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). Type V collagen N-propeptide removal is partial, and reported to be mediated by BMP-1 which cleaves between the proline/arginine-rich protein domain and the variable domain of the alpha1 chain and between the small and the large collagenous domain of alpha3 chain (refs. in Colige et al. 2005). Mutated propeptide collagen type V (COL5A1) is known to form abnormal trimers and non-functional fibrils (Takahara et al. 2002).