Brain fatty acid binding protein exhibits non-preferential and mutation-resistant binding towards fatty acids

Members of the fatty acid binding protein (FABP) family function as intracellular transporters of long chain fatty acids and other hydrophobic molecules to different cellular compartments. Brain fatty acid binding protein (FABP7) exhibits ligand-directed differences in cellular transport behavior. For example, when FABP7 binds to docosahexaenoic acid (DHA), the complex relocates to the nucleus and influences transcriptional activity, whereas FABP7 bound with monosaturated fatty acids remain in the cytosol. We used a variety of biophysical techniques to enhance understanding of ligand-directed transport. Specifically, we examine how FABP7 binds to fatty acids, including saturated stearic acid (SA), monounsaturated oleic acid (OA), and polyunsaturated DHA. We find that at 37{degrees}C FABP7 has near equivalent binding affinities for the fatty acids, while at lower temperatures, FABP7 exhibits a preference for the unsaturated fatty acids. Therefore, nuclear localization of the FABP7-DHA complex cannot be explained by binding preferences. Using NMR spectroscopy and molecular dynamics simulations, we observe that DHA uniquely affects the portal region of FABP7, which could enhance the complexs nuclear localization. Mutations to purported critical binding residues (R126L and Y128F) have little effect on fatty acid binding, with molecular dynamics simulations revealing that the bound fatty acid can adopt binding poses that can accommodate the mutations. SignificanceThis work studies FABP7 at physiological temperature and shows that nuclear localization of FABP7 cannot be initiated by tighter ligand interactions. Through biophysical experiments and simulations, we show ligand-dependent conformational changes, instead of binding affinities, are associated with certain biological outcomes. Extensive simulations reveal redundancy in available ligand binding conformations, which permits mutant-resistant binding. This suggests that these mutations do not affect ligand binding affinities, but changes in protein conformation and dynamics may result in disease associated cellular outcomes.