Biosynthesis of procollagens and collagens by tissue explants and matrix-free cells from embryonic chick cornea

Tendon and cornea of 17-day-old chick embryos primarily synthesize type I collagen as the major collagenous component nd type V collagen as a minor collagenous component. Matrix-free cells isolated from cornea and tendon synthesize and secrete mainly type I procollagen. A short lag period for the secretion of collagenous polypeptides into the medium of cell suspensions is observed. There is no significant difference in the synthesis and secretion of type I procollagen between matrix-free cells from tendon or cornea, except that cornea cells contain more free pro alpha 2(I) chains. The pro alpha 2(I) chain does not contain interchain disulfide linkages and can be separated from type I procollagen by ion exchange chromatography. However, no free pro alpha 2(I) chain can be detected in the medium of cell suspensions, suggesting that free pro alpha 2(I) is probably not secreted normally and may be rapidly degraded intracellularly. These results suggest that proper chain composition of type I procollagen is not regulated at the transcriptional or translational levels. Rather it is determined by the primary structure of the pro alpha chains and/or a posttranslational factor.