Binding and interactions of L-BABP to lipid membranes studied by molecular dynamic simulations.

Chicken liver bile acid-binding protein (L-BABP) is a member of the fatty acid-binding proteins super family. The common fold is a beta-barrel of ten strands capped with a short helix-loop-helix motif called portal region, which is involved in the uptake and release of non-polar ligands. Using multiple-run molecular dynamics simulations we studied the interactions of L-BABP with lipid membranes of anionic and zwitterionic phospholipids. The simulations were in agreement with our experimental observations regarding the electrostatic nature of the binding and the conformational changes of the protein in the membrane. We observed that L-BABP migrated from the initial position in the aqueous bulk phase to the interface of anionic lipid membranes and established contacts with the head groups of phospholipids through the side of the barrel that is opposite to the portal region. The conformational changes in the protein occurred simultaneously with the binding to the membrane. Remarkably, these conformational changes were observed in the portal region which is opposite to the zone where the protein binds directly to the lipids. The protein was oriented with its macrodipole aligned in the configuration of lowest energy within the electric field of the anionic membrane, which indicates the importance of the electrostatic interactions to determine the preferred orientation of the protein. We also identified this electric field as the driving force for the conformational change. For all the members of the fatty acid-binding protein family, the interactions with lipid membranes is a relevant process closely related to the uptake, release and transfer of the ligand. The observations presented here suggest that the ligand transfer might not necessarily occur through the domain that directly interacts with the lipid membrane. The interactions with the membrane electric field that determine orientation and conformational changes described here can also be relevant for other peripheral proteins.