The construction and structural analysis of a tethered planar lipid bilayer containing bacterial photosynthetic membrane proteins, light-harvesting complex 2 (LH2), and light-harvesting core complex (LH1-RC) is described and establishes this system as an experimental platform for their functional analysis. The planar lipid bilayer containing LH2 and/or LH1-RC complexes was successfully formed on an avidin-immobilized coverglass via an avidin-biotin linkage. Atomic force microscopy (AFM) showed that a smooth continuous membrane was formed there. Lateral diffusion of these membrane proteins, observed by a fluorescence recovery after photobleaching (FRAP), is discussed in terms of the membrane architecture. Energy transfer from LH2 to LH1-RC within the tethered membrane was observed by steady-state fluorescence spectroscopy, indicating that the tethered membrane can mimic the natural situation.

To reveal the structure-function relationship of membrane proteins, a membrane environment is often used to establish a suitable platform for assembly, functioning, and measurements. The control of the orientation of membrane proteins is the main challenge. In this study, the electron conductivity and photocurrent of a light-harvesting/reaction center core complex (LH1-RC) embedded in a lipid membrane were measured using conductive atomic force microscopy (C-AFM) and photoelectrochemical analysis. AFM topographs showed that LH1-RC molecules were well-orientated, with their H-subunits toward the membrane surface. Rectified conductivity was observed in LH1-RC under precise control of the applied force on the probe electrode (<600 pN). LH1-RC embedded in a membrane generated photocurrent upon irradiation when assembled on an electrode. The observed action spectrum was consistent with the absorption spectrum of LH1-RC. The control of the orientation of LH1-RC by lipid membranes provided well-defined conductivity and photocurrent.

In purple photosynthetic bacteria, light-harvesting complex 2 (LH2) and the light harvesting-reaction center complex (LH1-RC) play the key roles of capturing and transferring light energy and subsequent charge separation. These photosynthetic apparatuses form a molecular assembly; however, how the assembly influences the efficiency of energy conversion is not yet clear. To address this issue, direct observation of the assembly at the molecular level is necessary to analyze its function. In this study, we reconstituted photosynthetic membrane proteins into artificial lipid bilayers and directly observed their assembly by AFM. The absorption spectra of the reconstituted proteins showed characteristic Qy bands of bacteriochlorophyll a that were identical to those of intact proteins. AFM observation of the reconstituted membranes revealed that LH2 and LH1-RC were successfully assembled into the lipid bilayer, and their observed structures were in good agreement with corresponding crystallographic structures. Specifically, binary proteins, i.e., LH2/LH1-RC and LH2/LH1, which form a densely packed molecular assembly, could be clearly identified at the molecular level by this method of observation. Energy transfer from LH2 to LH1-RC in a reconstituted lipid bilayer was observed by steady-state fluorescence spectroscopy. Enhanced energy transfer was confirmed in the membrane phase compared to that in a homogeneous micellar solution. Such reconstituted molecular assemblies are useful experimental platforms to investigate the relationship between supramolecular arrays and function.