The collagen family of proteins: two distinct lines of evolution.

Fourteen well-defined collagen types are known in vertebrates and a few of their counterparts have been described in invertebrates. A single, ancestral collagen type might have been expected in the most primitive multicellular animals, the sponges. These organisms actually contain collagen in two forms, cross-striated fibrils and a large assembly of thin filaments constituting the spongin skeleton. A molecular biology study has revealed that these two forms correspond to two distinct genetic collagen types. Several short-chain collagen sequences have been deduced from their cDNA. They bear homologies with nematode cuticular collagens, with basement membrane collagens and with the type XIII collagen of vertebrates. The corresponding genes are expressed in cells secreting the spongin. A fibrillar collagen has also been characterized. Its structure at the C-terminal non-helical domain and its gene organization allowed a comparison with the vertebrate type XI fibrillar collagen, considered as an axial core contained in vertebrate larger fibrils.