Crystallisation and structure determination of the bacterial LH1-RC core complex

Z.-Y. Wang-Otomo, L.-J. Yu*, T. Kawakami
Faculty of Science, Ibaraki University, Japan; *Present address: Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan

Bacterial antenna apparatus provides a simplified model system ideally for studying the basic mechanism of photosynthetic light-harvesting and energy-transfer processes of solar energy conversion. While a number of high-resolution structures have been known for the RC and LH2, the structures of core antenna complex (LH1) from several species had remained at low resolutions for long time.

In this work, we describe purification, crystallisation and structure determination of the LH1-RC core complex from a thermophilic purple sulfur bacterium Thermochromatium tepidum. This complex is characterized by an enhanced thermostability and a LH1-Qy transition at 915 nm[1]. These properties have been shown to be regulated by Ca2+ ions[2,3]. Following crystallisation of the core complex (Mw: ~390,000)[1], its structure has been determined[4,5]. The structure reveals a closed arrangement of LH1 complex around the RC, and the LH1 BChl a molecules form a partially overlapping ring with a shorter Mg-Mg spacing compared to that of B850 in LH2. Structural evidence is provided for the possible ubiquinone pathway in the closed LH1 complex. The Ca2+-binding sites have been identified. Based on the published structure, spectroscopic and thermodynamic properties of the LH1 complex are addressed. Molecular mechanisms of the quinone transport, Ca2+-regulation and interaction between LH1 and RC will be discussed.

References

[1] H. Suzuki, et al., Biochim. Biophys. Acta 1767, 1057 (2007)
[2] Y. Kimura, et al., J. Biol. Chem. 283, 13867 (2008).
[3] Y. Kimura, et al., J. Biol. Chem. 284, 93 (2009).
[4] S. Niwa et al., Nature 508, 228 (2014).
[5] R. J. Cogdell & A. W. Roszak Nature 508, 196 (2014).