Chain composition of type IV collagen networks in basement membranes

Type IV collagen, the major component of basement membranes (BM), is a family of six very similar chains. This allows for many kinds (isoforms) of triple helical protomers. The protomers self-assemble into network-like supramolecular structures by dimerization through C-terminal (NCl) domains (resulting in six chains forming an NC1 hexamer), and tetramerization through the protomer N-terminal domain. Studies on the networks of seminiferous tubule BM (STBM) involved affinity chromatography to fractionate NCl hexamers excised by collagenase digestion. Three major hexamer populations were obtained that contained α 1(IV) and α 2(IV) chains, α 1(IV)- α 6(IV) chains, and α 3(IV)- α 6(IV) chains, respectively, which indicated three major separate type IV collagen networks in STBM. To study glomerular BM, we used selective proteolysis to separate large fragments of type IV collagen. These contained type IV collagen chains cross-linked between NCl domains, and α 1(IV) through α 5(IV) chains cross-linked between NC1 domains as well between triple helices. The latter set of fragments contained a subset of disulfidelinked α 3(IV), α 4(IV) and α 5(IV) chains. The existence of networks containing α 3(IV), α 4(IV) and α 5(IV) chains suggests a molecular basis for mutations in the gene encoding the α 5(IV) chain causing defective assembly of not only α 5(IV) chains, but also α 3(IV) and α 4(IV) chains in the GBM of patients with Alport syndrome. A significant result of these studies is that GBM contains three major type IV collagen networks: α 1(IV) · α 2(IV), α 1(IV)- α 5(IV) and α 3(IV) · α 4(IV) · α 5(IV), similar to those of STBM, which also contains α 6(IV) chains in the latter two networks.