Conformational changes of chicken liver bile acid-binding protein bound to anionic lipid membrane are coupled to the lipid phase transitions

Chicken liver bile acid-binding protein (L-BABP) binds to anionic lipid membranes by electrostatic interactions and acquires a partly folded state [Nolan, V., Perduca, M., Monaco, H., Maggio, B. and Montich, G. G. (2003) Biochim. Biophys. Acta 1611, 98–106]. We studied the infrared amide I′ band of L-BABP bound to dipalmitoylphosphatidylglycerol (DPPG), dimyristoylphosphatidylglycerol (DMPG) and palmitoyloleoylphosphatidylglycerol (POPG) in the range of 7 to 60 °C. Besides, the thermotrophic behaviour of DPPG and DMPG was studied in the absence and in the presence of bound-protein by differential scanning calorimetry (DSC) and infrared spectra of the stretching vibration of methylene and carbonyl groups. When L-BABP was bound to lipid membranes in the liquid-crystalline state (POPG between 7 and 30 °C) acquired a more unfolded conformation that in membranes in the gel state (DPPG between 7 and 30 °C). Nevertheless, this conformational change of the protein in DMPG did not occur at the temperature of the lipid gel to liquid-crystalline phase transition detected by infrared spectroscopy. Instead, the degree of unfolding in the protein was coincident with a phase transition in DMPG that occurs with heat absorption and without change in the lipid order.