THE FUNCTION OF THE NC1 DOMAINS IN TYPE-IV COLLAGEN

At its C terminus, the collagen IV molecule bears a globular NC1 domai n, to which two functions have been assigned. In the macromolecular ne twork of collagen IV, two molecules are connected via their NC1 domain s, which form a hexameric complex, stabilized by intermolecular disulf ide bonds. In addition, the NC1 domains are thought to be responsible for chain selection and assembly. In order to understand the role of t he NC1 domains during these steps, hexameric complexes were isolated a nd further investigated. SDS-polyacrylamide gel electrophoresis and We stern 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 reconstitutio n experiments. Only alpha 1(IV)NC1 dimers were able to reconstitute a hexameric complex. alpha 1(IV)-NC1 and alpha 2(IV)NC1 monomers as well as the alpha 2(IV)NC1 dimers showed only a low tendency to form compl exes. It is assumed that during formation of the collagen IV network, lateral aggregation of the molecules via the triple helical domains br ings the C termini of two molecules into close vicinity and that subse quently the weak interactions observed between the NC1 subdomains prov ide the correct alignment for a disulfide exchange. It is, however, qu estionable whether the low affinity between the NC1 subdomains alone i s sufficient for chain assembly and alignment of the alpha(IV) chains before molecule formation.