CLONING OF AN ANNELID FIBRILLAR-COLLAGEN GENE AND PHYLOGENETIC ANALYSIS OF VERTEBRATE AND INVERTEBRATE COLLAGENS

Arenicola marina possesses cuticular and interstitial collagens, which are mostly synthesised by its epidermis. A cDNA library was construct ed from the body wall. This annelid cDNA library was screened with a s ea-urchin-collagen cDNA probe, and several overlapping clones were iso lated. Nucleotide sequencing of these clones revealed an open reading frame of 2052 nucleotides. The translation product exhibits a triple h elical domain of 138 Gly-Xaa-Yaa repeats followed by a 269-residue-lon g C-terminal non-collagenous domain (C-propeptide). The triple helical domain exhibits an imperfection that has been previously described in a peptide produced by cyanogen bromide digestion (CNBr peptide) of A. marina interstitial collagen. This imperfection occurs at the same pl ace in the interstitial collagen of the vestimentiferan Riftia pachypt ila. This identifies the clone as coding for the C-terminal part of a fibrillar collagen chain. It was called FAm1 alpha, for fibrillar coll agen la chain of A. marina. The non-collagenous domain possesses a str ucture similar to carboxy-terminal propeptides of fibrillar pro-a chai ns, Only six conserved cysteine residues are observed in A. marina com pared with seven or eight in all other known C-propeptides. This provi des information on the importance of disulfide bonds in C-propeptide i nteractions and in the collagen-assembly process. Phylogenetic studies indicate that the fibrillar collagen 1 alpha chain of A. marina is ho mologous to the R. pachyptila interstitial collagen and that the FAm1 alpha gene evolved independently from the other a-chain genes, Complem entary analyses indicate that the vertebrate fibrillar collagen family is composed of two monophyletic subgroups with a specific position of the collagen type-V chains.