Structural analysis of the RC-LH1 photosynthetic core complex of Rhodospirillum rubrum using atomic force microscopy

The bacterium Rhodospirillum rubrum contains a simple photosynthetic system, in which the reaction center (RC) receives energy from the light harvesting LH1 complex. We have used high-resolution atomic force microscopy (AFM) to image 2D crystals of the RCLH1 complex of R. rubrum. The AFM topographs show that the RC-LH1 complex is ~94Å in height; the RC-H subunit protrudes from the cytoplasmic face of the membrane by 40Å and it sits 21Å above the highest point of the surrounding LH1 ring. In contrast, the RC on the periplasmic side is at a lower level than LH1, which protrudes from the membrane by 12Å. The RC-LH1 complex can adopt an irregular shape in regions of uneven packing forces in the crystal; this reflects a likely flexibility in the natural membrane, which might be functionally important by allowing the export of quinol formed as a result of RC photochemistry. Nanodissection of the RC by the AFM tip removes the RC-H subunit and reveals the underlying RC-L and -M subunits; LH1 complexes completely lacking the RC were also found, providing ideal conditions for imaging both rings of LH1 polypeptides for the first time by AFM. In addition we demonstrate the ellipticity of the LH1 ring at the cytoplasmic and periplasmic sides of the membrane, in both the presence and absence of the RC. These AFM measurements have been reconciled with previous EM and NMR data to produce a model of the RC-LH1 complex. 2 by gest on N ovem er 7, 2017 hp://w w w .jb.org/ D ow nladed from