AN ABUNDANT, TRANS-SPLICED MESSENGER-RNA FROM TOXOCARA-CANIS INFECTIVE LARVAE ENCODES A 26-KDA PROTEIN WITH HOMOLOGY TO PHOSPHATIDYLETHANOLAMINE-BINDING PROTEINS

A full-length mRNA encoding a secreted 26-kDa antigen of infective lar vae of the ascarid nematode parasite Toxocara canis has been identifie d. This was characterized as a 1,082-base pair clone highly abundant ( 0.8-1.9%) in cDNA prepared from infective stage larvae but absent from cDNA from adult male worms. Sequence analysis revealed an open readin g frame corresponding to a hydrophilic 263-amino acid residue polypept ide with a 20-residue N-terminal signal peptide, indicating that it is secreted. The 5' end of the cDNA was isolated by polymerase chain rea ction using a primer containing the nematode spliced leader sequence, SL1, showing that the mRNA is trans-spliced. The molecular mass of the putative protein with the signal peptide removed is 26.01 kDa, and an tibody to the recombinant protein expressed in bacterial vectors react s with a similarly sized protein in T. canis excretory/secretory (TES) products. An identical sequence was obtained from a genomic clone iso lated by expression screening with mouse antibody to TES. The 72 amino acid residues adjacent to the signal peptide form two homologous 36 r esidue motifs containing 6 cysteine residues; this motif is found also in the T. canis-secreted glycoprotein TES-120 and in genes of Caenorh abditis elegans. Sequence data base searches revealed significant simi larity to 7 other sequences in a newly recognized gene family of phosp hatidylethanolamine binding proteins that includes yeast, Drosophila, rat, bovine, simian, and human genes and a representative from the fil arial nematode Onchocerca volvulus. Assays with the T. canis recombina nt 26-kDa protein expressed as a fusion with maltose-binding protein h ave confirmed phosphatidylethanolamine-binding specificity for this no vel product.