Decorin (DCN) belongs to the small leucine-rich repeat proteoglycan family (SLRPs) which also includes biglycan, fibromodulin (Hedlund et al. 1994 - binding to collagen II), lumican and asporin (Hedbom & Heinegard 1993, Ezura et al. 2000). Fibromodulin and lumican bind the same site while the binding site for decorin is distinct (Hedbom & Heinegard 1993). All appear to be involved in collagen fibril formation and matrix assembly (Ameye & Young 2002, Kalamajski & Oldberg 2010). DCN consists of a core protein of approximately 40 kDa attached to a single chondroitin or dermatan sulfate glycosaminoglycan (GAG) chain. It interacts with collagen types I, II (Vogel et al. 1984), III (Witos et al. 2011), V (Whinna et al. 1993), VI (Bidanset et al. 1992) and XIV (Ehnis et al. 1997). It binds collagen I and II near the N-terminus, placing it at the 'd' band gap in the fibril structure (Kalamajski et al. 2007). The binding site for DCN on collagen XIV is in the NH2-terminal fibronectin type III repeat. In addition, an auxiliary binding site located COOH-terminally to this fibronectin type III repeat interacts with the glycosaminoglycan component of DCN.
DCN binding regulates fibrillogenesis (Vogel et al. 1984, Orgel et al. 2006). One molecule of DCN interacts with four to six collagen molecules. The interaction is between collagen and the core protein, not the GAG chain, and is more likely to involve the monomeric, not dimeric form (Orgel et al. 2009). Fibronectin (Winnemoller et al. 1991) and thrombospondin-1 (Winnemoller et al. 1992) are also DCN interactors. DCN acts as a sink for all three isoforms of TGF-Beta, binding them when already bound to collagen (Markmann et al. 2000). Degradation of DCN by matrix metalloproteinases MMP-2, -3 or -7 results in release of TGF-beta (Imai et al. 1997). In addition, DCN binds to EGFR (Iozzo et al. 1999) causing prolonged down-regulation of EGFR-mediated mobilization of intracellular calcium (Csordás et al. 2000).