Photon Factory Activity Report 2014 #32 (2015) B

1A, 17A, NE3A /2013G519

Crystal structure of the light-harvesting-reaction center core complex
from a thermophilic photosynthetic bacterium
L.-J. YU, T. KAWAKAMI and S. OTOMO*
Faculty of Science, Ibaraki University, Mito 310-8512, Japan
Introduction
In purple photosynthetic bacteria, the light energy isabsorbed by two types of light-harvesting complexes(LH1, LH2), and then is transferred efficiently to thereaction center (RC) where the primary charge separation
takes place across the membrane and a cyclic electrontransport chain occurs. The LH1 complex is locatedintimately around the RC with a fixed stoichiometric ratio
to form the so-called core complex (LH1-RC). LH1 is alarge oligomer of a basic structural unit composed of aheterodimer of two small integral membrane polypeptides(α and β, ca. 6 kDa) associated with bacteriochlorophyll(BChl) and carotenoid molecules. Thermochromatium(Tch.) tepidum is a thermophilic purple sulfur
photosynthetic bacterium originally isolated from a hot
spring in Yellowstone National Park. It grows
anaerobically at optimum temperatures of 48 – 50 ºC withan upper limit of 58 ºC, and contains an unusual LH1complex that absorbs maximally around 917 nm (Qytransition). In a series of investigations, we found that theTch. tepidum LH1 is highly stable at room temperature,and calcium ions are involved in both the enhanced
thermal stability and the large red shift of the LH1 Qytransition[1, 2].
The crystal structure of LH1-RC from Tch. tepidum has
been determined [3], in which the Ca-binding sites havebeen identified. Here, we report the structural features onthe Ca-binding pockets.

Materials and Methods
Crystallization of the native Tch. tepidum LH1-RC
complex was performed using sitting-drop vapordiffusion method at 20 ˚C. The protein solution wasmixed with the same volume of precipitant solution (20mM Tris-HCl, pH 7.5, 3 mM DPC, 50 mM CaCl2, 16%
w/v PEG3000).

Results and Discussion
Anomalous difference Fourier maps revealed that there
are 16 Ca-binding sites in the LH1 and one in the RC.
The Ca2+ ions in LH1 distribute in the middle between theinner and outer rings (see the Figure). The Ca2+-bindingsites are located in the C-terminal regions of the α- and βpolypeptides with both chains providing ligands for thecoordination. Each Ca2+ is coordinated by the main chain
oxygen atoms of α-Trp46, side chain of α-Asp49 and αAsn50, and the C-terminal carboxyl group of the β-Leu46
in the adjacent subunit. Water molecules, whose electron
densities have not been clearly identified, may also beinvolved in the ligation to form a heptacoordinatedstructure as favoured by the Ca2+ ion. Association of theα- and β-polypeptides through the Ca2+ stabilizes the LH1structure and is thought to account for its enhanced
thermostability. Since the binding network is positioned
close to the BChl a molecules, it could modify theconfiguration of the coupled pigments through the nearby
α-Trp46 and β-Trp45 whose side chains are hydrogenbonded to the C3-acetyl oxygen atoms of BChls a. The
Ca2+-binding effect is considered to have additional
contribution to the larger red-shift of the LH1 Qytransition in Tch. tepidum compared with the Qy
transitions (~ 880 nm) in other species.

References[1] Y. Kimura, et al., J. Biol. Chem., 283, 13867(2008)[2] Y. Kimura, et al., J. Biol. Chem., 284, 93(2009)[3] S. Niwa et al., Nature 508, 228(2014)*otomo@mx.ibaraki.ac.jp