The function of the NC1 domains in type IV collagen.

At its C terminus, the collagen IV molecule bears a globular NC1 domain, to which two functions have been assigned. In the macromolecular network of collagen IV, two molecules are connected via their NC1 domains, which form a hexameric complex, stabilized by intermolecular disulfide bonds. In addition, the NC1 domains are thought to be responsible for chain selection and assembly. In order to understand the role of the NC1 domains during these steps, hexameric complexes were isolated and further investigated. SDS-polyacrylamide gel electrophoresis and Western blot revealed disulfide-linked alpha 1 (IV)NC1 and alpha 2(IV)NC1 homodimers but no heterodimers. The hexamers were dissociated at low pH, separated into monomers and dimers, and submitted to reconstitution experiments. Only alpha 1(IV)NC1 dimers were able to reconstitute a hexameric complex. alpha(IV)-NC1 and alpha 2(IV)NC1 monomers as well as the alpha 2(IV)NC1 dimers showed only a low tendency to form complexes. It is assumed that during formation of the collagen IV network, lateral aggregation of the molecules via the triple helical domains brings the C termini of two molecules into close vicinity and that subsequently the weak interactions observed between the NC1 subdomains provide the correct alignment for a disulfide exchange. It is, however, questionable whether the low affinity between the NC1 subdomains alone is sufficient for chain assembly and alignment of the alpha(IV) chains before molecule formation.