SUBSTITUTION OF SERINE FOR GLYCINE-883 IN THE TRIPLE-HELIX OF THE PRO-ALPHA-1(I) CHAIN OF TYPE-I PROCOLLAGEN PRODUCES OSTEOGENESIS IMPERFECTA TYPE-IV AND INTRODUCES A STRUCTURAL-CHANGE IN THE TRIPLE-HELIX THATDOES NOT ALTER CLEAVAGE OF THE MOLECULE BY PROCOLLAGEN N-PROTEINASE

Type I procollagen secreted by dermal fibroblasts from an individual w ith osteogenesis imperfecta type TV was a mixture of normal molecules and molecules that were post-translationally overmodified. The individ ual was heterozygous for a G to A transition in the COL1A1 gene that r esulted in the substitution of serine for glycine 883 in one or both o f the pro alpha l(I) chains. The thermal stability of molecules contai ning overmodified chains was lower by 2 degrees C than that of normal molecules. However, following cleavage of the molecules with vertebrat e collagenase, the temperature of denaturation of the overmodified A f ragments (residues 1-775 of the helix did not contain the substitution ) was 2 degrees C greater than that of A fragments from normal molecul es. The rates of cleavage by procollogen N-proteinase (EC 3.4.214.14) (N-proteinase) of procollagen molecules in normal and osteogenesis imp erfecta samples were not significantly different. The procollagen mole cules in the osteogenesis imperfecta sample were also indistinguishabl e from those in control samples by rotary shadowing electron microscop y. The results suggest that this substitution of serine for glycine in the alpha 1(I) chain of procollagen, like the substitution of asparta te for the same glycine previously described (Lightfoot, S. J., Holmes , D. F., Brass, A., Grant, M. E., Byers, P. H., and Kadler, K. E. (199 2) J. Biol. Chem. 267, 25521-25528), can alter the structure of the tr iple helix N-terminal to the site of the substitution. However, in con trast to the aspartate for glycine substitution, the structural change is insufficient to delay the cleavage of the procollagen by N-protein ase and results in a mild rather than lethal phenotype.