PARENT SESSION

Symposium S4Aa Chlorophyll and bilin based antenna systems
Tuesday August 31st, 2004 2:40 PM-4:40 PM Room 511D
Chair: Roberto Bassi
Co-Chair: Herbert van Amerongen

The molecular basis of the low energy absorption forms of LHCI. Roberta Croce^*,1, Tomas Morosinotto^{2, 3}, Roberto Bassi^{2, 3}, ¹ Istituto di Biofisica - CNR, Povo - Trento, Italy² Dipartimento Scientifico e Tecnologico, Verona, Italy³ Université Aix-Marseille II, LGBP- Faculté des Sciences de Luminy - Département de Biologie - Case 901 - 163, Avenue de Luminy -, Marseille, France

ABSTRACT- Photosystem I is characterized by the presence of Chls which absorb at energies lower than the primary donor, P700 and yield the typical fluorescence emission peak at 735 nm. In higher plants most of the red forms are associated with the outer antenna, LHCI. The four gene products Lhca 1-4, encoding higher plants (Arabidopsis thaliana) Photosystem I antenna complexes, were overexpressed in bacteria and reconstituted in vitro. A mutation analysis at the putative Chl binding residues was performed by substituting with non-Chl ligand residues. A red-shifted emission peak was observed in all four WT complexes, albeit with different energies and amplitudes: the red-most form, emitting at 733 nm, is associated with Lhca4, while Lhca3 emits at 725 nm and Lhca1 as well as Lhca2 emit at 702 nm. The Lhca complexes belong to the Lhc family and they show high structure similarity with the antenna complexes of Photosystem II, in both protein conformation and pigment organization. Interestingly, the emission maxima of Lhcb complexes is around 681 nm, thus blue-shifted by 20-50 nm as compared to the Lhca proteins. It is thus likely that the red absorption associated to Lhca complexes is the result of small, local, changes in the structure of the complexes. The results of mutation analysis show that the red emission of all Lhca complexes originates from the low energy absorption band of an excitonic interaction involving Chls molecules bound to sites A5 and B5. The large energy difference between the red emission in Lhca3 and Lhca4 as compared to Lhca2 and Lhca1 was assigned to the presence of an asparagine as ligand for Chl A5 in Lhca3 and Lhca4 while an histidine is present in all others Lhc proteins. This substitution modulates the distance between the two coupled Chls, thus changing the interaction energy value. Modulation of the interaction strength in Lhca4 was also observed upon mutation at site B6, thus suggesting a role for the carotenoid in site L2 in stabilizing the conformation which leads to red forms. The differences in pigment-pigment interaction in Lhcb vs Lhca protein will be discussed in the light of the recent structure determination of LHCII and LHCI.

KEY WORDS: red forms, fluorescence, Lhca, mutation analysis