Fig. 2. (A) Linear structure of human collagen XV and XVIII 1 chains. The 1 chains of collagen XV and XVIII are structurally homologous; they define a new collagen subfamily, the multiplexin family, on the basis of their central triple-helical domain with multiple long interruptions (green boxes). They are also characterized by a long non-collagenous N-terminal-domain-containing Thrombospondin sequence motif with two splicing variants in human collagen XVIII and a long non-collagenous globular C-terminal domain or NC1 domain. (B) Functional sub-domains of human NC1(XVIII) and protease cleavage sites. The NC1 domain contains three functionally different subdomains: these domains consist of a N-terminal non-covalent trimerization domain necessary for the association of trimers, a hinge domain containing multiple sites sensitive to different proteases and an endostatin globular domain covering a fragment of 20 kDa with anti-angiogenic and anti-branching morphogenesis activities. Numerous enzymes can generate fragments containing endostatin. Cathepsin L and elastase are the most efficient, but in contrast to MMP cleavage leading to accumulation of endostatin, cathepsin L and B degrade the molecule [cleavage sites are indicated according to the data published by Ferreras (Ferreras et al., 2000)].

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