Sj-FABPc fatty-acid-binding protein of the human blood fluke Schistosoma japonicum: structural and functional characterization and unusual solvent exposure of a portal-proximal tryptophan residue

Sj-FABPc of the blood fluke of humans, Schistosoma japonicum, is a member o f the FABP/P2/CRBP/CRABP family of beta-barrel cytosolic fatty-acid-binding and retinoid-binding proteins. Sj-FABPc has at least eight different varia nts encoded by a single-copy polymorphic gene. In fluorescence-based assays , recombinant Sj-FABPc was found to bind 11-(dansylamino)undecanoic acid (D AUDA), inducing a shift in peak fluorescence emission from 543 to 493 nm. A similar spectral change was observed in dansyl-amino-octanoic acid (in whi ch the dansyl fluorophore is attached at the alpha-carbon rather than the o mega-carbon of DAUDA), indicating that the ligand enters entirely into the binding site. Sj-FABPc also bound the naturally fluorescent cis-parinaric a cid, as well as oleic acid and arachidonic acid, by competition, but not al l-trans-retinol. Dissociation constants were, for cis-parinaric acid, K-d = 2.5 +/- 0.1 mu M (mean +/- S.E.M.) and an apparent stoichiometry consisten t with one binding site per molecule of Sj-FABPc and, for oleic acid, K-i a pproximate to 80 nM. A deletion mutant from which alpha-11 was absent faile d to bind ligand. Sj-FABPc modelled well to known structures of the protein family; an unusually solvent-exposed Trp side chain was evident adjacent t o the presumptive portal through which ligand is thought to enter and leave . Intrinsic fluorescence analyses of Sj-FABPc and of the deletion mutant (f rom which Trp-27 is absent) confirmed the unusual disposition of this side chain. Virtually all members of the FABP/P2/CRBP/CRABP protein family have prominent hydrophobic side chains in this position, with the exception of l iver FABP and ileal FABP, which instead have charged side chains. Liver FAB P is known to be distinct from other members of the protein family in that it does not seem to contact membranes to collect and deposit its ligand. It is therefore postulated that the unusually positioned apolar side chains i n Sj-FABPc and others in the family are important in interactions with memb ranes or other cellular components.