The interaction of liver fatty-acid-binding protein (FABP) with anionic phospholipid vesicles: is there extended phospholipid anchorage under these conditions?

Abstract

Liver FABP (fatty-acid-binding protein) binds a variety of non-polar anionic ligands including fatty acids, fatty acyl CoAs, lysophospholipids and bile acids. Liver FABP is also able to bind to anionic phospholipid vesicles under conditions of low ionic strength, and membrane binding results in the release of bound ligand. However, the molecular interactions involved in binding to the phospholipid interface and the mechanism of ligand release are not known. Ligand release could be due to a significant conformational change in the protein at the interface or interaction of a phospholipid molecule with the ligand-binding cavity of the protein resulting in ligand displacement. Two portal mutant proteins of liver FABP, L28W and M74W, have now been used to investigate the binding of liver FABP to anionic phospholipid vesicles, monitoring changes in fluorescence and also fluorescence quenching in the presence of brominated lipids. There is a large increase in fluorescence intensity when the L28W mutant protein binds to vesicles prepared from DOPG (dioleoyl-sn-phosphatidylglycerol), but a large decrease in fluorescence intensity when the M74W mutant binds to these vesicles. The Br4-phospholipid prepared by bromination of DOPG dramatically quenches both L28W and M74W, consistent with the close proximity of a fatty acyl chain to the tryptophan residues. The binding of liver FABP to DOPG vesicles is accompanied by only a minimal change in the CD spectrum. Overall, the results are consistent with a molecule of anionic phospholipid interacting with the central cavity of the liver FABP, possibly involving the phospholipid molecule in an extended conformation.

IFABP portal region insertion during membrane interaction depends on phospholipid composition

“…This further supports the notion that electrostatic forces are essential for IFABP-membrane interactions. A similar effect was reported before for the non-collisional LFABP, for which an increase from 0 to 200 mM NaCl abolished completely its interaction with anionic vesicles [33].…”

“…This hypothesis urges to reanalyze by molecular dynamics the release of fatty acids from IFABP starting from the different configurations in which the protein is partially immersed in for each type of membrane. Similar studies were performed already in other members of the FABP family, like Liver FABP (LFABP), where membrane association was observed only under conditions of low ionic strength [33]. In this case, two portal Trp mutants at positions 28 and 74 (L28W and M74W) were employed in the analysis.…”

“…The conformational change that would promote the transfer of the ligand may involve both ionic and hydrophobic interactions with the membrane [10,31], but further studies are needed to understand how membrane interaction and conformational changes in the portal region are being translated in the exit/entry of the ligand to/from the phospholipid bilayer. An alternative was suggested by the analysis of LFABP-vesicles interaction, where a single acyl chain from a phospholipid of the membrane is believed to be inserted into the ligand binding cavity via the portal region, potentially promoting the release/upload of ligands [33].…”

NMR Studies Reveal the Role of Biomembranes in Modulating Ligand Binding and Release by Intracellular Bile Acid Binding Proteins

“…For example, the interaction of liver FABP with anionic phospholipids at low ionic strength was shown to involve both electrostatic and non-polar interactions. 29,30 A phospholipid molecule is then suggested to interact with the central cavity of the protein, replacing the fatty acid ligand. The interaction with the membrane appears to be mediated by residues located at the portal region of the protein, with the N-terminal region undergoing a conformational change.…”

Interaction of enterocyte FABPs with phospholipid membranes: Clues for specific physiological roles

“…In contrast, LFABP has been classically considered a “diffusional FABP,” based on FA transfer kinetic studies. Nevertheless, it has been demonstrated that LFABP can bind to small anionic phospholipid vesicles, albeit only under conditions of low ionic strength [22-24]. …”