Photon Factory Activity Report 2016 #34 (2017)

BL-1A, BL-17A, AR-NE3A/2015G513

Structural analysis of the metal-substituted light-harvesting-reaction
center core complexes
Tomoaki Kawakami, Long-Jiang Yu and Seiu Otomo
Faculty of Science, Ibaraki University, Mito 310-8512, Japan
1 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(a and b, 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 LH1
complex that absorbs maximally around 915 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. On the other hand, the Ca2+ ions in LH1
can be replaced by other divalent metal ions (e.g., Sr2+and Ba2+), resulting in a substituted LH1-RC with theLH1 Qy in a range of 880 ~ 890 nm [1, 2]. Here, wereport structure determination of the Sr- and Basubstituted LH1-RCs [4].
2 Experiment
Crystallization of the Sr- and Ba-LH1-RCs and postcrystallization treatment were carried out following thesame method as describe previously [3]. X-ray diffraction
data were collected at BL41XU and BL44XU of SPring-8,and BL1A, BL17A and NE3A of the Photon Factory at
100K. To identify the Sr2+- and Ba2+-binding sites,
datasets were acquired at three wavelengths for the SrLH1-RC and two wavelengths for the Ba-LH1-RC.
3 Results and Discussion
Anomalous difference Fourier maps obtained atwavelengths of 0.76 Å and 1.5 Å revealed strong electron
densities for the Sr- and Ba-LH1-RC crystals, respectively.
The strong densities disappeared at 0.78 Å (remote) for
the Sr-LH1-RC and largely reduced at 0.98 Å for the BaLH1-RC, confirming that they are truly from the Sr2+ andBa2+ ions, respectively. Both the Sr2+ and Ba2+ have thesame number and are located at essentially the same
positions in the substituted LH1-RCs. These locations areclearly different from, though close to, the Ca-bindingsites. Sixteen Sr2+ and Ba2+ ions are identified in the LH1
complexes. Conformational rearrangement induced by thesubstitution is limited to the metal-binding sites. Unlike
the Ca-LH1-RC, only the α-polypeptides are involved inthe Sr- and Ba-coordinations in LH1 (Figure 1). The
difference in the thermostability between these complexes
can be attributed to the different patterns of the network
formed by metal-binding. The Sr- and Ba-LH1-RCs forma single-ring network formed by the LH1 α-polypeptides
only, in contrast to the double-ring network composed ofboth α- and β-polypeptides in the Ca-LH1-RC. Basedon the structural information, a combined effect ofhydrogen-bonding, structural integrity and charge
distribution is considered to influence the spectral
properties of the core antenna complex.

Fig. 1: Schematic illustration showing the difference
between Sr(Ba)- and Ca-binding networks formed in
LH1s [4].

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)[4] L.-J. Yu, et al., Biochemistry 55, 6495(2016)*otomo@mx.ibaraki.ac.jp