Native cartilage matrix protein (CMP). A compact trimer of subunits assembled via a coiled-coil alpha-helix

Cartilage matrix protein (CMP), a major component of many types of cartilage, is a noncollagenous glycoprotein with a molecular mass of 148 kDa consisting of three identical subunits. With the aim of performing a more comprehensive characterization, we purified CMP in a native conformation from fetal bovine rib cartilage avoiding the denaturing solvents previously used. CMP could be selectively extracted with EDTA-containing buffer which indicates a divalent cation-dependent anchorage in the cartilage matrix. Determination of the amino-terminal sequence of the bovine protein confirmed its identity when compared with published cDNA sequences of chicken and human CMP. Electron microscopy revealed the presence of three ellipsoid subunits which are connected at one end. Sequence analysis indicated the presence of a coiled-coil alpha-helical assembly domain formed by the COOH-terminal end of the subunits. The trimeric structure was retained after complete reduction under native conditions which shows that the coiled-coil domain is stable also in the absence of interchain disulfide bonds.