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J. Biol. Chem., Vol. 278, Issue 49, 49223-49229, December 5, 2003

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The Nature of a Chlorophyll Ligand in Lhca Proteins Determines the Far Red Fluorescence Emission Typical of Photosystem I^*

Tomas Morosinotto, Jacques Breton¶, Roberto Bassi||, and Roberta Croce**

From the Dipartimento Scientifico e Tecnologico, Università di Verona, Strada Le Grazie, 15-37134 Verona, Italy, the Université Aix-Marseille II, Laboratoire de Genetique et Biophysique des Plantes, Faculté des Sciences de Luminy, Département de Biologie, Case 901-163, Avenue de Luminy, 13288 Marseille, France, the ^¶Service de Bioénergétique, Bât. 532, Commissariat á l'Energie Atomique, Saclay, 91191 Gif-sur-Yvette Cedex, France, and the ^**Consiglio Nazionale delle Ricerche, Istituto di Biofisica, Via Sommarive 18, 38050 Povo (Trento), Italy

Photosystem I of higher plants is characterized by a typically long wavelength fluorescence emission associated to its light-harvesting complex I moiety. The origin of these low energy chlorophyll spectral forms was investigated by using site-directed mutagenesis of Lhca1–4 genes and in vitro reconstitution into recombinant pigment-protein complexes. We showed that the red-shifted absorption originates from chlorophyll-chlorophyll (Chl) excitonic interactions involving Chl A5 in each of the four Lhca antenna complexes. An essential requirement for the presence of the red-shifted absorption/fluorescence spectral forms was the presence of asparagine as a ligand for the Chl a chromophore in the binding site A5 of Lhca complexes. In Lhca3 and Lhca4, which exhibit the most red-shifted red forms, its substitution by histidine maintains the pigment binding and, yet, the red spectral forms are abolished. Conversely, in Lhca1, having very low amplitude of red forms, the substitution of Asn for His produces a red shift of the fluorescence emission, thus confirming that the nature of the Chl A5 ligand determines the correct organization of chromophores leading to the excitonic interaction responsible for the red-most forms. The red-shifted fluorescence emission at 730 nm is here proposed to originate from an absorption band at 700 nm, which represents the low energy contribution of an excitonic interaction having the high energy band at 683 nm. Because the mutation does not affect Chl A5 orientation, we suggest that coordination by Asn of Chl A5 holds it at the correct distance with Chl B5.

Received for publication, August 19, 2003 , and in revised form, September 21, 2003.

^* This work was funded by Minestero dell'Istruzione Università e Ricerca (MIUR) Progetti Fondo per gli Investimenti della Ricerca di Base (FIRB) Grants RBAU01E3CX and RBNE01LACT and by the European Community's Human Potential Program Grant HPRN-CT-2002-00248 (PSICO). 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.

^|| To whom correspondence should be addressed. Tel.: 39-0458027916; Fax: 39-0458027929; E-mail: bassi{at}sci.univr.it.

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