A TYPE-I COLLAGEN WITH SUBSTITUTION OF A CYSTEINE FOR GLYCINE-748 IN THE ALPHA-1(I) CHAIN COPOLYMERIZES WITH NORMAL TYPE-I COLLAGEN AND CAN GENERATE FRACTALLIKE STRUCTURES

Type I procollagen was purified from cultured fibroblasts of a proband with a lethal variant of osteogenesis imperfecta. The protein was a mixture of normal procollagen and mutated procollagens containing a substitution of cysteine for glycine in either one pro-alpha-1(I) chain or both pro-alpha-1(I) chains, some or all of which were disulfide-linked through the cysteine at position alpha-1-748. The procollagen was then examined in a system for generating collagen fibrils de novo by cleavage of the pCcollagen to collagen with procollagen C-proteinase [Kadler et al. (1987) J. Biol. Chem. 262, 15696-15701]. The mutated collagens and normal collagens were found to form copolymers under a variety of experimental conditions. With two preparations of the protein that had a high content of alpha-1(I) chains disulfide-linked through the cysteine alpha-1-748, all the large structures formed had a distinctive, highly branched morphology that met one of the formal criteria for a fractal. Preparations with a lower content of disulfide-linked alpha-1(I) chains formed fibrils that were 4 times the diameter of control fibrils. The formation of copolymers was also demonstrated by the observation that the presence of mutated collagens decreased the rate of incorporation of normal collagen into fibrils. In addition, the solution-phase concentration at equilibrium of mixtures of mutated and normal collagens was 5-10-fold greater than that of normal collagen. Therefore, the mutated and normal collagens synthesized by the proband's fibroblasts were similar enough in structure to copolymerize into the same fibrils, and because of copolymerization, there was a distortion of normal fibril morphology, a delay in fibril formation, and a decrease in the net amount of collagen incorporated into fibrils. The results were consistent with a recently proposed model for growth of collagen fibrils from pointed tips [Kadler et al. (1990) Biochem. J. 268, 339-343].