Fibrillar collagen is synthesized in the ER as procollagen with N- and C- terminal propeptides flanking the collagenous domain (Bellamy & Bornstein 1971). These propeptides, particularly the C-propeptide, inhibit fibril formation (Kadler et al. 1987). Removal of propeptides is generally described as an extracellular process but can occur within the cell. Procollagen processing in tendon fibroblasts was initiated within the secretory pathway with the N-propetides removed first, in the ER or an intermediate between the ER and Golgi. The C peptides were removed later, probably at the cell membrane-ECM interface (Canty-Laird et al. 2012).
Collagen C-propeptides are cleaved by the tolloid family metalloproteinases bone morphogenic protein 1 (BMP1)/mammalian tolloid (mTLD), tolloid-like 1 (TLL1) and TLL2.
Procollagen types I-III are cleaved by BMP1/mTLD (Chicken types I and II, human type III, by chicken enzyme, Hojima et al. 1985, human types I, II, human enzyme, Scott et al. 1999), TLL-1 (human types I, II, human enzyme, Scott et al. 1999), and TLL-2 in the presence of PCOLCE (PCPE-1, Pappano et al. 2003, Petropoulou et al. 2005). Type V C-propeptide removal is mediated by furin-like proprotein convertases and/or BMP-1 depending on the chain type (Kessler et al. 2005).