Selective assembly of photosynthetic antenna proteins into a domain-structured lipid bilayer for the construction of artificial photosynthetic antenna systems: structural analysis of the assembly using surface plasmon resonance and atomic force microscopy

Two types of photosynthetic membrane proteins, the peripheral antenna complex (LH2) and the core antenna/reaction center complex (LH1-RC), play an essential role in the primary process of purple bacterial photosynthesis, that is, capturing light energy, transferring it to the RC where it is used in subsequent charge separation. Establishment of experimental platforms is required to understand the function of the supramolecular assembly of LH2 and LH1-RC molecules into arrays. In this study, we assembled LH2 and LH1-RC arrays into domain-structured planar lipid bilayers placed on a coverglass using stepwise combinations of vesicle-to-planar membrane formation and vesicle fusion methods. First, it was shown that assembly of LH2 and LH1-RC in planar lipid bilayers, through vesicle-to-planar membrane formation, could be confirmed by absorption spectroscopy and high resolution atomic force microscopy (AFM). Second, formation of a planar membrane incorporating LH2 molecules made by the vesicle fusion method was corroborated by AFM together with quantitative analysis by surface plasmon resonance (SPR). By combining planar membrane formation and vesicle fusion, in a stepwise manner, LH2 and LH1-RC were successfully organized in the domain-structured planar lipid membrane. This methodology for construction of LH2/LH1-RC assemblies will be a useful experimental platform with which to investigate energy transfer from LH2 to LH1-RC where the relative arrangement of these two complexes can be controlled.