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J. Biol. Chem., Vol. 283, Issue 1, 487-495, January 4, 2008

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Structural and Functional Analysis of the Antiparallel Strands in the Lumenal Loop of the Major Light-harvesting Chlorophyll a/b Complex of Photosystem II (LHCIIb) by Site-directed Mutagenesis^*

Cheng Liu¹, Yajie Zhang¹, Derong Cao^¶, Yikun He^||, Tingyun Kuang^||, and Chunhong Yang^**2

From the Key Laboratory of Photosynthesis and Environmental Molecular Physiology, Institute of Botany, Chinese Academy of Sciences, 20 Nanxincun, Beijing, 100093, China, the ^¶College of Chemistry, South China University of Technology, Guangzhou 510640, China, the ^||College of Life Sciences, Capital Normal University, Beijing 100037, China, the ^**Institut für Allgemeine Botanik, Johannes-Gutenberg-Universität Mainz, D-55099 Mainz, Germany, and the Graduate University of Chinese Academy of Sciences, Beijing 100049, China

The light-harvesting chlorophyll a/b-binding protein of photosystem II (LHCIIb) fulfills multiple functions, such as light harvesting and energy dissipation under different illuminations. The crystal structure of LHCIIb at the near atomic resolution reveals an antiparallel strands structure in the lumenal loop between the transmembrane helices B/C. To study the structural and functional significances of this structure, three amino acids (Val-119, His-120, and Ser-123) in this region have been exchanged to Phe, Leu, and Gly, respectively, and the influence of the mutagenesis on the structure and function of LHCIIb has been investigated. The results are as follows. 1) Circular dichroism spectra of the mutations reveals that the antiparallel strands in the lumenal region are very important for adjusting pigment conformation in the neoxanthin domain of LHCIIb. Although the mutagenesis causes only a slight loss of the Neo binding in the complexes (V119F, 0.09; S123G, 0.19; and H120L, 0.27), it imparts remarkable changes to the pigment conformation. 2) Substituting Ser-123 with Gly results in a higher susceptibility to photodamage, an increased tendency to aggregate, and enhanced fluorescence quenching induced by the medium acidification. These results demonstrate that this antiparallel strands domain plays an important role in regulating the pigment conformation and in adjusting the aggregation and the fluorescence yield of LHCIIb.

Received for publication, February 12, 2007 , and in revised form, October 10, 2007.

^* This work was supported by National Science Foundation of China Grant 30470149 and Knowledge Innovation Program of the Chinese Academy of Sciences Grant KSCX2-YW-R-137. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Table S1 and Figs. S1 and S2.

¹ Both authors contributed equally to this work.

² To whom correspondence should be addressed: Key Laboratory of Photosynthesis and Environmental Molecular Physiology, Institute of Botany, Chinese Academy of Sciences, 20 Nanxincun, Beijing, 100093, China. Tel.: 86-10-62836252; E-mail: yangch{at}ibcas.ac.cn.

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