Chicken Liver Bile Acid-Binding Protein is in a Compact Partly Folded State at Acidic pH. Its Relevance to the Interaction with Lipid Membranes

VERÓNICA NOLAN, MASSIMILIANO PERDUCA, HUGO MÓNACO AND GUILLERMO MONTICH

BIOCHEMISTRY

American Chemical Society

Lugar: Washington; Año: 2005 vol. 44 p. 8486 - 8493

0006-2960

Chicken liver bile acid-binding protein (formerly known as chicken liver basic fatty acid-binding protein) binds to anionic lipid membranes acquiring a partly folded state [Nolan, V. Perduca, M., Monaco, H., Maggio, B. and Montich, G. (2003) Biochim. Biophys. Acta 1611, 98-106]. To understand the mechanisms of its interactions with membranes we have investigated the presence of partly folded states in solution. Using fluorescence spectroscopy of the single Trp residue, circular dichroism in the far- and near- UV, Fourier transform infrared spectroscopy and size-exclusion chromatography we found that L- BABP was unfolded at pH 2.5 and low ionic strength, although retaining some of its secondary structure. Addition of 0.1 M NaCl at pH 2.5 or decreasing the pH to 1.5 produced a compact partly folded state, similar to that observed when the protein is bound to the lipid membrane, with a partial increase of secondary structure and none of  tertiary structure. Fluorescence emission spectra of this state indicate that the Trp residue is within an environment of low polarity, similar to the native state. This environment is not produced by the insertion of the Trp into soluble aggregates as revealed by size-exclusion chromatography and fluorescence anisotropy. The compact partly folded state can undergo thermal unfolding at lower temperature and with less cooperativity than the native protein. The presence of partly folded states under acidic conditions in solution suggests the possibility that membrane binding of L-BABP occurs via this state.