As the collagen peptide chain is translocated across the membrane of the endoplasmic reticulum, intrachain disulfide bonds are formed within the N- and C-propeptides. This allows the triple helical domain to form a nucleation point at its C-terminal end and ensures correct alignment of the chains (Engel & Prockop 1991). Protein disulfide isomerase (P4HB) catalyzes the formation of both intra- (Bulleid & Freedman 1988) and inter-chain disulfide bonds (Koivu & Myllylä 1987). In addition, PDI acts as a molecular chaperone, interacting with monomeric collagen propeptide chains to prevent premature assembly or aggregation (Wilson et al. 1998).