Characterization of a cDNA encoding RP43, a CUB-domain-containing protein from the tube of Riftia pachyptila (Vestimentifera), and distribution of its transcript.

A major 43 kDa protein from the protective tube of Riftia pachyptila (Vestimentifera), named RP43, was partly microsequenced after isolation by SDS/PAGE from the protein fraction of tubes collected around the hydrothermal vents at the East Pacific Rise. On the basis of the partial peptide sequences obtained, experiments using reverse-transcriptase-mediated PCR and rapid amplification of cDNA ends led to the complete cDNA sequence. Analysis of deduced amino acid sequence of RP43 showed the presence of CUB domains (100-110-residue-spanning domains first reported in the complement subcomponents C1r/C1s, epidermal-growth-factor-related sea urchin protein and bone morphogenetic protein 1) that seem to be involved in protein-protein and glycosaminoglycan-protein interactions. This peculiarity strongly suggests that RP43 might have a crucial role in tightening the different elements of the worm tube. However, the absence of chitin-binding motifs inclines us to favour a role in protein-protein interactions during assembly of the tube components. The RP43 mRNA was found to be present in specific epidermal cells from the worm body wall but never in the chitin-synthesizing gland cells. This unexpected result clearly indicates that the major tube protein is synthesized in specialized areas of the outer epithelium and that at least two different tissues are involved in the synthesis of the exoskeleton.