PARENT SESSION
Posters P3B Photosynthetic acclimation: Ecophysiology, diverse environments. Abstracts (479-522)

Structure of Chlamydomonas LHCI-PSI supercomplex during state transitions. Joanna Kargul^*,1, Jon Nield¹, Sam Benson¹, Ayumi Kanno⁴, Maria Turkina⁵, Alexander Vener⁵, Jun Minagawa⁴, Jim Barber¹, ¹ Wolfson Laboratories, London, UK⁴ The Institute of Low Temperature Science, Japan⁵ Division of Cell Biology, Linkoping, Sweden

ABSTRACT- Light excitation of both photosystem I and II is regulated by a process known as "state transitions" ensuring the highest efficiency of photosynthetic electron transport. A mobile pool of light-harvesting complex II (LHCII) is believed to be involved in this regulation in response to N-terminal phosphorylation/dephosphorylation. Using electron microscopy and single particle analysis of isolated State 1 and State 2 LHCI-PSI supercomplexes, we identified changes in the LHCI-PSI structure during state transitions in the green alga Chlamydomonas reinhardtii. Previously we reported the first low resolution 3D map of this supercomplex isolated from cells grown in State 1 light conditions [1]. This structure has now been refined with respect to the number and positioning of the associated LHCI antenna subunits taking advantage of the recent publication of the X-ray crystal structure of higher plant LHCI-PSI [2]. Furthermore, negatively-stained State 2 LHCI-PSI particles allowed for the identification of additional protein density adjacent to the PsaH core subunit. The identity of the protein(s) within this region was investigated by mass spectrometry in conjunction with the mutant analysis. We propose a novel mechanism by which LHCII antenna subunits might associate with the PSI core using a PsaH docking site during State 1-to-State 2 transition. References [1] Kargul, J., Nield, J. and Barber, J. (2003) J. Biol. Chem., 278, 16135-16141. [2] Ben-Shem, A., Frolow, F. and Nelson, N. (2003) Nature, 426, 630-635.

KEY WORDS: Chlamydomonas, LHCII, state transitions, PSI