Architecture and Function of Membrane Proteins in Planar Supported Bilayers: A Study with Photosynthetic Reaction Centers

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Abstract
We present a simple and convenient method for creating fluid supported bilayers which contain oriented and functional photosynthetic reaction centers (RCs).1 The supported bilayers are prepared by fusion of proteoliposomes with a glass surface. The proteoliposomes are prepared by spontaneous insertion of RCs into preformed small, unilamellar vesicles. The RCs in these vesicles are shown to be oriented with the cytochrome c binding surface on the outside and the H-subunit facing inside. Upon fusion to glass surfaces, the RCs remain functional and highly oriented, with the cytochrome c binding surface exposed to the bulk solution. The RCs in the supported bilayers are at a surface density of order 1011 RCs/cm2. The quality of the supported lipid bilayer is characterized by epifluorescence microscopy and the long-range lateral mobility of the lipids by fluorescence recovery after photobleaching. We demonstrate that homogeneous, fluid bilayers can be prepared over large areas (e.g., 1 cm2) of clean glass surfaces. The lipids in these supported bilayers are laterally mobile, and their diffusion coefficient agrees with values obtained in other fluid bilayer systems. This fluidity is unaffected by the presence of RCs; however, the RCs bearing a site-specific fluorescent label are immobile, despite retaining their charge separation and cytochrome c binding properties. We speculate that this results from interactions between the globular domain of the H-subunit and the glass substrate. Because of the unique spectroscopic and functional signatures associated with intact RCs, this system is one of the best characterized examples of a transmembrane protein in a supported bilayer at a nonbiological interface.