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J. Biol. Chem., Vol. 279, Issue 15, 15067-15075, April 9, 2004

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Hydrogen Bonding in a Model Bacteriochlorophyll-binding Site Drives Assembly of Light Harvesting Complex^*

Lee G. Kwa, Adela García-Martín, Attila P. Végh¶||, Brigitte Strohmann, Bruno Robert¶, and Paula Braun**

From the Department Biologie I der Universität München, Botanik, 80638 München, Germany, the ^¶Section de Biophysique des Fonctions Membranaires, Unite de Recherche Associee CNRS 2096, C.E. Saclay 91191 Gif sur Yvette Cedex, France, and the ^||Institute of Biophysics and Radiation Biology, Semmelweis University, Faculty of Medicine, H-1088, Puskin 9, Budapest, Hungary

In this study, the contribution of intramembrane hydrogen bonding at the interface between polypeptide and cofactor is explored in the native lipid environment by use of model bacteriochlorophyll proteins. In the peripheral antenna complex, LH2, large portions of the transmembrane helices, which make up the dimeric bacteriochlorophyll-binding site, are replaced by simplified, alternating alanine-leucine stretches. Replacement of either one of the two helices with the helices containing the model sequence at a time results in the assembly of complexes with nearly native light harvesting properties. In contrast, replacement of both helices results in the loss of antenna complexes from the membrane. The assembly of such doubly modified complexes is restored by a single intramembrane serine residue at position -4 relative to the liganding histidine of the -subunit. In situ analysis of the spectral properties in a series of site-directed mutants reveals a critical dependence of the model complex assembly on the side chain of the residue at this position in the helix. A hydrogen bond between the hydroxy group of the serine and the 13¹ keto group of one of the central bacteriochlorophylls of the complexes is identified by Raman spectroscopy in the model antenna complex containing one of the alanine-leucine helices. The additional OH group of the serine residue, which participates in hydrogen bonding, increases the thermal stability of the model complexes in the native membrane. Intramembrane hydrogen bonding is thus shown to be a key factor for the binding of bacteriochlorophyll and assembly of this model cofactor-polypeptide site.

Received for publication, November 13, 2003 , and in revised form, January 21, 2004.

^* 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.

These authors contributed equally to this work.

^** To whom correspondence should be addressed. Tel.: 49-89-17861-294; Fax: 49-89-17861-185; E-mail: braun{at}botanik.biologie.uni-muenchen.de.

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