Procollagen: biological scission of amino and carboxyl extension peptides

Procollagen, the triple-stranded precursor of chick embryo skull bone collagen, contains two pro alpha1 and one pro alpha2 chains. We find that each of these is a collagen chain with both an NH2-terminal and a COOH-terminal extension peptide. The NH2-peptide of pro alpha1 contains cysteine and differs from the NH2-peptide of pro alpha2. The three NH2-peptides are cut off, giving a disulfide-linked intermediate, named altered procollagen; then the disulfide-linked COOH-peptides, which contain cysteine and tryptophan, are cut off, leaving collagen. Procollagen, altered procollagen, and COOH-peptide were isolated. Collagenase digestion of procollagen gave both NH2- and COOH-peptides, while altered procollagen gave only COOH-peptides. The following results of sequential, in vitro labeling at 37 degrees and of specific cleavage of procollagen proved the structure: [(NH2-peptide)-collagen-(COOH-peptide)]3 with interstrand S-S links between only the COOH-peptides. (i) The COOH-peptides of pro alpha chains were labeled with [3H]proline before the remainders of the chains; (ii) [35S]cysteine appeared in the COOH-peptides of completed covalent molecules 5 min earlier than in the NH2-peptides; (iii) tadpole tail collagenase, which cuts native collagen into triple-stranded 3/4 pieces containing the NH2 termini and 1/4 pieces containing the COOH ends, cuts procollagen into 3/4 pieces with NH2-peptides attached and 1/4 pieces attached to the disulfide-linked COOH-peptides. The COOH-peptides of pro alpha 1 and pro alpha2 were labeled in a 2:1 ratio at 4 min, indicating simultaneous translation of pro alpha1 and pro alpha2.