Type II procollagen, the precursor form of a genetically distinct type of collagen present in hyaline cartilages, has previously been shown to contain extension peptides both at the amino‐terminal and carboxy‐terminal ends of the molecule. Here, the synthesis of type II procollagen was studied by pulse‐labeling chick embryo sterna with [14C]proline for 10 min, and the incorporation of radioactivity was then inhibited by the addition of cycloheximide and unlabeled proline. At the end of a 10‐min pulse, most of the 14C‐labeled protein was [14C]procollagen. If the incubation was continued for 30–120 min after the inhibition of the protein synthesis, an increasing fraction of [14C]procollagen was converted to [14C]collagen, as monitored by polyacrylamide slab gel electrophoresis in dodecylsulfate. Two intermediate forms containing either the amino‐terminal or carboxy‐terminal extension peptides were also detected. In further studies the site of the conversion process was examined by comparing the rate of conversion with the time required for secretion of the procollagen molecules out of the cells. Comparison of the time of conversion with the time of secretion, as estimated in intact cartilage by radioautographic techniques, or determined as secretion of hydroxy[14C]proline by matrix‐free cartilage cells incubated under identical conditions, indicated that the secretion started significantly earlier than the conversion. Also, the conversion of the [14C]procollagen secreted by the matrix‐free cartilage cells continued in the medium after separation of the cells by centrifugation. Furthermore, incubation of intact cartilages with α,α'‐dipyridyl, L‐azetidine‐2‐carboxylic acid, cis‐4‐hydroxy‐L‐proline, or colchicine, agents which, by independent mechanisms, delay the secretion of procollagen, also inhibited the conversion. The results suggest that the conversion of type II procollagen to collagen occurs in the extracellular space, the amino‐terminal and carboxy‐terminal extensions beind removed without a preferential sequence. Copyright © 1979, Wiley Blackwell. All rights reserved
