Flexibility and Size Heterogeneity of the LH1 Light Harvesting Complex Revealed by Atomic Force Microscopy

Abstract

Previous electron microscopic studies of bacterial RC-LH1 complexes demonstrated both circular and elliptical conformations of the LH1 ring, and this implied flexibility has been suggested to allow passage of quinol from the Q B site of the RC to the quinone pool prior to reduction of the cytochrome bc 1 complex. We have used atomic force microscopy to demonstrate that these are just two of many conformations for the LH1 ring, which displays large molecule-to-molecule variations, in terms of both shape and size. This atomic force microscope study has used a mutant lacking the reaction center complex, which normally sits within the LH1 ring providing a barrier to substantial changes in shape. This approach has revealed the inherent flexibility and lack of structural coherence of this complex in a reconstituted lipid bilayer at room temperature. Circular, elliptical, and even polygonal ring shapes as well as arcs and open rings have been observed for LH1; in contrast, no such variations in structure were observed for the LH2 complex under the same conditions. The basis for these differences between LH1 and LH2 is suggested to be the H-bonding patterns that stabilize binding of the bacteriochlorophylls to the LH polypeptides. The existence of open rings and arcs provides a direct visualization of the consequences of the relatively weak associations that govern the aggregation of the protomers (␣ 1 ␤ 1 Bchl 2 ) comprising the LH1 complex. The demonstration that the linkage between adjacent protomer units is flexible and can even be uncoupled at room temperature in a detergent-free membrane bilayer provides a rationale for the dynamic separation of individual protomers, and we may now envisage experiments that seek to prove this active opening process.Photosynthetic organisms harvest light energy and convert it to a chemically useful form, using light harvesting (LH) 1 and reaction center (RC) complexes. In the purple photosynthetic bacteria, the reaction center, which is the site of photochemistry, receives excitation energy from the light harvesting LH1 complex, which receives energy in turn from the LH2 complex (reviewed in Ref. 1). The atomic structure of the Rhodopseudomonas acidophila LH2 complex (2) and the cryo-electron microscopy (EM) structure of the Rhodobacter sphaeroides complex (3) both revealed a circular arrangement of nine protomers, each consisting of an ␣ and a ␤ polypeptide. The LH2␣ polypeptides formed an inner ring, with the ␤ ring outermost; in all, 27 bacteriochlorophyll (Bchl) molecules are bound to this structure (2). More recent work has established that LH1 surrounds the RC using an arrangement of 16 ␣␤ protomers and 32 Bchls (4) when there is no prulifloxacin PufX protein. In other bacteria, an LH1 ring of 15 ␣␤ protomers, together with either PufX or a putative PufX homologue (W), form a continuous ring of protein around the RC (5, 6). The demonstration of both circular and elliptical forms of this LH1 complex provided evidence for its flexibility (4). This property of the LH1 complex wa...

Reconstitution and AFM Observation of Photosynthetic Membrane Protein Assembly in Planar Lipid Bilayers

“…Ring structures (d = 10-11 nm) were clearly observed. The ring size is in good agreement with the reported value [18]. The absorption spectra of the LH1+LH2 coexisting membrane are shown in Fig.…”

Three-dimensional Reconstruction of a Membrane-bending Complex

“…Subsequent AFM studies of the RC-LH1 complex from Rsp. rubrum, as well as that from Blastochloris viridis, confirmed the flexibility of LH1 (17)(18)(19).…”

“…Subsequent AFM studies of the RC-LH1 complex from Rsp. rubrum, as well as that from Blastochloris viridis, confirmed the flexibility of LH1 (17)(18)(19).The core complex of Rhodobacter (Rba.) sphaeroides, the most intensively researched photosynthetic bacterium, is a dimer (8, 20 -22).…”

Investigation of the Effects of Different Carotenoids on the Absorption and CD Signals of Light Harvesting 1 Complexes

“…The effect may be enhanced by other causes, such as high-order structural changes (changes in the shape of the rings) [36][37][38]65,66 or variations in the protein environment of the pigments. However, our experimental results show that some of the effect must be due to the aforementioned interactions, and this conclusion is supported by theoretical calculations in the companion paper.…”