Substitutions for glycine α1-637 and glycine α2-694 of type I procollagen in lethal osteogenesis imperfecta: The conformational strain on the triple helix introduced by a glycine substitution can be transmitted along the helix

Two substitutions for glycine in the triple-helical domain were found in type I procollagen synthesized by skin fibroblasts from two probands with lethal osteogenesis imperfecta. One was a substitution of valine for glycine α1-637, and the other was a substitution of arginine for glycine α2-694. The effects of the mutations on the zipper-like folding of the collagen triple helix were similar, since there was post-translational overmodification of the collagenase A fragments (amino acids 1-775) but not of more COOH-terminal fragments of the protein. The mutations differed markedly, however, on their effects on thermal unfolding of the triple helix. The collagenase A fragment from the collagen containing the arginine α2-694 substitution was cleaved at about amino acid 700 when incubated with trypsin at 30-35 °C. Therefore, there was microunfolding of the triple helix at a site close to the glycine substitution. Surprisingly, however, the collagenase A fragment with the valine α1-637 substitution was also cleaved at about amino acid 700 under the same conditions. The results, therefore, demonstrated that although most glycine substitutions delay folding of the triple helix in regions that are NH₂-terminal to the site of the substitution, the effects on unfolding can be transmitted to regions that are COOH-terminal to the site of the glycine substitution.