Δ98Δ, a functional all-β-sheet abridged form of intestinal fatty acid binding protein

Abstract

Intestinal fatty acid binding protein (IFABP) is a 15 kDa intracellular lipid-binding protein exhibiting a ́-barrel fold that resembles a clamshell. The ́-barrel, which encloses the ligand binding cavity, consists of two perpendicular five-stranded ́-sheets with an intervening helix-turn-helix motif between strands A and B. Δ98Δ (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 β-barrel, Δ98Δ remains soluble and stable in solution. Spectroscopic analyses by circular dichroism, ultraviolet absorption, and intrinsic fluorescence indicate that the fragment retains substantial β-sheet content and tertiary interactions. In particular, the environment around W82 is identical in both Δ98Δ 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 Δ98Δ is similar to that of IFABP and 16% larger than that calculated from its molecular weight (11 kDa). The monomeric status of Δ98Δ was further confirmed by chemical cross-linking experiments. Although lacking 25% of the amino acids of the parent protein, in the presence of GdnHCl, Δ98Δ unfolds through a cooperative transition showing a midpoint at 0.90 M. Remarkably, it also preserves binding activity for fatty acids (K d = 5.1 μM for oleic acid and Kd = 0.72 μM for trans-parinaric acid), a fact that exerts a stabilizing effect on its structure. These cumulative evidences show that Δ98Δ adopts a monomeric state with a compact core and a loose periphery, being so far the smallest structure of its kind preserving binding function.