Folding and assembly of type X collagen mutants that cause metaphyseal chondrodysplasia-type schmid. Evidence for co-assembly of the mutant and wild-type chains and binding to molecular chaperones.

Schmid metaphyseal chondrodysplasia results from mutations within the COOH-terminal globular domain (NC1) of type X collagen, a short chain collagen expressed in the hypertrophic region of the growth plate cartilage. Previous in vitro studies have proposed that mutations prevent the association of the NC1 domain of constituent chains of the trimer based upon a lack of formation of a trimeric structure that is resistant to dissociation with sodium dodecyl sulfate. To examine the effect of mutations on folding and assembly within a cellular context, bovine type X cDNAs containing analogous disease causing mutations Y598D, N617K, W651R, and wild-type were expressed in semi-permeabilized cells. We assessed trimerization of the mutant chains by their ability to form a collagen triple helix. Using this approach, we demonstrate that although there is an apparent lower efficiency of association of the mutant NC1 domains, they can drive the formation of correctly aligned triple helices with the same thermal stability as the wild-type collagen. When epitope-tagged mutant and wild-type collagen were co-expressed, heterotrimers could be detected by sequential immunoprecipitation. Both wild-type and mutant type X chains were found in association with the molecular chaperones protein disulfide isomerase and Hsp 47. The implications of these findings on the likely mechanism of Schmid metaphyseal chondrodysplasia will be discussed.