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

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