Heparin binding of laminin: contribution of the triple helix in the rod domain to the formation of cryptic and active sites in the globular domain.

Laminin, a multidomain glycoprotein in basement membranes, interacts with heparin through the terminal globular domain (G domain) of its long arm. The interaction with heparin is thought to be important for modulation of basement membrane assembly and adhesion to cells. The G domain contains two different heparin binding activities: a strong one in the proximal portion and a moderate one in the distal portion of globule. The proximal activity was found to be weak in fragment E8 and in the intact laminin long arm, whose three-chain moieties are joined together in a triple-helical coiled-coil. Dissociation of the A chain of E8 from its B chain moieties by denaturation and chain separation resulted in the exposure of heparin binding activity the in the globular domain. Furthermore, when a recombinant G domain with a distal alpha-helical domain was intercalated into the B chains of E8, the triple alpha-helix was reconstituted and heparin binding was considerably reduced. This data provides evidence for the influence of non-heparin binding rod domain on the binding activity in the G domain.