Collagen is synthesized in the endoplasmic reticulum (ER) as procollagen, which is the precursor protein that bears propeptide domains at either end of the triple helical domain. The processes by which procollagen is synthesized in the lumen of the ER include unique steps that are not found in the biosynthesis of globular proteins. First, each polypeptide chain of procollagen (proα-chains) finds its correct partners, which enables the formation of the distinct types of procollagen. Second, triple helix-formation of long Gly-X-Y repeats starts at a defined region, which results in the formation of a correctly aligned triple helix and thereby prevents mis-staggering. The most characteristic step is the formation of the triple helix. This step involves specific post-translational modifications, in particular, the prolyl 4-hydroxylation of the Y-position amino acids that stabilizes the triple helical conformation. The formation of the triple helix is a slow process compared to the folding of globular proteins, including cis-trans isomerization of the many prolyl and hydroxyprolyl peptide bonds. Recent advances have indicated that these processes are assisted by a set of the ER-resident molecular chaperones, such as protein disulfide isomerase (PDI), peptidyl prolyl cis-trans isomerases (PPIases), heat-shock protein (Hsp)47, and prolyl 4-hydroxylase (P4-H). The intracellular trafficking of procollagen molecules has also been shown to involve a pathway distinct from that utilized by small secretory proteins.
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