Schmid's metaphyseal chondrodysplasia mutations interfere with folding of the C-terminal domain of human collagen X expressed in Escherichia coli

Human collagen X contains a highly conserved 161-amino acid C-terminal non-triple helical domain that is homologous to the C-terminal domain of collagen VIII and to the Clq module of the human C1 enzyme. We have expressed this domain (residues 545-680) in Escherichia coli as a glutathione S-transferase fusion protein, The purified fusion protein trimerizes spontaneously in vitro, and after thrombin cleavage, the purified C-terminal domain trimer (46.2 kDa) is extremely stable and trypsin-resistant. Mutations within the C terminal domain have been observed in patients with Schmid's metaphyseal chondrodysplasia (SMCD), Some of these mutations (Y598D, G618V, W651X, or H669X; X is the stop codon) were constructed by site directed mutagenesis. Each mutation had identical consequences regarding the fusion protein: 1) absence of trimeric formation, 2) copurification of the similar to 60-kDa GroEL chaperone protein, and 3) sensitivity of the monomeric fusion protein to trypsin digestion. These results show that the C-terminal domain of collagen X is sufficient to produce a very stable and compact trimer in the absence of collagen Gly-X-Y repeats. Moreover, mutations causing SMCD interfere in this system with the correct folding of the C-terminal domain. The existence of a similar mechanism in chondrocytes might explain the relative homogeneity of phenotypes in SMCD despite the diversity of mutations.