Crystal Structure of Allophycocyanin from Red Algae Porphyra yezoensis at 2.2-Å Resolution*

Jin-Yu Liu, Tao Jiang, Ji-Ping Zhang, and Dong-Cai LiangDagger

From the National Laboratory of Biomacromolecules, Institute of Biophysics, Chinese Academy of Sciences, Chaoyang District, Beijing 100101, China

    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES

The crystal structure of allophycocyanin from red algae Porphyra yezoensis (APC-PY) at 2.2-Å resolution has been determined by the molecular replacement method. The crystal belongs to space group R32 with cell parameters a = b = 105.3 Å, c = 189.4 Å, alpha  beta  = 90°, gamma  = 120°. After several cycles of refinement using program X-PLOR and model building based on the electron density map, the crystallographic R-factor converged to 19.3% (R-free factor is 26.9%) in the range of 10.0 to 2.2 Å. The r.m.s. deviations of bond length and angles are 0.015 Å and 2.9°, respectively.

In the crystal, two APC-PY trimers associate face to face into a hexamer. The assembly of two trimers within the hexamer is similar to that of C-phycocyanin (C-PC) and R-phycoerythrin (R-PE) hexamers, but the assembly tightness of the two trimers to the hexamer is not so high as that in C-PC and R-PE hexamers.

The chromophore-protein interactions and possible pathway of energy transfer were discussed. Phycocyanobilin 1alpha 84 of APC-PY forms 5 hydrogen bonds with 3 residues in subunit 2beta of another monomer. In R-PE and C-PC, chromophore 1alpha 84 only forms 1 hydrogen bond with 2beta 77 residue in subunit 2beta . This result may support and explain great spectrum difference exists between APC trimer and monomer.

    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES

Phycobilisomes are large supramolecular aggregates attached to the stromal side of the thylakoid membrane in cyanobacteria, red algae, and cryptomonads. These supramolecular aggregates are light-harvesting protein pigment complexes that are composed of phycobiliproteins and linker proteins. Based on the absorption of visible light, the phycobiliproteins can be divided into three main groups: phycoerythrin (PE)1 or phycoerythrocyanin (PEC), phycocyanin (PC), and allophycocyanin (APC). With the help of linker proteins, phycobiliproteins form the two distinct structural domains of phycobilisome, the core and the rods. The core, which is composed of three or more core cylinders associated by APC discs, is in proximity of the reaction centers, whereas the rods are attached on the core and are composed of PC discs in the middle and PE or PEC discs on the tip. Light energy is transferred from PE or PEC via PC to APC and finally to the reaction centers (1).

The crystal structures of several phycobiliproteins have been solved; among them, three are PEs (2-4), three are C-phycocyanins (C-PCs) (5-7), one is PEC:PEC from Mastigocladus laminosus (8), and one is APC:APC from Spirulina platensis (9). All these structures are very similar. The basic building block is an alpha beta monomer composed of alpha  and beta  subunits (R-phycoerythrin (R-PE) and B-phycoerythrin (B-PE) have a third subunit gamma  in the center of the molecule); three alpha beta monomers are arranged around a 3-fold symmetry axis to form an (alpha beta )3 trimer or two (alpha beta )3 trimers, which are assembled face to face into an (alpha beta )6 hexamer.

The crystal structure of APC is very special compared with other phycobiliproteins. First, the spectrum difference between APC trimer and its monomer is very large. When APC monomers aggregate to trimer, the absorption spectrum has a 40-nm red shift; the CD spectrum also changes a great deal, and exiton interaction in the trimer of APC was suggested (10), whereas the spectrum difference between C-PC monomer and its trimer is not so large as in APC, although phycocyanin has the same alpha 84PCB and beta 84PCB as APC.

Second, the functional unit of APC was thought to be a trimer, whereas the function unit of other phycobiliproteins were hexamer (alpha beta )6 or (alpha beta )6gamma . Brejc and co-workers solved the structure of APC-SP from blue alga S. platensis (9) in the unit cell of APC-SP crystal; two trimers are associated in a "back to back" manner that might represent the assembly state of APC in nature. Red alga is higher than blue alga in evolution, so it would be interesting to know the packing of APC from red alga in the unit cell and in nature.

Third, in PE and PC, the two trimeric discs are superimposed along a 3-fold axis, but in PC and APC the two discs are connected perpendicularly. The pathway of energy transfer between PC and APC is still unknown.

The red algae Porphyra yezoensis is an algae that exists widely in nature. Its phycobilisomes contain R-PE, C-PC, and APC. In this paper we report the crystal structure of APC from P. yezoensis (APC-PY) at 2.2-Å resolution. The organization of APC trimers in the core cylinders of phycobiliproteins and the pathway of energy transfer were discussed.

    EXPERIMENTAL PROCEDURES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES

Crystallization and data collection of APC-PY was reported earlier (11). The crystals of APC-PY belong to space group R32 with parameters a = b = 105.3 Å, c = 189.4 Å, alpha  = beta  = 90°, and gamma  = 120°.

Molecular replacement using program AMoRe (12) was carried out using the 2.3-Å structure of APC-SP as a model. Model cell parameters were a = b = c = 150.0 Å, alpha  = beta  = gamma  = 90°, integrate radius was 30 Å, and rotation function calculation gave a rather high coefficient solution, alpha  = 60.07, beta  = 3.06, gamma  = 88.03, Cc = 20.0. The orientations and positions of one alpha beta in the asymmetric unit were determined by the translation function with a high correlation coefficient of 66.9%. The R-factor in the range from 10 to 4 Å was 36.1%. After rigid-body refinement, R-factor dropped to 33.1%, and the correlation coefficient increased to 71.2%. The packing of molecules in the unit cell was reasonable.

The structure was refined using X-PLOR (13). The consensus sequence was used for the initial model building. Fourier transform and electron density were first calculated in the resolution range of 10 Å to 3.5 Å. Residues that could not be fitted into the electron density map were omitted from the phase calculation in the next refinement cycle. After several cycles of rigid body, positional refinement, and manual model adjustment, the R-factor dropped to 25.4%, and a 2Fo-Fc Fourier map looked quite good. Then the resolution was extended to 2.2 Å. After the chromophores were fitted in the map and followed by several cycles of positional refinement and model adjustment, the electron density improved further. At this stage almost all side chains were well defined except those on the surface. Residue exchanges were carried out at this stage according to the omit map. After several cycles of positional refinement and model adjustment, the R-factor was converged to 24.0%, the individual B-factors were then refined, and the R-factor dropped to 21.5%. 169 water molecules were added to the model according to the Fo-Fc and 2Fo-Fc maps, and the final R-factor of the model was 19.3% (R-free factor was 26.9%) in the range of 10 Å to 2.2 Å.

    RESULTS AND DISCUSSION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES

Amino Acid Sequence-- Because the amino acid sequence of APC from P. yezoensis is still unknown, the following six APC amino acid sequences were used to get a consensus sequence for model building. Among these sequences, four are from cyanobacteria, Anabaena cylindrica (14), Calotrix PCC7601 (15), Fischerella PCC7603 (16), and Synechococcus PCC6301 (17), and two are from red algae, Aglaothamnion neglectum (18) and Cyanidium caldarium (19). The alignment of these six sequences is shown in Table I.

                              
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Table I
The sequence alignment of the APCs
The alignment of alpha  and beta  subunit sequences is shown of APCs from A. cylindrica (ANCY), Calotrix PCC7601 (CALO), Synechococcus PCC6301 (SYNO), M. Laminosus (MALA), C. caldarium (CYCA), and A. neglectum (AGNE). The consensus sequence (CONS) was derived from the alignment. The model sequence (SEQ) is based on the electron density. Conserved amino acids are marked in bold letters.

Quality of the Model-- The final crystallographic R-factor for APC-PY model is 19.3%, in the range of 10.0 Å to 2.2 Å. The Luzzati plot gives a mean positional error of 0.26 Å (20). The r.m.s. deviations of bond lengths and bond angles are 0.015 Å and 2.9°, respectively. The quality of the final model is summarized in Table II. The Ramachandran plot shows that all dihedral angles fall into most favored or allowed regions with the only exception of beta 77Thr (Fig. 1) (21), which has a conserved unusual dihedral angle in all known phycobiliprotein structures. In APC-PY, the N atom of beta 77Thr forms a hydrogen bond with OD (the oxygen atom in the ring D of chromophore) oxygen atom of alpha 84PCB. The electron density of this residue is well defined in APC-PY. The consensus amino acid sequence (Table I) was used to build the initial model and later modified according to the electron density map. In the 2.3-Å-resolution crystal structure of APC-SP (9), 28 residues were not well defined with 102 atoms of zero occupancy; these residues are alpha 25 Asp, alpha 35Glu, alpha 36Arg, alpha 49Glu, alpha 50Arg, alpha 53Lys, alpha 54 Gln, alpha 76 Tyr, alpha 79 Asp, alpha 120Lys, alpha 127Glu, beta 2 Gln, beta 10Asn, beta 17Lys, beta 20 Asp, beta 25 Gln, beta 35Glu, beta 36Leu, beta 39Arg, beta 50Asn, beta 58Lys, beta 65 Asp, beta 68Arg, beta 116Lys, beta 117Glu, beta 131 Gln, beta 138Glu, beta 150Lys. The fit of these residues to our electron density map is better in APC-PY; most of them behave well at 1sigma density level (Fig. 2); others behave well at 0.7sigma density level except alpha 76 in the loop, which has a density at 0.5sigma density level.

                              
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Table II
Parameters of refined APC-PY model


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Fig. 1.   Ramachandran plot of the APC-PY residues. Glycine residues are marked as squares. Nonglycine residues are marked as crosses. beta 77 (277) is in an unusual region. PSI and PHI are dihedral angles psi  and phi .


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Fig. 2.   Omit electron density map of residues beta 35Glu and beta 36Leu in APC-PY.

Comparing the crystallographic sequences of the final model of APC-PY and APC-SP, there are 37 nonidentical residues, 25 in the alpha  subunit and 12 in the beta  subunit (Table III). In comparison with other APC sequences, the 37 residues of APC-PY are more conserved than those of APC-SP. For example, alpha 52Val and alpha 61Gln of APC-PY are identical to other known sequences. The electron density of these two residues in APC-PY are well defined.

                              
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Table III
Sequence comparison of APC-PY and APC-SP

Molecular Structure-- The asymmetric unit of APC-PY contains alpha  and beta  subunit. The alpha  subunit is composed of 160 residues, and the beta  subunit contains 161 residues. Three alpha beta monomers are arranged around a 3-fold axis to form a disc shaped (alpha beta )3 trimer of 30 Å in thickness and 110 Å in diameter with a cave in the center. The alpha  and beta  subunits in the alpha beta monomer have similar structures, with nine alpha -helices (X, Y, A, B, E, F, F', G, H) separated by irregular loops (Fig. 3).


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Fig. 3.   Ribbon representation of APC alpha  subunit (a), APC beta  subunit (b).

The three-dimensional structure of APC-PY alpha  and beta  subunits are very similar to the known structure of APC-SP. The intersubunit interactions within the (alpha beta ) monomer and the (alpha beta )3 trimer are also very similar to these of APC-SP and other phycobiliproteins. In the (alpha beta ) monomer of APC-PY, the ionic- and polar-interacting residues between the two subunits are alpha 3 Ser-beta 3 Asp, alpha 13 Asp-beta 94 Tyr, alpha 13 Asp-beta 110Arg, alpha 17Arg-beta 97 Tyr, alpha 18 Tyr-beta 93Arg, beta 13 Asp-alpha 93Arg, beta 13 Asp-alpha 97 Tyr, beta 18 Tyr-alpha 89 Asp.

In the APC-PY crystal, two trimers associate face to face into the (alpha beta )6 hexamer through a crystallographic dyad perpendicular to the triad. There are three (alpha beta )6 hexamers in a unit cell, locating at (0,0,0), (2/3, 1/3, 1/3), and (1/3, 2/3, 2/3) (Fig. 4a). The assembly of two trimers in this hexamer is completely different from that of APC-SP. In APC-SP crystal, the two trimers are associated loosely through beta  subunits in a "back to back" manner (Fig. 4b) in the hexamer (9), but in APC-PY crystal, the two trimers in the hexamer contact through alpha  subunits, and the assembly of the two trimers is much tighter than that in APC-SP.


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Fig. 4.   a, packing of APC-PY in the unit cell. b, packing of APC-SP in the unit cell.

The assembly of the hexamer in the APC-PY crystal is similar to that of C-PC from Fremylla diplosiphon (C-PC-FR) and R-PE from Polysiphonia urceolata (R-PE-PU) hexamers; two (alpha beta )3 trimers associate face to face in the hexamer. The alpha  subunits provide the contacting surface, and the two trimers fit complementarily in the hexamer.

Despite the similarity in assembly in APC-PY, C-PC-FR, and R-PE-PU hexamers, the superposition of the Calpha atoms of APC-PY and C-PC-FR hexamers shows that the assembly of the two trimers in APC-PY hexamer is obviously looser than that in C-PC-FR and R-PE-PU hexamers. The calculated accessible areas between the trimers in C-PC-FR and R-PE-PU hexamers are about 5900 and 6900 Å2. In APC-PY hexamer, this value is about 3200 Å2, which is much bigger than that in APC-SP (600 Å2); thus, the APC-PY hexamer can be described as a "loose hexamer." The interactions between the trimers in APC-PY hexamer are different from those in C-PC-FR and R-PE-PU hexamers.

First, the number of the residues involved in the interactions between the two trimers in APC-PY hexamer is smaller than that in C-PC-FR and R-PE-PU hexamers, indicating a weaker association. This is consistent with the calculated accessible areas between the two trimers in C-PC-FR, R-PE-PU, and APC-PY hexamers. The special polar network present in C-PC of Agmenellum quadruplaticum (C-PC-AQ), formed by residues 1beta 46 Asn-6alpha 164 Asn-1alpha 21 Asn-6alpha 161 Glu-6alpha 33 Glu-6alpha 30 Arg (6) is not conserved in APC-PY hexamer. In addition, the electrostatic interactions between 1alpha 2Lys-6alpha 23Glu, 1alpha 17Arg-6alpha 108 Asp, and 1alpha 120Arg-4alpha 174 C-terminal carboxyl group, which were suggested to be involved in the hexamer formation in C-PC-FR (22), are also not present in APC-PY hexamer. Furthermore, the comparison of APC-PY with C-PC-FR and R-PE-PU reveals that all the conserved polar and ionic interactions between the two trimers in C-PC-FR and R-PE-PU hexamers are not present in APC-PY hexamer.

Despite the above difference, the interactions that maintain APC-PY as a loose hexamer seem still to be the polar and charged interactions between the two trimers. In APC-PY hexamer, the polar and charged interactions are 1alpha 25 Asp-6alpha 37Arg, 1alpha 22 Gly-6alpha 26Arg, 1alpha 25 Asp-6alpha 161Glu, 1alpha 25 Asp-6alpha 165 Tyr, and 1alpha 28Lys-6alpha 147 Asp. In APC-SP, only a few polar and charged interactions (<4 Å) exist between the two trimers, such as 1beta 65 Asp-6beta 131 Gln and 1beta 120Asn-6beta 120Asn, indicating a very loose packing.

Second, in C-PC-FR and R-PE-PU hexamers, the trimer-trimer association is mediated almost exclusively by polar and charged residues (6), but in the APC-PY hexamer, some hydrophobic residues are also involved, such as alpha 21 Pro, alpha 22 Gly, alpha 104 Val, alpha 164 Phe, beta 42 Ala, and beta 46 Ala.

Chromophores alpha 84PCB and beta 84PCB-- In APC, two phycocyanobilins are covalently bound to cysteine residues at position alpha 84 and beta 84 (Fig. 5). Both chromophores are well defined in APC-PY (Fig. 6). The geometry and protein environment of these two chromophores resemble those of APC-SP. The alpha 84 PCB chromophores have a protein environment similar to that of beta 84PCB. The polar and ionic protein-chromophore interactions in alpha 84PCB and beta 84PCB are shown in Table IV.


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Fig. 5.   Chemical structure of PCB.


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Fig. 6.   The coincidence of chromophores in APC-PY with 2Fo-Fc electron density map alpha 84PCB (a), beta 84PCB (b).

                              
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Table IV
The polar and ionic protein-chromophore interactions in APC-PY (Å)

Chromophores alpha 84PCB and beta 84PCB have similar hydrophobic environment; there are three aromatic residues close to alpha 84, such as alpha 90 Tyr, alpha 91 Tyr, and alpha 119 Tyr, and three close to beta 84, such as beta 90 Tyr, beta 91 Tyr, and beta 119 Tyr. In C-PC-FR, alpha 90 and alpha 91 are all Tyr, and beta 90 and beta 91 are all Ile. In R-PE-PU, alpha 90 and alpha 91 are His and Tyr, respectively, and beta 90 and beta 91 are all Ile. But in all known APCs, alpha 90, alpha 91, beta 90, and beta 91 are all Tyr. So the microenvironment of alpha 84 and beta 84 in APC-PY is similar to that in C-PC-FR and R-PE-PU, indicating that alpha 84PCB and beta 84PCB have similar conformation and spectrum character. beta 90 Tyr and beta 91 Tyr stabilize the beta 84PCB ring D conformation, which may make PCB have different spectrum characteristics in APC and C-PC.

Energy Transfer-- There are 12 PCBs in APC-PY hexamer; the arrangement of these chromophores is shown in Fig. 7. The theory of shot-distance exiton interaction (23) and long distance dipole-dipole resonance mechanism (24) has been used to explain the energy transfer rate between chromophores.


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Fig. 7.   The chromophores of APC-PY hexamer.

Inside trimer of APC-PY, the distance between 1beta 84PCB and 2beta 84PCB in APC-PY is about 34 Å and that between 1alpha 84PCB and 2beta 84PCB is about 21 Å. These values are similar to those in C-PCs. The chromophores are too far away to have exiton interaction. It is also difficult to explain why exiton interaction exists in APC but not in C-PC. Our study of chromophore-protein interactions and comparison of microenvironments in R-PE-PU, C-PC-FR, and APC-PY show that almost all the chromophore-protein interactions exist within the same monomer (alpha beta ), the only exception being alpha 84PEB in R-PE-PU, which forms a hydrogen bond with beta 77 Thr in another monomer. In C-PC-FR, the situation is the same as in R-PE-PU. However, it is different in APC-PY; its alpha 84PCB forms five hydrogen bonds with the residues in other monomer, such as alpha 84PCB O2B-2beta 62 Tyr OH, O1C-2beta 62 Tyr N, O1C-2beta 67 Thr OG1, O2C-2beta 67 Thr OG1, and OD-2beta 77 Thr N (see Table IV). We believe this difference may explain why the spectrum of APC changes greatly when its monomers associate to trimer. In APC-SP, distances of alpha 84PCB O2B-2beta 62 Tyr OH, O1C-2beta 62 Tyr N, O1C-2beta 67 Thr OG1, O2C-2beta 67 Thr OG1, and OD-2beta 77 Thr N are all within the distance of hydrogen bond formation. As we know, beta 62 Tyr and beta 67 Thr are close to chromophore beta 84PCB and may control the conformation of chromophore and bridge between alpha 84 PCB and beta 84 PCB to make the exiton interaction occur.

The distances of chromophores between the two trimers in APC-PY hexamer are similar to those in C-PC-FR and R-PE-PU hexamers. Based on the 1.9-Å resolution crystal structure, the possible pathway of energy transfer within and between the two trimers of R-PE-PU were discussed (25). There are three pairs of short distance interactions between two trimers, such as 1alpha 84 right-arrow 4alpha 84,1alpha 140a right-arrow 6beta 155, and 1beta 155 right-arrow 6beta 155. alpha 84PEB is on the inner surface of R-PE-PU, and 1alpha 84PEB right-arrow 4alpha 84PEB may be the dominant energy transfer pathway between the two trimers. Similar energy pathways (1alpha 84PCB right-arrow 4alpha 84PCB) also exists in C-PC-FR hexamers (26). In C-PC-FR, R-PE-PU, and APC-PY hexamers, the distances between 1alpha 84 (C10 atom) and 4alpha 84 (C10 atom) are 27.5, 28.7, and 30.3 Å, respectively, which are comparable. Therefore, the distance between the two chromophores of APC-PY seems adequate for effective energy transfer.

In addition to the energy pathway composed of chromophores, the aromatic pathway formed by aromatic residues may play an important role in energy transfer. The energy transfer from chromophores to aromatic residues vice versa can be explained by exiton interaction mechanism, because the distances between some chromophores and aromatic residues, such as alpha 84PCB-alpha 90 Tyr, alpha 84PCB-alpha 91 Tyr, beta 84PCB-beta 90 Tyr, beta 84PCB-beta 91 Tyr are very short (~4 Å). Förster's dipole-dipole resonance transfer can occur between different aromatic residues rather than between chromophores and aromatic residues, because the overlap integral between chromophore absorption (lambda max congruent  650 nm for fluorescence spectrum) and aromatic residue emission (lambda max congruent  300 nm for fluorescence spectrum and lambda max congruent  400 nm for phosphorescence spectrum) is quite small. In APC-PY there are two areas abundant in aromatic residues as shown in Fig. 8. One is close to chromophore alpha 84PCB and composed of alpha 164 Phe, alpha 165 Tyr, alpha 166 Phe, alpha 168 Tyr, alpha 90 Tyr, alpha 91 Tyr, alpha 97 Tyr, alpha 119 Tyr, and beta 18 Tyr. Another is close to chromophore beta 84 and composed of beta 165 Tyr, beta 166 Phe, beta 168 Tyr, beta 90 Tyr, beta 91 Tyr, beta 94 Tyr, beta 97 Tyr, beta 119 Tyr, and alpha 18 Tyr. alpha 164 Phe is involved in the hydrophobic interactions between the two trimers. The aromatic residues in the alpha  subunit and the beta  subunit have high homology and similar locations. Other aromatic residues are on the periphery of the disc, such as alpha 76 Tyr, alpha 60 Phe, beta 76 Tyr, beta 62 Tyr, beta 30 Tyr, beta 31 Phe, alpha 30 Phe, and beta 81 Tyr; among them alpha 76 Tyr and beta 62 Tyr may mediate the energy transfer between 1alpha 84PCB and 2beta 84PCB. Because PC and APC are connected perpendicularly in vivo, aromatic residues on the periphery may mediate the energy transfer from the chromophores of PC to those of APC.


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Fig. 8.   The location of aromatic residues in APC-PY.

Functional Unit-- In the core cylinders of phycobilisomes, several APC trimers are close together, but the association manner of these APC trimers is still unknown. Based on dissociation experiments, it was suggested that allophycocyanin does not form hexamers (27), because almost all the residues involved in the trimer-trimer aggregation in C-PC-AQ and C-PC-FR hexamers are not conserved in APC. Similar conclusions were reported later (6, 9). In the APC-PY hexamer, all the interactions involved in the formation of C-PC-AQ and C-PC-FR hexamers and all the conserved polar and charged interactions in C-PC-FR and R-PE-PU hexamers are not present, but APC-PY can still associate face to face to form a hexamer, which is maintained by some polar and charged interactions, different from those in C-PC-FR and R-PE-PU. Because the distances of chromophores between the two trimers in this hexamer are also adequate for effective energy transfer, we assume that the loose hexamer may represent the basic unit of APC in physiological conditions. It is possible that linker proteins may help to stabilize the loose hexamers.

    ACKNOWLEDGEMENT

We thank Professor Lu-Lu Gui, Institute of Biophysics, Chinese Academy of Sciences and Professor You-Shang Zhang, Institute of Biochemistry, Chinese Academy of Sciences for their support and concern.

    FOOTNOTES

* This work was supported by Chinese Academy of Sciences (KJ85-04-40) and the National Natural Science Foundation of China (39630090).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Dagger To whom correspondence should be addressed: National Laboratory of Biomacromolecules, Institute of Biophysics, Chinese Academy of Sciences, 15 Datun Rd., Chaoyang District, Beijing 100101, China. Tel.: 86-10-64889867; Fax: 86-10-64889867; E-mail: dcliang{at}sun5.ibp.ac.cn.

    ABBREVIATIONS

The abbreviations used are: PE, phycoerythrin; APC, allophycocyanin; PC, phycocyanin; C-PC, C-phycocyanin; PEC, phycoerythrocyanin; APC-PY, allophycocyanin from P. yezoensis; APC-SP, APC from S. platensis; C-PC-AQ, C-PC from A. quadruplaticum; C-PC-FR, C-PC from F. diplosiphon; R-PE-PU, R-PE from Polysiphonia urceolata; PCB, phycocyanobilin; alpha , 1beta , 2alpha , 2beta ,  ... 6beta stand for the individual subunits of different monomers within the hexamer (according to Schirmer et al. (6)); r.m.s., root mean square.

    REFERENCES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES
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