7334
Biochemistry 2002, 41, 7334-7343
Chromophore Organization in the Higher-Plant Photosystem II Antenna Protein CP26 Roberta Croce, Giusy Canino, Francesca Ros, and Roberto Bassi* Dipartimento Scientifico e Tecnologico-Facolta` di Scienze MM.FF.NN., Strada Le Grazie 15, 37134 Verona, Italy ReceiVed February 27, 2002; ReVised Manuscript ReceiVed April 12, 2002
ABSTRACT: The chlorophyll a/b-xanthophyll-protein CP26 complex belongs to the Lhc protein family. It binds nine chlorophylls and two xanthophylls per 26.6 kDa polypeptide. Determination of the characteristics of each binding site is needed for the understanding of functional organization of individual proteins belonging to the photosystem II supramolecular complex. The biochemical and spectroscopic features of native CP26 are presented here together with identification of pigment binding and energy transitions in different sites. The analysis has been performed via a new approach using recombinant CP26 complexes in which the chromophore content has been experimentally modified. Data were interpreted on the basis of homology with CP29 and LHCII complexes, for which detailed knowledge is available from mutation analysis. We propose that one additional Chl b is present in CP26 as compared to CP29 and that it is located in site B2. We also found that in CP26 three chlorophyll binding sites are selective for Chl a, one of them being essential for the folding of the pigment-protein complex. Two xanthophyll binding sites were identified, one of which (L1) is essential for protein folding and specifically binds lutein. The second site (L2) has lower selectivity and can bind any of the xanthophyll species present in thylakoids.
The Chl a/b-xanthophyll-protein complexes belong to the Lhc multigene family (1), providing components of the antenna system of higher plants. These are involved in two important aspects of photosynthesis: (i) light absorption and transfer of the excitation energy to reaction centers for charge separation and (ii) photoprotection mechanisms that involve thermal dissipation of excess Chl1 a*, aimed at preventing photoinhibition and photobleaching (2). Although a growing body of information about the mechanisms underlying both functions has recently been obtained (3, 4), the precise understanding of structure-function relations is still far for being satisfactory. A model for the structure of Lhc proteins has been derived from electron crystallographic analysis of the major LHCII complex (5), revealing the organization of chromophores, which are bound to three transmembrane helices and a small amphiphilic helix. Two xanthophyll molecules have been shown to be cross-braced to helices A and B. Eight or nine Chl molecules appear to be coordinated to nucleophilic amino acid residues, while four or five additional Chls and two xanthophylls are held in place by interactions with other pigments. Further biochemical analysis yielded a total complement of eight Chl a molecules, five Chl b molecules, and four xanthophylls per polypeptide in the case of the major LHCII complex (6-8). Although Lhc proteins share a high degree of homology, differences can * To whom correspondence should be addressed. Telephone: 0039045-8027916. Fax: 0039-045-8027929. E-mail:
[email protected]. 1 Abbreviations: Chl, chlorophyll; DCCD, dichlorocyclohexylcarbodiimmide; IEF, isoelectrofocalization; L, lutein; LT, low temperature; N, neoxanthin; nCP26, native CP26; NPQ, non-photochemical quenching; rCP26, recombinant CP26; RT, room temperature; V, violaxanthin; Z, zeaxanthin.
be recognized in both their polypeptide and chromophore moieties. Thus, phosphorylation sites have been found in Lhcb1, Lhcb2, and Lhcb4 gene products, although they are located in different positions within the N-terminal, stromally exposed, domain (9, 10). Protonation sites, located on the lumenal loops of Lhcb4 (11) and Lhcb5 (12), have been detected by DCCD binding, which partially overlap Ca2+ binding sites (13). The chromophore composition of Lhc proteins is also variable. Chl/polypeptide ratios of 8 (Lhcb4), 9 (Lhcb5), 10 (Lhcb6 and LHCI), and 12 (Lhcb1) have been reported (6, 14-16). Purification of individual Lhc proteins from thylakoids is difficult due to their very similar physicochemical properties. This problem has been overcome by in vitro reconstitution of recombinant apoproteins overexpressed in bacteria (1719), thus allowing functional studies on individual gene products. Ultrafast spectroscopic measurements (20-23) yielded a detailed knowledge of the energy transfer processes (24) and of the effects of incorporating xanthophyll cycle pigments on the energy conservation and/or dissipation properties of Lhc proteins (4, 25-28). Studies of Lhcb1 and Lhcb4 proteins by mutation analysis (6, 29-31) showed that important spectroscopic (i.e., functional) differences between Lhc products are associated with the occupancy and/or vacancy of individual pigment binding sites and by the possibility of a particular site to be occupied by a different Chl or xanthophyll species. In this work, we have studied the chromophore organization of CP26, the product of the nuclear gene Lhcb5, encoding a 247-amino acid polypeptide. This monomeric subunit is located between the CP43 subunit of the PSII
10.1021/bi0257437 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/11/2002
Chromophore Organization in CP26 reaction center and the S trimer of LHCII (32). Its primary structure is highly homologous to LHCII, thus suggesting a similar folding. Moreover, seven of eight Chl-binding residues in LHCII are conserved in CP26 but for the ligand of Chl in the B6 site, which is a glutamic acid rather than a glutamine, a characteristic shared with CP29. Nevertheless, CP26 binds nine Chls (six Chl a and three Chl b molecules) (14, 33) and one Chl b in addition with respect to CP29. Lutein, violaxanthin, and neoxanthin are bound in a total of approximately 2 mol/mol of polypeptide (34, 35), although a value of 3 was also reported for the spinach complex (33). An interesting question is whether violaxanthin is bound to a high-affinity site (28, 36) or to a low-affinity peripheral site (33) similar to the V1 site in LHCII (8). This is relevant for the understanding of the mechanism of thermal dissipation due to the following findings: (i) CP26 binds zeaxanthin under high-light conditions (37) leading to fluorescence quenching (28), and (ii) it possesses two binding sites for the NPQ inhibitor DCCD (38), suggesting it can be active in xanthophyll cycle-mediated NPQ and may respond to a low lumenal pH. We have studied CP26, either purified from leaves or recombinant. The affinity of pigment binding sites was probed by biochemical and spectroscopic analysis of complexes reconstituted in the presence of different ratios between chromophores. The results were interpreted by comparison with the properties of two homologous proteins, namely, CP29 and LHCII, the chromophore organization of which is known from previous work. As a result, we propose a model for the occupancy of chromophore binding sites in CP26. MATERIALS AND METHODS Purification of the NatiVe Complex. Native CP26 (nCP26) was purified from a maize grana preparation (39) by IEF and sucrose gradient ultracentrifugation as previously reported (40). Reconstituted Complex. The apoprotein was obtained as reported in ref 36. The reconstitution and purification of the reconstituted complexes were performed as described in ref 19. Complexes with a modified Chl a/b and carotenoid composition were obtained by changing either the Chl a/b ratio or the carotenoid species in the pigment mixes used for reconstitution. Pigment Analysis and Pigment/Protein Stoichiometry. The pigment complement of the holoprotein was analyzed by HPLC (41) and fitting of the acetone extract with the spectra of the individual pigments (see below for a description of the fitting program). The protein was quantified by ninhidrin method (42) and by SDS-PAGE analysis (15). Spectroscopy. The absorption spectra at RT were recorded with an SLM-Aminco DK2000 spectrophotometer, in 10 mM Hepes (pH 7.5), 20% glycerol, and 0.06% DM. A 0.4 nm step was used. LT spectra were obtained as reported in ref 43. The fluorescence emission spectra were measured on a Jasco FP-777 fluorimeter in a 1 cm × 1 cm cuvette and corrected for the instrumental response. The samples were excited at 440, 475, and 500 nm. The bandwidths were 5 nm in excitation and 3 nm in emission. All fluorescence
Biochemistry, Vol. 41, No. 23, 2002 7335 spectra were measured at 0.01 OD at the maximum of the Qy transition. LD spectra at RT and LT were obtained as described in refs 44 and 45 using samples oriented by the polyacrylamide gel squeezing technique. The measurements at the two temperatures were obtained on the same sample. The CD spectra were measured at 10 °C on a Jasco 600 spectropolarimeter. The OD of the samples was 1 at the maximum in the Qy transition for all complexes, and the samples were in the same solution described for the absorption. The measurements were performed in a 1 cm cuvette. All the spectra presented were normalized to the same polypeptide concentration, based on the Chl binding stoichiometry (46). Denaturation temperature measurements were performed by following the decay of the CD signal at 679 nm when increasing the temperature from 20 to 80 °C with a time slope of 1 °C/min and a resolution of 0.2 °C. The signal (CD vs T) was fitted with a sigmoid function. Data Analysis. The spectra of the acetone extracts of the protein were recorded in the visible range from 350 to 750 nm with the SLM-Aminco DW 2000 spectrophotometer as described above. The following procedure was used to fit the experimental spectra in terms of pigment spectral forms. (a) A library of spectra of pure pigments (Chl a, Chl b, lutein, neoxanthin, violaxanthin, and zeaxanthin) was obtained by recording spectra of pure pigments in 80% acetone. (b) These spectra were combined to obtain a fit of the spectrum of the acetone extracts from pigment-protein complexes by using homemade software based on Microsoft Excel and Visual Basic utilities. The software used the “solver” module of Excel to find first the least-squares solution of the polynomial combination that best fit the spectrum in the 600-750 nm region, thus determining the Chl a/b ratio. Once a satisfactory fitting in this region where only Chls absorb was obtained, the xanthophyll content was determined via the difference spectrum (original spectrum - Chl a+b calculated spectrum) in the Soret region (410-510 nm). To choose between possible nonunique solutions (because of the similarity of the absorption spectra of xanthophyll species), only solutions fitting the HPLC-determined ratio between xanthophyll species were accepted. The same software has been adapted to fit the absorption spectra in the Qy region of holoproteins. Spectral forms of Chl a and Chl b in the Lhc protein environment (47) were used instead of 80% acetone spectra. In the description of CP26 spectra, nine absorption forms were used, three Chl b and six Chl a molecules, in agreement with the pigment/ polypeptide stoichiometry. In all cases, the Chl a/b ratio calculated from the fitting was in agreement with the value obtained from pigment analysis within 0.05. RESULTS NatiVe CP26 Complex. The CP26 complex was purified from maize thylakoids by preparative IEF and sucrose gradient ultracentrifugation (40). When analyzed by SDSPAGE, the preparation exhibited a single Coomassie-stained band at ∼28 kDa. Pigment content was determined upon extraction with buffered 80% acetone, and analysis was carried out by a combined approach of HPLC and fitting of the spectrum of acetone extracts with the spectra of individual
7336 Biochemistry, Vol. 41, No. 23, 2002
Croce et al.
Table 1: Pigment Content of Samples with Different Chl a/b Ratiosa sample
Chl a/b mixb
Chl a/b
Chl a
Chl b
Neo
Viola
Lute
Chl tot
Chl/car
nCP26 rCP26-2.7 rCP26-2.25 rCP26-2.08 rCP26-1.7 rCP26-1.1 rCP26-0.23 rCP26-0.11
8 5.5 3 1 0.3 0.05
