Reconstitution of the folding pathway of collagen in a cell-free system: formation of correctly aligned and hydroxylated triple helices.

We describe here a cell-free system which will carry out the initial stages in the synthesis, post-translational modification and assembly of type-X collagen. The mRNA coding for bovine type-X collagen was synthesized in vitro and translated in a rabbit reticulocyte lysate to yield a protein that was collagenase sensitive and could be immunoprecipitated with antibodies raised to purified avian type-X collagen. When type-X collagen was synthesized in the absence of added microsomes or in the presence of canine pancreas microsomes, the translation products showed partial resistance to digestion with pepsin but were completely degraded with a mixture of chymotrypsin and trypsin, suggesting that only incorrectly aligned non-native collagen molecules were synthesized under these conditions. When the protein was synthesized in the presence of microsomes derived from avian fibroblasts or a human fibrosarcoma cell line, the translocated product migrated as a diffuse band characteristic of hydroxylated collagen. The synthesized polypeptides were also resistant to both pepsin and trypsin/chymotrypsin digestion, demonstrating the formation of correctly aligned native collagen. Furthermore, the collagen polypeptides assembled into higher-order structures, possibly trimers, which were stabilized by interchain disulphide bonds. The collagen helix synthesized in vitro had a melting temperature of 41 degrees C which is comparable with the protein synthesized in vivo, further demonstrating that the polypeptides were hydroxylated and that the triple helix formed was correctly aligned.