Structural and functional domains of collagen: a comparison of the protein with its gene

The covalent protein structures of the homologous chains alpha 1 (I), alpha 2 (I) and alpha 1 (III) are known. Recently the structure of the alpha 2 (I) gene, at least the number and size of its exons has been almost completely elucidated. About 60% of the pro alpha-chain amino acid sequence is involved in the formation of the collagen triple helix. In the protein a repeating D unit is present which is important for the self assembly of the molecules into fibrils. In the gene, no obvious relationship between the 54 base pairs long exons and the repeating D unit could be found. This led to the conclusion that the region of the pro alpha chains involved in triple helix formation evolved first by repeated tandem duplications of an ancestral 54 base pairs exon and that the D repeat in the protein evolved independently of the exon structure of the gene. However, in other important functional regions of the pro alpha-chains that are not involved in helix formation, there is a good correlation with the gene exon structure.