PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

Thermo-optically induced reversible reorganizations in LHCII. Significance of thermal instability, role of LHCII-only domains in thylakoids. Jens Holm^{1, 2}, Sashka Krumova^{1, 3}, Zsuzsanna Várkonyi¹, Svetla Todinova³, Subrimanyan Rajagopal¹, Dorthe Posselt², Stefka Taneva³, Győző Garab^*,, ¹ Institute of Plant Biology, Biological Research Center, Hungarian Academy of Sciences, Szeged, Hungary² IMFUFA, Dept. of Mathematics and Physics, Roskilde University, Roskilde, Denmark³ Institute of Biophysics, Bulgarian Academy of Sciences, Sofia, Bulgaria

ABSTRACT- Earlier we have shown that granal thylakoid membranes as well as loosely stacked lamellar aggregates of the main light harvesting chlorophyll a/b complex of photosystem II (LHCII) are capable of undergoing light-induced reversible reorganizations that are (largely and fully) independent of the photochemical activity of the membranes; the changes are approximately linearly proportional to the intensity of the excess light, a unique feature that is potentially very important in the protection of plants against excess excitation [Barzda et al., 1996, Biochemistry 35: 8981]. In thylakoids exposed to high light, three consecutive transitions have been identified: (i) unstacking of membranes, (ii) a lateral desorganization of the macrodomains, and (iii) monomerization of LHCII trimers; [Garab et al. 2002, Biochemistry 41: 15121, Dobrikova et al. Biochemistry 42: 11272]. These changes have been assigned to originate from a novel, biological thermo-optic mechanism: fast thermal transients arising from dissipated excitation energy lead to elementary structural transitions in the close vicinity of the dissipation centers due to the presence of built-in thermal instability in the (macro-) assembly of complexes. Hence, with this mechanism the changes are confined to the (very small) volumes of the ultrafast heat transients; this lends a local structural flexibility in an otherwise structurally stabile system. In this work we show (i) correlation between the thermal instability of isolated LHCII, with different lipid contents, and their ability to undergo light-induced reversible reorganizations, and (ii) a close similarity between the light-induced reversible transients and thermally induced difference spectra; further, we also show that (iii) the light induced reversible CD transients (CD, circular dichroism) of the lamellar aggregates of the isolated LHCII are virtually indistinguishable from the CD transients in thylakoids, which suggests that LHCII-only domains play a key role in the structural flexibility and light-adaptation of thylakoid membranes.

KEY WORDS: structural changes, thermo-optic mechanism, photoprotection, LHCII