Fibril-forming collagens are synthesized in precursor form, procollagens, with N-and C-
terminal propeptide extensions. The C-propeptides direct chain association during
intracellular assembly of the procollagen molecule from its three constituent polypeptide
chains. Following or during secretion into the extracellular matrix, propeptides are cleaved
by specific procollagen proteinases, thereby triggering fibril formation. The recent
determination of the low-resolution structure of the C-propeptide trimer gives insights into the …