Mutations that substitute serine for glycine alpha 1-598 and glycine alpha 1-631 in type I procollagen. The effects on thermal unfolding of the triple helix are position-specific and demonstrate that the protein unfolds through a series of cooperative blocks.

Cultured skin fibroblasts from two probands with lethal variants of osteogenesis imperfecta synthesized type I procollagen that was posttranslationally over-modified. Analysis of cDNAs and genomic DNAs from the two probands demonstrated that proband I had a single-base mutation that converted the codon for glycine alpha 1-631 to a codon for serine, and proband II had a single-base mutation that converted the codon for glycine alpha 1-598 to a codon for serine. Although the two serine-for-glycine substitutions were separated by only 35 residues, they had markedly different effects on the thermal unfolding of the collagen triple helix as assayed by brief protease digestion. The type I procollagen from proband I (serine alpha 1-631) had an essentially normal temperature for thermal unfolding. In contrast, type I procollagen from proband II (serine alpha 1-598) was cleaved to readily identifiable intermediate fragments of about 630 residues at 20 degrees C. With procollagens from both probands, collagenase A fragments containing the first 775 amino acids of the alpha chain domains had a lowered temperature for thermal unfolding as assayed by brief protease digestion. The collagenase A fragments from proband I were cleaved to intermediates of about 600 amino acids at 36 degrees C and to fragments of about 510 residues at 37 degrees C. The collagenase A fragments from proband II were cleaved to intermediates of about 630 residues at 32 degrees C, to fragments of about 600 residues at 36 degrees C, and to fragments of about 510 at 37 degrees C. The fragments of about 510 residues from both mutated procollagens were more stable to protease digestion than the collagenase A fragments of 775 residues from normal type I collagen. The results demonstrate that the effects of glycine substitutions on the thermal unfolding of type I collagen are highly position-specific. They also provide direct evidence for previous indications that the triple helix of the protein undergoes micro-unfolding of a series of relatively independent "cooperative blocks" in the predenaturation range of temperatures.