Conformational change of Liver Bile Acid Binding Protein bound to anionic lipid membrane is coupled to the lipid phase transition

DECCA, MB; PERDUCA, M; MONACO, H; MONTICH, GG

Rosario

Congreso; Sociedad Argentina de Biofísica (SAB); 2006

Sociedad Argentina de Biofísica (SAB)

L-BABP is a member of a large family of proteins that share a common structural motif and the capacity to bind non polar compounds. We have described previously that L-BABP binds to anionic lipid membranes and acquires a partly unfolded state in the interface (1). The binding is driven by electrostatic interactions: once the protein is bound to anionic lipids in the absence of added salt, it can be released from the membrane, and the native secondary structure recovered, by an increase in the ionic strength. In this work we describe that the conformation of L-BABP bounded to anionic lipid membranes is influence by the physical state of the lipids. The global protein secondary structure was studied by the infrared amide I’ band. In D2O solution, L-BABP displays an FTIR spectrum with a large absorption band centred at 1623 cm-1, which is asigned to b-sheet and smaller bands at 1650 cm-1, assigned to a-helix in agreement with the know crystallographic structure (2). In general, the FTIR spectra of L-BABP bound to anionic lipid membranes, shows a broader amide I´, a decreased amount of b-sheet, and an increased amount of unordered structure (1640 cm-1) as compared with the protein in solution. Nevertheless, we observed that the conformation of L-BABP is distinctively different when it is bound to lipids in the gel or fluid phase. L-BABP bound to the lipids in the gel phase (LUV of DMPG and DPPG in the temperature range below the phase transition) acquired more structure as compared to the lipid in the liquid crystalline phase (POPG in the whole rage of temperatures or DMPG above the main phase transition). The increase in the temperature from 9 ºC to 30 ºC did not produce changes in the FTIR spectra of L-BABP bound to lipid that remains in the same phase during this temperature range (POPG remaining in the liquid phase and DPPG remaining in the gel phase). Instead, a noticeable change occurred for L-BABP bound to DMPG when the lipid changed from the gel to liquid phase, indicating that such conformational change is coupled to the lipid phase transition. The Fourier self deconvolution and fitting procedure revealed that this global change in the spectra was due to an increase in the band component due to unordered structure and a decrease in the band component due to b-sheet.