Intestinal fatty acid binding protein (IFABP) is a 15 kDa intracellular lipid-binding protein exhibiting a beta-barrel fold that resembles a clamshell. The beta-barrel, which encloses the ligand binding cavity, consists of two perpendicular five-stranded beta-sheets with an intervening helix-turn-helix motif between strands A and B. Delta98delta (fragment 29-126 of IFABP) was obtained either in its recombinant form or by limited proteolysis with clostripain. Despite lacking extensive stretches involved in the closure of the beta-barrel, delta98delta remains soluble and stable in solution. Spectroscopic analyses by circular dichroism, ultraviolet absorption, and intrinsic fluorescence indicate that the fragment retains substantial beta-sheet content and tertiary interactions. In particular, the environment around W82 is identical in both delta98delta and IFABP, a fact consistent with the conservation in the former of all the critical amino acid residues belonging to the hydrophobic core. In addition, the Stokes radius of delta98delta is similar to that of IFABP and 16% larger than that calculated from its molecular weight (11 kDa). The monomeric status of delta98delta was further confirmed by chemical cross-linking experiments. Although lacking 25% of the amino acids of the parent protein, in the presence of GdnHCl, delta98delta unfolds through a cooperative transition showing a midpoint at 0.90 M. Remarkably, it also preserves binding activity for fatty acids (Kd = 5.1 microM for oleic acid and Kd = 0.72 microM for trans-parinaric acid), a fact that exerts a stabilizing effect on its structure. These cumulative evidences show that delta98delta adopts a monomeric state with a compact core and a loose periphery, being so far the smallest structure of its kind preserving binding function.

A number of naturally occurring mutations of human apolipoprotein A-I (apoA-I) have been associated with hereditary amyloidoses. The molecular mechanisms involved in amyloid-associated pathology remain largely unknown. Here we examined the effects of the Arg173Pro point mutation in apoA-I on the structure, stability, and aggregation propensity, as well as on the ability to bind to putative ligands. Our results indicate that the mutation induces a drastic loss of stability, and a lower efficiency to bind to phospholipid vesicles at physiological pH, which could determine the observed higher tendency to aggregate as pro-amyloidogenic complexes. Incubation under acidic conditions does not seem to induce significant desestabilization or aggregation tendency, neither does it contribute to the binding of the mutant to sodium dodecyl sulfate. While the binding to this detergent is higher for the mutant as compared to wt apoA-I, the interaction of the Arg173Pro variant with heparin depends on pH, being lower at pH 5.0 and higher than wt under physiological pH conditions. We suggest that binding to ligands as heparin or other glycosaminoglycans could be key events tuning the fine details of the interaction of apoA-I variants with the micro-environment, and probably eliciting the toxicity of these variants in hereditary amyloidoses.

The design of b-barrels has always been a formidable challenge for de novo protein design. For instance, a persistent problem is posed by the intrinsic tendency to associate given by free edges. From the opposite standpoint provided by the redesign of natural motifs, we believe that the intestinal fatty acid binding protein (IFABP) framework allows room for intervention, giving rise to abridged forms from which lessons on b-barrel architecture and stability could be learned. In this context, D98D (encompassing residues 29-126 of IFABP) emerges as a monomeric variant that folds properly, retaining functional activity, despite lacking extensive stretches involved in the closure of the b-barrel. Spectroscopic probes (fluorescence and circular dichroism) support the existence of a form preserving the essential determinants of the parent structure, albeit endowed with enhanced flexibility. Chemical and physical perturbants reveal cooperative unfolding transitions, with evidence of significant population of intermediate species in equilibrium, structurally akin to those transiently observed in IFABP. The recognition by the natural ligand oleic acid exerts a mild stabilizing effect, being of a greater magnitude than that found for IFABP. In summary, D98D adopts a monomeric state with a compact core and a loose periphery, thus pointing to the nonintuitive notion that the integrity of the b-barrel can indeed be compromised with no consequence on the ability to attain a native-like and functional fold.Keywords: IFABP; truncated variant; b-barrel protein folding; redesign of natural motifs; equilibrium folding intermediates; circular dichroism; fluorescence spectroscopy Additional Supporting Information may be found in the online version of this article.Abbreviations: ANS, 1-anilino naphthalene-8-sulfonic acid; apo-or holo-, prefixes that denote the absence or presence of fatty acid ligand, respectively; CMFE, center of mass of fluorescence emission; GdnHCl, guanidinium hydrochloride; IFABP, intestinal fatty acid binding protein; D98D, a truncated variant of IFABP corresponding to the fragment 29-126 of the parent protein.