PhotosynthesisResearch 44: 139-148, 1995. (g) 1995KluwerAcademicPublishers. Printedin the Netherlands. Regular paper

Sequence conservation of light-harvesting and stress-response proteins in relation to the three-dimensional molecular structure of LHCII Beverley R. Green1 & Werner Ktihlbrandt2 IDepartment of Botany, University of British Columbia, Vancouver, B.C., Canada V6T 1Z4; 2European Molecular Biology Laboratory, Meyerhofstr. 1, 6900 Heidelberg, Germany Received 10 January 1995;acceptedin revisedform 16 February1995

Key words: chlorophyll, chl alb (CAB) protein, light-harvesting antenna, LHCII, structure prediction, sequence evolution, gene family, early light-inducible protein

Abstract The structure of pea light-harvesting complex LHCII determined to 3.4 A resolution by electron crystallography ( Ktihlbrandt, Wang and Fujiyoshi (1994) Nature 367:614-621) was examined to determine the relationship between structural elements and sequence motifs conserved in the extended family of light-harvesting antennas (Chl a/b, fucoxanthin Chl a/c proteins) and membrane-intrinsic stress-induced proteins (ELIPs) to which LHCII belongs. It is predicted that the eukaryotic ELIPs can bind at least four molecules of Chl. The one-helix prokaryotic ELIP of Synechococcus was modelled as a homodimer based on the high degree of conservation of residues involved in the interactions of the first (B) and third (A) helices of LHCII.

Abbreviations: CAB - Chl a/b-binding; ELIP - early light-inducible protein; FCP - fucoxanthin-Chl a/c protein; Lutl, Lut2 - lutein molecules 1 and 2 Introduction The protein sequences of all 10 members of the Chl a/b (CAB) family of light-harvesting antennas show a number of highly conserved motifs (Green and Pichersky 1994; Green et al. 1991). Many of these motifs are conserved in the fucoxanthin-Chl a/c antenna proteins (FCPs) of chromophyte algae. Related proteins which appear to act as Chl a light-harvesting antennas in rhodophyte algae have been detected immunologically (Wolfe et al. 1994), which implies that they also share some conserved sequences. Some of the conserved motifs are also found in the early lightinducible proteins (ELIPs), a class of stress-induced thylakoid membrane proteins (Meyer and Kloppstech 1984). Along with the psbS protein of CP22 (Funk et al. 1994) all these related proteins make up the extended family of antennas and stress-response proteins. The determination of the molecular structure of LHCII, the major Chl a/b-protein complex of higher

plants, to 3.4/~ resolution, indicated the roles of several of these conserved features (Ktihlbrandt et al. 1994). The most striking feature of the 3.4 A model was the 2-fold symmetry relating the first and third transmembrane helices (Helices B and A). This supported the suggestion first made on the basis of sequence comparisons, that these two helices were related as the result of an ancient gene duplication (Hoffman et al. 1987). The structure of LHCII (Ktihlbrandt et al. 1994) shows that the most highly conserved residues either provide 5th ligands for the Mg atoms of Chls al, a2, a4 and a5 or are part of the pattern of small and large residues on the contact surfaces which are necessary for close packing of these helices (Figs. 1-3). A unique feature of this symmetry-related pair are the reciprocal ionic bonds between Glu on one helix and Arg on the other that serve to cross-link the two helices. The Glu carboxyl groups also provide ligands for Chls a 1 and a4. A third ion pair in the middle helix (Helix C) binds Chl b5. In this case, the ion pair is formed between

Fig. I. Sideview of the atomicmodelof the LHCII monomer(Kiihlbrandtet al. 1994) showingthe alpha-carbontrace of the polypeptideand bound pigments. Sequencemotifs in the loop regions and the amphipathichelix D (lowerright) that are highly conservedamongall CABs are shown in red. Note that only the upper side of helix D is conserved. Membrane-spanninghelices are in the order A, B, C from right to left. The N-terminusis at the upper right, the C-terminusat the lowerright hand side. Chl tetrapyrrolesassignedto Chl a are shown in cyan; those assigned to Chl b in green. The two luteinmoleculesare yellow. the two residues separated by one turn of the helix. Two other Chls were assigned to specific sidechain ligands: Chla3 to Gln197 in the third helix and Chlb6 to G l n l 3 1 in the second helix (Fig. 2). In addition to the Chls, two molecules of the carotenoid lutein appear to play an important structural role in linking the flanking regions at the opposite ends of helices B(1) and A(3), and by providing a path for quenching deleteri-

ous chlorophyll triplets. Chlorophylls were assigned to Chl a or Chl b based on their relative proximity to the carotenoids (K0hlbrandt et al. 1994), but this assignment has yet to be confirmed experimentally. We have now undertaken a detailed examination of the structure of LHCII in relation to amino acid sequence motifs conserved in the CAB family, as well as motifs that are either conserved or not conserved

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Sequence conservation of light-harvesting and stress-response proteins in relation to the three-dimensional molecular structure of LHCII.

The structure of pea light-harvesting complex LHCII determined to 3.4 Å resolution by electron crystallography (Kühlbrandt, Wang and Fujiyoshi (1994) ...
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