Coupled plasmon-waveguide resonance spectroscopy studies of the cytochrome b6f/plastocyanin system in supported lipid bilayer membranes.

The incorporation of cytochrome (cyt) b6f into a solid-supported planar egg phosphatidylcholine (PC) bilayer membrane and complex formation with plastocyanin have been studied by a variant of surface plasmon resonance called coupled plasmon-waveguide resonance (CPWR) spectroscopy, developed in our laboratory. CPWR combines greatly enhanced sensitivity and spectral resolution with direct measurement of anisotropies in refractive index and optical extinction coefficient, and can therefore probe structural properties of lipid-protein and protein-protein interactions. Cyt b6f incorporation into the membrane proceeds in two stages. The first occurs at low protein concentration and is characterized by an increase in total proteolipid mass without significant changes in the molecular order of the system, as demonstrated by shifts of the resonance position to larger incident angles without changing the refractive index anisotropy. The second stage, occurring at higher protein concentrations, results in a decrease in both the mass density and the molecular order of the system, evidenced by shifts of the resonance position to smaller incident angles and a large decrease in the membrane refractive index anisotropy. Plastocyanin can bind to such a proteolipid system in three different ways. First, the addition of plastocyanin before the second stage of b6f incorporation begins results in complex formation between the two proteins with a KD of approximately 10 microM and induces structural changes in the membrane that are similar to those occurring during the second stage of complex incorporation. The addition of larger amounts of plastocyanin under these conditions leads to nonspecific binding to the lipid phase with a KD of approximately 180 microM. Finally, the addition of plastocyanin after the completion of the second phase of b6f incorporation results in tighter binding between the two proteins (KD approximately 1 microM). Quantitation of the binding stoichiometry indicates that two plastocyanin molecules bind tightly to the dimeric form of the cyt b6f complex, assuming random insertion of the cytochrome into the bilayer. The structural basis for these results and formation of the proteolipid membrane are discussed.