Delayed triple helix formation of mutant collagen from patients with osteogenesis imperfecta.

The kinetics of triple helix formation of procollagen I were measured in normal human fibroblast cultures and cell strains from six patients with osteogenesis imperfecta (OI), a heritable connective tissue disorder. After a 4-minute pulse-labelling with [35S]methionine, the appearance of protease-resistant and thus helical collagen molecules was followed for variable chase times. In control cells, 50% of the molecules were fully triple-helical after 14 minutes. In the six OI cell strains harbouring a single Gly-->Cys substitution at positions 94, 223, 526, 691 and 988 in the helical domain of the alpha 1(I)-chain, formation of full-length protease-resistant molecules containing two mutant alpha 1(I)-chains as judged by the appearance of disulphide-linked alpha 1(I)-dimers was delayed by 5 to 60 minutes. The delay inversely correlated with the thermal stability of abnormal collagen molecules containing alpha 1(I)-dimers. Folding time and melting temperature of procollagen I in the sixth cell strain with a Gly-->Cys substitution at position 1017, outside the triple helical region in the C-terminal telopeptide, were normal. Here, we demonstrate the hitherto postulated delay in the zipper-like folding of collagen molecules harbouring Gly-->Cys substitutions in the alpha 1(I)-chain affecting the helical part of the molecule.