Type IV collagen is the most abundant structural basement membrane (BM) component, providing a scaffold for other major BM proteins such as laminin (Charonis et al. 1985, 1986). There are six different genes encoding type IV collagen chains, alpha-1 to alpha-6(IV) with distinct tissue distributions. Three alpha chains fold to form the triple helical unit of collagen IV. Three chain combinations have been identified, alpha-1X2 alpha-2(IV), alpha-3,alpha-4, alpha-5(IV) and alpha-5X2, alpha-6(IV) (Borza et al. 2001). The first is the major form, found in all basement membranes, the other types have more restricted distributions.
Collagen IV forms a lattice network rather than extended fibrils. It can be digested by MMP2 (Liotta et al. 1981, Salo et al. 1983, Bergers et al. 2000, Monaco et al. 2006), MMP3 (Okada et al. 1986, Wilhelm et al. 1987, Bejarano et al. 1988, Nicholson et al. 1989), MMP7 (Miyazaki et al. 1990, Murphy et al. 1991), MMP9 (Moll et al. 1990, Morodomi et al. 1992, Murphy et al. 1991, Watanabe et al. 1993, Bergers et al. 2000), MMP10 (Nicholson et al. 1989) and MMP12 (Chandler et al. 1996).