Lipid-induced conformational transitions of beta-lactoglobulin.
Bovine beta-lactoglobulin (betaLG) provides an excellent model protein system for beta-to-alpha conformational change, but its behavior varies when the change is induced by alcohols, surfactants, or lipid vesicles. Here the interaction and orientation of betaLG in association with various artificial lipid vesicles at neutral and acidic pH have been studied by use of several complementary spectroscopic techniques. Circular dichroism (CD) and Fourier transform infrared (FTIR) spectra demonstrated that betaLG acquires a non-native alpha-helical structure upon binding with anionic lipids, while zwitterionic lipids do not have a significant effect on its conformation. The degree of induced alpha-helix depends on the lipid concentration and is strongly affected by the charge of the protein and lipids as well as the ionic strength of the solution. Near-UV CD and Trp emission spectra revealed that the tertiary structure of lipid-bound betaLG is highly expanded but not completely disrupted. Fluorescence quenching together with a Trp emission blue shift showed that the Trp residues remain largely shielded from the solvent when interacting with DMPG, which would be consistent with at least some portions of betaLG having been inserted into the lipid membrane. The orientations of the alpha-helix and beta-sheet axes in membrane-bound betaLG were found to be parallel and perpendicular, respectively, to the membrane film normal, as determined by use of polarized attenuated total reflection (ATR) FTIR spectra. Our findings reveal that the lipid-induced beta-to-alpha transition in betaLG, accompanied by a substantial disruption in tertiary structure, is mainly driven by strong electrostatic interactions. Once the tightly packed betaLG is disrupted, hydrophobic residues become exposed and available for insertion into the lipid bilayer, where hydrophobic interaction with the lipids may play a role in stabilizing the helical components.