges in cyclotides might be facilitated by a protein disulphide isomerase (PDI) recently isolated from O. affinis,129 whereas backbone cyclisation is mediated by an asparaginyl endopepti-dase.130,131 Support for the latter proposal comes from the fact that a C-terminal Asn residue is essential for cyclisation of the peptide backbone. Violacin... 

The p(25-35) interacts with the native protein, destroys its native conformation, and forms pathological aggregates. Its action is exactly opposite to that of a chaperone, which helps proteins fold correctly. Hence, p(25-35) is termed as an antichaperone, protein disulphide isomerase (PDI), a folding catalyst and chaperone, under certain conditions, can facilitate the... 

In addition to its role as the P-subunit of PHY, PDI acts independently by catalysing thiol/protein disulphide interchange. The role of PDI as the P-subunit in prolyl 4-hydroxylase is not related to its disulphide isomerase activity and experiments where the vertebrate PDI was mutated in both thioredoxin-like active domains had no effect on tetramer assembly (Vuori et al., 1992). PDI appears to function as a molecular chaperone, retaining the a-subunits in the correct catalytically active, non-aggregated form in the ER-lumen (John et al, 1993). Dissociation of the P-subunits results in insoluble aggregates of the a-subunits, analogous to a-subunits expressed in the absence of PDI. An additional function of PDI in the complex is to maintain the ER luminal location of the a-subunits, since deletion of the ER retention signal from PDI results in the secretion of the complex (Vuori et al., 1992). 

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