Denatured acid-soluble collagens from bovine and rat skins contain fractions which do not elute in the salt gradient in the usual carboxyrnethyl (CM)-cellulose chromatographic systems at 40°. These fractions are eluted with 6.0 m urea. Once isolated, these fractions show enhanced aggregation and renaturation properties typical of the crosslinked collagens. They can be dissociated into their constituent peptide chains by dénaturation at 60° and the chains separated by CM-cellulose chromatography at elevated temperature. The chains so prepared have compositions different from the a 1 and al components and, as demonstrated by disc gel electrophoresis and analytical ultracentrifugation have molecular weights 10-30% higher than α1 and α1. The composition data lead to the suggestion that the urea-eluted fractions represent a chains and polymers containing peptide extensions of noncollagen character directly adjoining the peptide chain backbones. The ureä-eluted fractions renature to native fibril form and, in the presence of ATP, form very thin segment long spacing type precipitates showing a marked 300 A head-to-tail overlap in contrast to the usual segment long spacing single spool precipitates seen in unfractionated acid-soluble collagens. It is proposed that the peptide extensions assist in the alignment and organization of monomeric collagen into the limiting microfibrils characteristic of native collagen fibers.
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