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J. Biol. Chem., Vol. 280, Issue 21, 20612-20619, May 27, 2005

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Pigment-Pigment Interactions in Lhca4 Antenna Complex of Higher Plants Photosystem I^*

Tomas Morosinotto, Milena Mozzo¶, Roberto Bassi, and Roberta Croce¶||

From the Dipartimento Scientifico e Tecnologico, Università di Verona, Strada Le Grazie, 15-37134 Verona, Italy, the ^¶Istituto di Biofisica, CNR, Trento, c/o ITC via Sommarive 18, 38100 Povo, Trento, Italy, and the Université Aix-Marseille II, LGBP-Faculté des Sciences de Luminy, Département de Biologie-Case 901-163, Avenue de Luminy, 13288 Marseille, France

The red-most fluorescence emission of photosystem I (733 nm at 4 K) is associated with the Lhca4 subunit of the antenna complex. It has been proposed that this unique spectral feature originates from the low energy absorption band of an excitonic interaction involving chlorophyll A5 and a second chlorophyll a molecule, probably B5 (Morosinotto, T., Breton, J., Bassi, R., and Croce, R. (2003) J. Biol. Chem. 278, 49223–49229). Because of the short distances between chromophores in Lhc proteins, the possibility that other pigments are involved in the red-shifted spectral forms could not be ruled out. In this study, we have analyzed the pigment-pigment interactions between nearest neighboring chromophores in Lhca4. This was done by deleting individual chlorophyll binding sites by mutagenesis, and analyzing the changes in the spectroscopic properties of recombinant proteins refolded in vitro. The red-shifted (733 nm) fluorescence peak, the major target of this analysis, was lost upon mutations affecting sites A4, A5, and B5 and was modified by mutating site B6. In agreement with the shorter distance between chlorophylls A5 and B5 (7.9 Å) versus A4 and A5 (12.2 Å) in Lhca4 (Ben-Shem, A., Frolow, F., and Nelson, N. (2003) Nature 426, 630–635), we conclude that the low energy spectral form originates from an interaction involving pigments in sites A5 and B5. Mutation at site B6, although inducing a 15-nm blue-shift of the emission peak, maintains the red-shifted emission. This implies that chromophores responsible for the interaction are conserved and suggests a modification in the pigment organization. Besides the A5–B5 pair, evidence for additional pigment-pigment interactions between chlorophylls in sites B3–A3 and B6–A6 was obtained. However, these features do not affect the red-most spectral form responsible for the 733-nm fluorescence emission band.

Received for publication, January 20, 2005 , and in revised form, March 22, 2005.

^* This work was supported by Fondi per gli Investimenti della Ricerca di Base Grants RBAU01E3CX and RBNE01LACT, European Communities Human Potential Program Grant HPRN-CT-2002-00248 (to R. B.), and by Provincia Autonoma di Trento Grant SAMBAx2, and European Union, Human Resources and Mobility Activity Contract MRTN-CT-2003-505069 (to R. C.). 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.: 39-0461405360; Fax: 39-0461405372; E-mail: croce{at}itc.it.

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