Sequence-divergent units of the ABA-1 polyprotein array of the nematode Ascaris suum have similar fatty-acid- and retinol-binding properties but different binding-site environments

Polyproteins comprise long polypeptides that are posttranslationally cleave d into proteins of different function, or tandemly repetitive polypeptides which are processed into multiple versions of proteins which are presumed t o have the same function. In the latter case the individual units of the po lyprotein can differ substantially in sequence. Identity of function betwee n the different units therefore cannot be assumed. Here we have examined th e ABA-1 polyprotein allergen of the parasitic nematode Ascaris suum and fou nd it to contain units which show a 50 % difference in amino acid sequence. The parasite therefore produces at least two radically different forms of the allergen encoded within the polyprotein array. In fluorescence-based li gand-binding assays, recombinant polypeptides representing the two forms (d esignated ABA-1A1 and ABA-1B1) showed similar binding affinities for a rang e of fluorescent active-site probes [retinol, dansylundecanoic acid, dansyl -DL-alpha-amino-octanoic acid, cis-parinaric acid (cPnA)] and for the non-s pecific hydrophobic surface probe 8-anilinonaphthalene-1-sulphonic acid. Ho wever, the molecular environments in the active sites are markedly differen t, as indicated by disparate fluorescence emission peaks and intensities of bound probes. CD showed that the proteins have similar secondary structure s but differ in susceptibility to chemical denaturation/unfolding by guanid inium chloride. Both retain a single conserved tryptophan residue in a char acteristic non-polar environment, as revealed by extreme fluorescence blue shift. Thus the gross differences in sequence of the two proteins are not r eflected in their ligand-binding specificities but in their binding-site en vironments.