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Fig. 1. (A) Linear structure of human collagen IV {alpha} chains. Six different genes encode collagen IV {alpha} chains; each polypeptide is composed of three distinct domains: a cysteine-rich N-terminal 7S domain, a central triple-helical domain with multiple small interruptions (green boxes) and a globular C-terminal non-collagenous NC1 domain. The NC1 and central triple-helical domains are of an equivalent size, whereas the 7S domains are shorter in the case of {alpha}3, {alpha}4, {alpha}5 and {alpha}6 compared with {alpha}1 and {alpha}2. On the basis of sequence homology these different chains can be divided in two groups, the {alpha}1-like ({alpha}1, {alpha}3, {alpha}5) and the {alpha}2-like ({alpha}2, {alpha}4, {alpha}6). Assembly of collagen IV {alpha} chains. (B) The assembly of trimers is dependent first on the association of the NC1 domains, then the triple-helical structure forms and 7S domains are covalently associated. Four trimers interact through their 7S domains in a spider-shaped structure, and two trimers interact head to head through their NC1 domains, forming a sheet structure. Several trimers can also lace together along their triple-helical domain, thickening the structure (Timpl et al., 1981).