Mapping of binding sites for nidogens, fibulin-2, fibronectin and heparin to different IG modules of perlecan

Perlecan, a major basement membrane proteoglycan, has a complex modular structure designed for the binding of many cellular and extracellular ligands. Its domain IV, which consists of a tandem of immunoglobulinlike modules (IG2-IG15), is rich in such binding sites, which have been mapped to different modules obtained by recombinant production. Heparin/sulfatide binding was restricted to IG5 and shown to depend on four arginine residues that are close in space in beta strands B and E of the C-type IG fold. The nidogen-1 and nidogen-2 isoforms bind to IG3 with high affinity (K-d similar to 10 nM). This interaction depends on the globular nidogen domain G2 and is crucial for the formation of ternary complexes with laminins. Two loops of IG3 located between beta strands B/C and F/G, which are spatially close, make a major contribution to binding. Fibronectin binding was localized to IG4-5 and fibulin-2 binds to IG2 and IG13-15 with different affinities. This implicates a complex cluster of heterotypic interaction sites apparently important for the supramolecular organization of perlecan in tissues. (C) 2001 Academic Press.