Protein-membrane interaction and ligand transfer to membranes from intestinal fatty acid binding proteins (FABPs) employing natural ligands

Liver and Intestinal fatty acid binding proteins (FABPs) are small cytosolic proteins presumably involved in the uptake and targeting of fatty acids (FA) to intracellular organelles and metabolic pathways. All further sites of metabolism of long chain FA involve membrane proteins. The objective of this work was to analyze FABP-membrane interaction and FA transfer from FABPs to artificial membranes, in order to better understand the specific physiological roles of I- and LFABP in the enterocyte. We employed two different methodologies: photocrosslinking studies and fluorescence-based FA transfer analysis employing natural ligands. The results of the photocrosslinking studies indicate that apo-IFABP interacts with membranes to a greater extent than holo-IFABP, while the opposite is observed for LFABP, probably indicating that IFABP could be delivering FA to membranes, whereas LFABP may be interacting to remove FA from membranes. Additionally, the interaction of IFABP is greatly increases with negatively charged vesicles, but is not affected by the charge in the vesicles for LFABP. To deepen our understanding of the LFABP FA targeting role, we have employed a tryptophan containing mutant at position 28 (L28W), whose fluorescence is enhanced upon FA binding. This gives us the chance to study both the ligand–protein interaction and protein-to-membrane ligand transfer using various physiological ligands instead of the analogues we have been employing previously. So far, our results of the binding properties of the L28W mutant with oleic acid, under physiological ionic strength, are consistent with a 2 site cooperative mechanism. We have also analyzed the transfer of oleic acid from L28W to phosphatidylcholine vesicles which showed that L28W will let us perform kinetic studies of FA transfer from protein to membranes under conditions which are closer to the physiological, and hence further our knowledge of the specific function/s of LFABP