The ABA-1 allergen of Ascaris lumbricoides: sequence polymorphism, stage and tissue-specific expression, lipid binding function, and protein biophysical properties

The ABA-1 protein of Ascaris lumbricoides (of humans) and Ascaris suum (of pigs) is abundant in the pseudocoelomic fluid of the parasites and also app ears to be released by the tissue-parasitic larvae and the adult stages. Th e genes encoding the polyprotein precursor of ABA-1 (aba-1) were found to b e arranged similarly in the two taxa, comprising tandemly repeating units e ncoding a large polyprotein which is cleaved to yield polypeptides of appro ximately 15 kDa which fall into 2 distinct classes, types A and B. The poly protein possibly comprises only 10 units. The aba-1 gene of A. lumbricoides is polymorphic, and the majority of substitutions observed occur in or nea r predicted loop regions in the encoded proteins. mRNA for ABA-1 is present in infective larvae within the egg, and in all parasitic stages, but was n ot detectable in unembryonated eggs. ABA-1 mRNA was confined to the gut of adult parasites, and not in body wall or reproductive tissues. Recombinant protein representing a single A-type unit for the A. lumbricoides aba-1 gen e was produced and found to bind retinol (Vitamin A) and a range of fatty a cids, including the pharmacologically active lipids lysophosphatidic acid, lysoplatelet activating factor, and there was also evidence of binding to l eukotrienes. It failed to bind to any of the anthelmintics screened. Differ ential Scanning Calorimetry showed that the recombinant protein was highly stable, and unfolded in a single transition at 90.4 degrees C. Analysis of the transition indicated that the protein occurs as a dimer and that the di mer dissociates simultaneously with the unfolding of the monomer units.