Binding of recombinant rat liver fatty acid-binding protein to small anionic phospholipid vesicles results in ligand release: A model for interfacialbinding and fatty acid targeting

A number of intracellular proteins bind to negatively charged phospholipid membranes, and this interfacial binding results in a conformational change that modulates the activity of the protein. Using a fluorescent fatty acid analogue, 11-[5-(dimethylamino)naphthalenesulfonyl]undecanoic acid (DAUDA), it is possible to demonstrate the release of this ligand from recombinant rat liver FABP in the presence of phospholipid vesicles that contain a sign ificant proportion of anionic phospholipids. The Ligand release that is obs erved with anionic phospholipids is sensitive to the ionic strength of the assay conditions and the anionic charge density of the phospholipid at the interface, indicating that nonspecific electrostatic interactions play an i mportant role in the process. The stoichiometric relationship between anion ic phospholipid and liver FABP suggests that the liver FABP coats the surfa ce of the phospholipid vesicle. The most likely explanation for Ligand rele ase is that interaction of FABP with an anionic membrane interface induces a rapid conformational change, resulting in a reduced affinity of DAUDA for the protein. The nature of this interaction involves both electrostatic an d nonpolar interactions as maximal release of liver FABP from phospholipid vesicles with recovery of ligand binding cannot be achieved with high salt and requires the presence of a nonionic detergent. The precise interfacial mechanism that results in the rapid release of ligand from L-FABP remains t o be determined, but studies with two mutants, F3W and F18W, suggest the po ssible involvement of the amino-terminal region of the protein in the proce ss. The conformational change linked to interfacial binding of this protein could provide a mechanism for fatty acid targeting within the cell.