Cartilage oligomeric matrix protein (COMP, thrombospondin-5) is a 524-kDa pentameric glycoprotein expressed primarily in cartilage, tendon, ligament and synovium. In adult cartilage, COMP is located primarily in the inter-territorial matrix between chondrocytes (Murphy et al. 1999). The mature protein is pentameric with each monomer linked to its neighbour by a disulphide bond, located at the amino terminus of the protein (Hedbom et al. 1992, Morgelin et al. 1992). COMP binds directly to collagen types I, II and IX (Rosenberg et al. 1998, Thur et al. 2001) at the fibril periphery. In addition it binds fibronectin (FN1) (Di Cesare et al. 2002), matrilins 1, 3 and 4 (Mann et al. 2004), and through the glycosaminoglycans heparan sulphate and chondroitin sulphate to aggrecan (Hauser et al. 1996, Chen et al. 2007).
Mutations in COMP lead to pseudoachondroplasia and multiple epiphyseal dysplasia (Jackson et al. 2012). COMP binding to FN1 and probably to other partners requires the presence of the divalent cations Ca2+, Mg2+ or Mn2+. Each COMP subunit binds approximately 10 calcium ions (Chen et al. 2000).