THE THROMBOSPONDIN-LIKE CHAINS OF CARTILAGE OLIGOMERIC MATRIX PROTEIN ARE ASSEMBLED BY A 5-STRANDED ALPHA-HELICAL BUNDLE BETWEEN RESIDUE-20 AND RESIDUE-83

The N-terminal fragment of rat cartilage oligomeric matrix protein (COMP), comprising residues 20-83, was over-expressed in E. coli and purified under non-denaturing conditions. The fragment forms pentamers similar to the assembly domain of the native protein. Its five chains can be covalently linked in vitro by oxidation of cysteines 68 and 71. The fragment adopts a predominantly alpha-helical structure as judged by circular dichroism spectroscopy. On the basis of these findings we propose the model of a five-stranded alpha-helical bundle for the assembly domain of COMP. The studied sequence is conserved in thrombospondins 3 and 4 thus raising the possibility that these proteins are also pentamers.