Analysis of a cDNA-derived sequence of a novel mannose-binding lectin, codakine, from the tropical clam Codakia orbicularis

This work relates to the characterisation of the predominant gill protein of the white clam, Codakia orbicularis (Linné, 1758), which harbours endosymbiotic sulphur-oxidising chemoautotrophic bacteria. Total RNA was extracted from the clam to perform 3'rapid amplification of cDNA ends (3'RACE) using degenerate oligonucleotides prepared from a partial sequence of a predominant protein of about 14kDa, termed codakine. The partial peptide sequence was obtained by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and Edman degradation. Clones isolated from the cDNA library and containing the gene of interest were used in polymerase chain reaction (PCR). The PCR and RACE-PCR products were sequenced to determine the entire coding DNA sequence of codakine. BLAST analysis revealed about 23% to 29% sequence identity between codakine and various animal C-type lectins that are often involved in symbiosis and immune defences. Codakine also contains the motifs and domains of Ca(2+)-dependent C-type lectins, and in particular the tripeptide EPN motif frequently found in mannose-binding lectins (MBLs). Analysis of the protein by affinity chromatography on a mannose-agarose column is consistent with the findings that codakine is a dimeric Ca(2+)-dependent MBL of about 29kDa. Based on the present results, it is hypothesised that this novel C. orbicularis gill protein is involved in the recognition of symbiotic and pathogenic bacteria.