. Using a strategy of charge reversal mutagenesis, the potential role of specific cationic residues in promoting interfacial binding of FABP to anionic phospholipid vesicles has been investigated. Cationic residues chosen included those within the ␣-helical region (Lys-20, Lys-31, and Lys-33) and those that make a significant contribution to the positive surface potential of the protein (Lys-31, Lys-36, Lys-47, Lys-57, and Arg-126). Only three cationic residues make a significant contribution to interfacial binding, and these residues (Lys-31, Lys-36, and Lys-57) are all located within the ligand portal region, where the protein may be predicted to exhibit maximum disorder. The binding of tryptophan mutants, F3W, F18W, and C69W, to dioleoylphosphatidylglycerol vesicles, containing 5 mol% of the fluorescent phospholipid dansyldihexadecanoylphosphatidylethanolamine, was monitored by fluorescence resonance energy transfer (FRET). All three mutants showed enhanced dansyl fluorescence due to FRET on addition of phospholipid to protein; however, this fluorescence was considerably greater with the F3W mutant, consistent with the Nterminal region of the protein coming in close proximity to the phospholipid interface. These results were confirmed by succinimide quenching studies. Overall, the results indicate that the portal region of liver FABP and specifically Lys-31, Lys-36, and Lys-57 are involved in the interaction with the interface of anionic vesicles and that the N-terminal region of the protein undergoes a conformational change, resulting in DAUDA release.Liver fatty acid-binding protein (FABP) 1 is a member of a family of structurally related small (14 -15 kDa) cytosolic lipid binding proteins that also include intestinal, heart (muscle), adipocyte, ileal, keratinocyte, and brain FABP (for recent reviews, see Refs. 1-6). The exact physiological functions of these proteins are unclear, although it is generally thought that they may have a potential role in the uptake and targeting of fatty acids to various intracellular organelles and metabolic pathways. All further sites of metabolism of long chain fatty acids in the cell involve membrane proteins. For a targeting role to operate, the FABP must interact with an intracellular structure such as a membrane interface or receptor/docking protein.A process involving membrane binding has been advocated for intestinal, muscle, and adipose FABP where model fluorescence studies have led to the proposal of a collisional mechanism for explaining the FABP-mediated transfer of fatty acids between phospholipid membranes and vesicles (reviewed in Ref. 3). However, such a process was not observed to operate for liver FABP under the same assay conditions, and an aqueous phase diffusion mechanism is proposed, not requiring interaction of the protein with membrane surfaces (3). In contrast, using different assay conditions we have reported the apparent binding of liver FABP to anionic vesicles, monitored as the release of the fluorescent fatty acid ligand (DAUDA) from the protei...