The role of cysteine residues in the folding and association of the COOH-terminal propeptide of types I and III procollagen.

Procollagen chains assemble in a type-specific manner forming either homo- or heterotrimers. The molecular mechanisms underlying procollagen chain selectivity are unknown, although it is thought that the C-propeptide (COOH-terminal propeptide) is responsible for directing chain recognition and assembly. To define the processes involved in chain selection we reconstituted the initial stages of procollagen folding and assembly in a cell-free system. Using human pro-alpha 1(III) and pro-alpha 2(I) chains as prototypes of chains that are either capable or incapable of forming homotrimeric molecules, respectively, we constructed two minigenes (p alpha 1(III) delta 1 and p alpha 2(I) delta 1) that lacked most of the triple helical domains. The minigenes were transcribed in vitro and translated in a rabbit reticulocyte lysate supplemented with microsomal membranes under conditions that favored disulfide bond formation. Both pro-alpha 1(III) delta 1 and pro-alpha 2(I) delta 1 chains formed intrachain disulfide bonds within the C-propeptide. However, only pro-alpha 1(III) delta 1 chains were able to self-associate forming homotrimers stabilized by interchain disulfide bonds. The C-propeptide of the pro-alpha 1(III) chain contains 8 cysteine residues (Cys-1-8). We used a site-directed mutagenesis to investigate the role of specific cysteine residues in trimer formation and found that substitution of serine for Cys-1, Cys-2, Cys-3, and Cys-4 prevented interchain disulfide bonding and trimerization. Furthermore, mutations in Cys-1 and Cys-4 also prevented intrachain disulfide bond formation within the C-propeptide. The C-propeptide of the pro-alpha 2(I) chain contains only 7 cysteine residues, lacking cysteine at position 2. Substitution of the existing Ser residue with Cys did not produce a homotrimeric phenotype, indicating that additional recognition signals are required to determine chain selection.