Redox- and pH-dependent association of plastocyanin with lipid bilayers: effect on protein conformation and thermal stability

The effect of electrostatic interactions on the conformation and thermal st ability of plastocyanin (Pc) was studied by infrared spectroscopy. Associat ion of any of the two redox states of the protein with positively charged m embranes at neutral pH does not significantly change the secondary structur e of Pc. However, upon membrane binding, the denaturation temperature decre ases, regardless of the protein redox state. The extent of destabilization depends on the proportion of positively charged lipid headgroups in the mem brane, becoming greater as the surface density of basic phospholipids incre ases. In contrast, at pH 4.8 the membrane binding-dependent conformational change becomes redox-sensitive. While the secondary structures and thermal stabilities of free and membrane-bound oxidized Pc are similar under acidic conditions, the conformation of the reduced form of the protein drasticall y rearranges upon membrane association. This rearrangement does not depend on electrostatic interactions to occur, since it is also observed in the pr esence of uncharged lipid bilayers. The conformational transition, only obs erved for reduced Pc, involves the exposure of hydrophobic regions that lea ds to intermolecular interactions at the membrane surface. Membrane-mediate d partial unfolding of reduced Pc can be reversed by readjusting the pH to neutrality, in the absence of electrostatic interactions. This redox-depend ent behavior might reflect specific structural requirements for the interac tion of Pc with its redox partners. (C) 2000 Elsevier Science B.V. All righ ts reserved.