Homologies between the non-collagenous C-terminal (NC1) globular domains of the alpha 1 and alpha 2 subunits of type-IV collagen.

The non-collagenous C-terminal globular domain (NC1) of type-IV collagen has the dual role of initiating triple-helix formation among the subunits and of crosslinking two collagen molecules during basement-membrane meshwork formation. By cloning a cDNA for the NC1 domain of the alpha 2(IV) collagen chain, we have found a high degree of homology (63% for nucleotides, 66% for amino acids) between the NC1 of the alpha 2 and alpha 1 chains of type-IV collagen. All cysteine residues are conserved. This high degree of homology is not found within the helical portion where the homology is 41% for amino acids (only 14% if the obligatory glycine is not used for this analysis). We propose that this high degree of homology within the non-collagenous domain indicates a close evolutionary relationship maintained by functional restraints between the two chains of type IV collagen.