Complete structure of the chicken alpha 2(VI) collagen gene.

Type VI collagen is a hybrid molecule consisting of a short triple helix flanked by two large globular domains. These globular domains are composed of several homologous repeats which show a striking similarity to the collagen-binding motifs found in von Willebrand factor. The alpha 2(VI) subunit contains three of these homologous repeats termed D1, D2 and D3. We have isolated and characterized the entire gene for chicken alpha 2(VI) collagen. This gene, which is present as a single copy in the chicken genome, is 26 kbp long and comprises 28 exons. All exons can be classified in three groups. (a) The triple-helical domain is encoded by 19 short exons (27-90 bp) separated by introns of phase class 0. These exons are multiples of 9 bp and encode an integral number of collagenous Gly-Xaa-Yaa triplets. (b) The homologous repeats D1-D3 are encoded by one or two very long exons each (153-1578 bp). These exons are separated by introns of phase class 1. (c) The homologous repeats and the collagen sequence are linked to each other by three short adapter segments which are each encoded by a single exon (21-46 bp). The modular nature of the polypeptide is thus clearly reflected by the mosaic structure of its gene. The size of the exons and the phase class of the introns suggest that the alpha 2(VI) gene evolved by duplication and shuffling of two different primordial exons, one of 9 bp encoding a collagen Gly-Xaa-Yaa triplet and one of 600 bp encoding the precursor of the homologous repeats.