Partly folded states of bovine carbonic anhydrase interact with zwitterionic and anionic lipid membranes

The acidic, partly folded states of bovine carbonic anhydrase II (BCAII) we re used as an experimental system to study the interactions of paltry denat ured proteins with lipid membranes. The pH dependence of their interactions with palmitoyloleoyl phosphatidylcholine (POPC) and palmitoyloleoyl phosph atidylglycerol (POPG) membranes was studied. A filtration binding assay sho ws that acidic partly folded states of BCAII bind to POPC membranes. Fluore scence emission spectra from Trp residues of the bound protein are slightly shifted to shorter wavelength and can be quenched by a water-soluble quenc her of fluorescence, indicating that the binding occurs without deep penetr ation of Trp residues into the membrane. The content of p-structures of the protein in solution, as revealed by FT-IR spectroscopy, decreases in the p artly folded states and the binding to POPC membrane occurs without further changes of secondary structure. In the presence of 0.1 M NaCl, a partly fo lded state self-aggregates and does not bind to POPC membrane. At acidic pH , BCAII binds to POPG membranes both at high and low ionic strength. The bi nding to the anionic lipid occurs with protein self-aggregation within the lipid-protein complexes and with changes in the secondary structure; large blue shifts in the fluorescence emission spectra and the decrease in the ex posure to water-soluble acrylamide quencher of Trp fluorescence strongly su ggest that BCAII penetrates the hydrocarbon domain in the POPG-protein comp lexes. (C) 2000 Elsevier Science B.V. All rights reserved.