cDNA cloning of galectins from third stage larvae of the parasitic nematode Teladorsagia circumcincta.

A monoclonal antibody raised to a Teladorsagia circumcincta 31-33 kDa doublet antigen was used to immunoscreen a T. circumcincta cDNA expression library. Sheep antibodies eluted from the proteins expressed by two clones immunopositive with the monoclonal antibody specifically recognised the doublet antigen on Western blots of third stage larval extract, confirming that these clones coded for the antigen. Database searches revealed high levels of similarity with beta-galactoside-binding lectin-like proteins (Ga1BPs or galectins) from Caenorhabditis elegans and Onchocerca volvulus. By analogy with these sequences, both T. circumcincta cDNA clones contain the full-length protein coding region. The native doublet proteins could be preferentially extracted from homogenates of third stage larvae with lactose and could be affinity purified on an asialofetuin column, confirming the identity of these bands as galectins. Reverse transcriptase-polymerase chain reaction amplification using a primer based on the C. elegans Spliced Leader SL1 sequence showed that the corresponding T. circumcincta mRNAs are also trans-spliced at their 5' ends. While there are considerable nucleotide differences between the two clones, the majority are located in the non-coding regions. Within the coding region there are 87 nucleotide differences but only three of these result in amino acid substitutions.