Nidogen mediates the formation of ternary complexes of basement membrane components

Using a recombinant nidogen we have probed the calcium binding potential of various nidogen domains, examined the binding of nidogen to various basement membrane proteins and assessed the ability of nidogen to mediate the formation of ternary complexes between laminin and heparan sulfate proteoglycan and collagen IV and laminin. The results of these experiments indicate that the Ca+2 binding is on the rod-like domain with additional binding observed on the N-terminal G1 domain. With regard to the role of nidogen in mediating complex formation among basement membrane components it was demonstrated that nidogen effectively promotes the formation of a ternary complex between laminin and collagen IV, with both of these components interacting independently with nidogen. Similarly, nidogen mediates a ternary complex formation between laminin and proteoglycan. Interestingly, the interaction between proteoglycan and nidogen is through the protein core of the proteoglycan. We have localized the major interaction sites on nidogen with the proteoglycan core and collagen IV to a region on the globular G2 domain while the C-terminal globe G3 binds to laminin. Ca+2 binding does not appear to be important in either of the binary or ternary complex formations. The data reported allow us to hypothesize that, via the multiple interactions of nidogen with other basement membrane components, nidogen plays a crucial structural role in basement membrane organization and stabilization.