Removal of amino-terminal and carboxy-terminal extension peptides from procollagen during synthesis of chick embryo tendon collagen

Matrix-free chick embryo tendon cells were incubated with ¹⁴C]proline for 60 minutes and protein synthesis was stopped by the addition of cycloheximide. Newly synthesized collagen precursors recovered in the incubation medium were mostly intact procollagen molecules which contain both amino-terminal and carboxy-terminal extensions. If the cells were further incubated for 2 hours in the presence of cycloheximide, most of the procollagen was converted to precursor molecules which were devoid of amino-terminal extensions. Removal of the carboxy-terminal extensions from procollagen was not observed. Similar experiments with intact tendons demonstrated that procollagen synthesized by the intact tissues $\text{in}\text{vitro}$ was readily converted to an intermediate form devoid of amino-terminal extensions and then to collagen. The results suggest that the removal of the amino-terminal and carboxy-terminal extensions from procollagen is catalyzed by two separate enzymic activities.