Type IX collagen is heterotrimeric, interacting covalently with type II collagen fibril surfaces, suggesting that they represent macromolecular bridges between fibrils and other cartilage matrix components (Eyre et al. 1987, Olsen 1997). Degradation of type IX (and type II) collagen is seen at the onset of inflammatory arthritis (Kojima et al. 2001).
Collagen type IX is cleaved by MMP3 (Okada et al. 1989, Eyre et al. 1991, Wu et al. 1991) and MMP13 (Knauper et al. 1997).