Type XI collagen is a heterotrimer with the composition (1 alpha, 2 alpha, 3 alpha) retaining non-triple-helical domains.

Three collagen chains, 1 alpha, 2 alpha, and 3 alpha, have previously been identified in the cartilaginous extracellular matrix and have been referred to collectively as type XI collagen. The structure of type XI is poorly defined. Neither the organization of these collagen chains into trimeric molecules nor the extent of proteolytic processing has been adequately determined. Formaldehyde-derived covalent cross-links were introduced into the native molecules. As judged by velocity sedimentation, the cross-links formed were predominantly intramolecular and led to the formation of covalent trimeric molecules. Chromatographic analysis of trimers is most consistent with a heterotrimer (1 alpha, 2 alpha, 3 alpha) being the predominant form. To investigate the structure of type XI as it occurs in the matrix, sterna from chick embryos treated with beta-aminopropionitrile were solubilized without proteolysis with pepsin. Electrophoretic analysis revealed that all three chains of type XI retain non-triple-helical domains. Some heterogeneity was observed in the size of the 1 alpha chain. Metabolic labeling and long term chase experiments suggested that this heterogeneity in the size of 1 alpha may be due to slow or incomplete posttranslational proteolytic processing.