Two-dimensional structure of light harvesting complex II (LHII) from the purple bacterium Rhodovulum sulfidophilum and comparison with LHII from Rhodopseudomonas acidophila

AbstractBackground: Within the membranes of photosynthetic bacteria, up to three types of light harvesting complexes (LHI, LHII, LHIII) are found. These complexes absorb photons and transfer the excitation energy to the photosynthetic reaction centre. The LH complexes comprise units that contain α and β polypeptides with associated pigment molecules.Results The structure of LHII complex from Rhodovulum sulfidophilum has been examined to a resolution of 7 å using electron microscopy. The complex is a nonamer containing nine αβ subunits. These are arranged in two radially symmetric concentric cylinders, with the nine α chains positioned in the inner cylinder and the nine β chains forming the outer cylinder. The 18 transmembrane helices are readily observed in the projection maps, along with 18 additional peaks attributed to the pigment molecules.Conclusion The determination of more structures of LH complexes will uncover the full extent of the variability of the oligomerization states in different bacteria and also in the native membrane. The analysis of two-dimensional crystals allows a rapid determination of key structural features and the oligomeric state of the complex. Comparison of our structure determined by electron microscopy with the recently solved X-ray structure indicates that the results of the two methods are complementary