I. Embryonic-chick tendon cells were pulse-labelled for 4 min with [14C]proline and the 14C-labelled polypeptides were chased with unlabelled proline for up to 30 min. Isolation of subcellular fractions during the chase period and their subsequent analysis for bacterial collagenase-susceptible 14C-labelled peptides demonstrated the transfer of procollagen polypeptides from rough to smooth microsomal fractions and thence to the extracellular medium. Parallel analyses of Golgi-enriched fractions indicated the involvement of this organelle in the secretory pathway of procollagen. Sodium dodecylsulphate/polyacrylamide-gel electrophoresis of the 14C-labelled polypeptides present in the Golgi-enriched fractions demonstrated that the procollagen polypeptides were all present as disulphide-linked pro-gamma components. 2. When similar kinetic studies of the intracellular transport of procollagen were conducted with embryonic-chick cartilage cells almost identical results were obtained, but the rate of translocation of cartilage procollagen was significantly slower than that observed for tendon procollagen. 3. When hydroxylation of procollagen polypeptides was inhibited by alphaalpha′-bipyridyl, the nascent polypeptides accumulated in the rough microsomal fraction. 4. When cells were pulse-labelled for 4min with [14C)proline and the label was chased in the presence of colchicine, secretion of procollagen was inhibited and an intracellular accumulation of procollagen 14C-labelled polypeptides was observed in the Golgi-enriched fractions. 5. The energy-dependence of the intracellular transport of procollagen was demonstrated in experiments in which antimycin A was found to inhibit the transfer of procollagen polypeptides from rough to smooth endoplasmic reticulum. 6. It is concluded that procollagen follows the classical route of secretion taken by other extracellular proteins.

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