Fig. 1. (A) Linear structure of human collagen IV chains. Six different genes encode collagen IV 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 3, 4, 5 and 6 compared with 1 and 2. On the basis of sequence homology these different chains can be divided in two groups, the 1-like (1, 3, 5) and the 2-like (2, 4, 6). Assembly of collagen IV 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).

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