The Crystal Structure of a Novel Unmethylated Form of C-phycocyanin, a Possible Connector Between Cores and Rods in Phycobilisomes*
Noam Adir
and
Natalia Lerner
From the
Department of Chemistry and Institute of Catalysis, Science and
Technology, Technion, Israel Institute of Technology, Technion City, Haifa
32000, Israel
Received for publication, March 20, 2003
, and in revised form, April 11, 2003.
 |
ABSTRACT
|
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A novel fraction of c-phycocyanin from the thermophilic cyanobacterium
Thermosynechcoccus vulcanus, with an absorption maxima blue-shifted
to 612 nm (PC612), has been purified from allophycocyanin and
crystallized. The crystals belong to the P63 space group with cell
dimensions of 153 Å x 153 Å x 59 Å with a single
(
) monomer in the asymmetric unit, resulting in a solvent content
of 65%, and diffract to 2.7 Å. The PC612 crystal structure
has been determined by molecular replacement and refined to a crystallographic
R-factor of 20.9% (Rfree = 27.8%). The crystal
packing in this form shows that the PC612 form of phycocyanin does
not associate into hexamers and that its association with adjacent trimers in
the unit cell is very different from that found in a previously determined
structure of the normal form of T. vulcanus phycocyanin, which
absorbs at 620 nm. Analysis of the PC612 structure shows that the
subunits, which typically form the interface between two trimers
within a hexamer, have a high degree of flexibility, as indicated by elevated
B-factors in portions of helices B, E, and G. Examination of
calculated electron density omit maps shows that unlike all other structures
of phycobiliproteins determined so far, the Asn
72 residue is
not methylated, explaining the blue-shift in its absorption spectra. On the
basis of the results presented here, we suggest that this new form of trimeric
phycocyanin may constitute a special minor component of the phycobilisome and
may form the contact between the phycocyanin rods and the allophycocyanin
core.
 |
INTRODUCTION
|
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Cyanobacteria and red algae efficiently harvest light used for
photo-induced electron transfer by a variety of pigment-protein complexes
called phycocbilisomes
(PB)1 (reviewed in
Refs.
15).
The PB is the largest type of all photosynthetic antenna pigment-protein
complexes attached to photosynthetic reaction centers. The sizes of such
complexes vary between species and growth conditions and can easily reach
molecular masses in excess of 2 MDa. The self-association of the protein
subunits has been studied and documented for a variety of species and shows
different complex forms. All pigment-binding protein species show a canonical
first level quaternary structure of the association of two pigment-binding
subunits termed
and
into the basic (
) monomer. The
monomers further assemble into higher organizational levels of
(
)3 trimers and (
)6 hexamers.
The trimers (and hexamers) are round disks with dimensions of about 110
x 30 Å (or 60 Å for the hexamers). The hexamer encloses a
large internal cavity with triangular shaped openings on both sides. The most
prevalent PB forms, found in many cyanobacteria, are made up of a core
assembly of three hexamers of allophycocyanin (APC) that are arranged with
their ring planes perpendicular to the membrane surface directly above
Photosystem II complexes (4).
On this core are arranged six rod-like structures, made up of phycocyanin (PC)
hexamers. In some species, and under certain environmental conditions,
additional phycoerythrin (or other phycobiliprotein variants) hexamers attach
at the terminal ends of the PC rods
(12). The planes of the rings
that make up the hexamers in the rods are perpendicular to both the APC core
and to the membrane, so that in essence the APC hexamer disks bind the PC rods
by their outer circumference. In addition to the pigment-binding
pycobiliproteins, a number of linker proteins have been found associated with
PB components (6,
7). It has been suggested that
these linker proteins occupy positions running through the internal cavities
of the disks and may play roles in complex stabilization, rod-core assembly,
and in inducing the directionality of energy transfer toward Photosystem II.
Light energy trapped by the most prevalent pigments (phycoerythrobilin,
max = 560 nm; phycocyanobilin,
max = 620
nm) traverses down through the rods to the APC pigments
(
max = 652 nm) and from these pigments to the chlorophyll
pigments of the reaction center (
max = 674680 nm)
(3,
4).
A significant number of PB protein structures have been determined over the
past few years
(825).
All of these structures show a very high degree of similarity in the overall
structures, the details of the pigment surroundings, the solvent interactions,
and the protein residue positions in each structure, and thus have given us an
excellent molecular view of the constituents of the PBs. However, a true
structural description of how these different components interact is still
lacking, due to the fact that the structures of each of the components has
been determined separately. Because there appears to be a certain degree of
correlation between the formation of hexamers and rods during the
crystallization process and the actual PB rods, it has been suggested that the
overall packing of the crystal unit cell can help identify energy transfer
pathways between pigments at all levels, including inter-rod energy transfer
(9,
20,
21,
24). Biochemical, biophysical,
and electron microscopic studies of the PB have been interpreted and promoted
a number of models of the entire PB
(5,
2629).
These studies were performed on a variety of PB forms, from different species,
with different numbers of APC core disks. There are, however, geometric
problems with the schematic models of the PB structure. To assemble six PC
rods around the three APC disks, interpenetration of the rods must occur,
because the circumference if the core is significantly smaller than the sum of
the rod diameters. Interpenetration of the circumference of one PC hexamer
into the molecular circumference of an adjacent hexamer would necessitate the
presence of large cavities in the circumference. Analysis of the crystal
structures of many PC hexamers has not shown such cavities to exist,
indicating that such interpenetration is not possible.
We have recently determined the structure of the PC component of
phycobilisomes from the thermophilic cyanobacteria Thermosynchococcus
vulcanus (Tv-PC, formally Synechococcus vulcanus) at
high resolution (1.6 Å, Protein Data Bank code 1KTP
[PDB]
; see Ref.
25). This structure, along
with a room-temperature structure at lower resolution (2.5 Å, Protein
Data Bank code 1I7Y
[PDB]
; see Ref.
23) allowed the detailed
analysis of a number of important structural details pertaining to the
structure, function, and stability of this protein.
In the present study we have identified and determined the structure of a
novel form of Tv-PC, which we believe may prove to be a functional
and structural link between the rods and cores of the phycobilisomes, helping
to avoid the interpenetration problem in PB assembly.
 |
EXPERIMENTAL PROCEDURES
|
---|
Protein Isolation, Characterization, and
CrystallizationPhycocyanin with an absorption maxima of 612 nm
from the thermophilic cyanobacterium T. vulcanus (PC612)
was isolated by the following procedure.
Cyanobacterial cells were grown at 55 °C for 34 days with 5%
CO2 in air added continuously to the growth medium. Cells were
harvested by centrifugation, washed in Buffer A (20 mM HEPES, pH
8.0) and then treated with 1 mg/ml lysozyme (Sigma) at 55 °C for 60 min.
The cells were then ruptured using a Yeda Pressure cell using 25 atmospheres
of N2. Broken cell debris was separated from the photosynthetic
membranes by centrifugation, and the membranes were then pelleted. The
supernatant contained large amounts of PC absorbing at 620 nm
(PC620). The membranes were then treated with Buffer A with 2
M KCl, which removed the remaining phycobiliproteins. Following
removal of the membranes by centrifugation, the soluble fraction was treated
with polyethylene glycol 4000 to precipitate contaminating PC620,
and fractions that absorbed at 651 nm (APC) were pooled. The APC-rich fraction
was further fractionated by anion-exchange chromatography (DEAE, Toyohaas),
using Buffer A as the mobile phase. A salt gradient of 0300
mM NaCl in Buffer A was used to separate between different protein
fractions, with a fraction containing PC612 eluting at 130
mM NaCl and APC eluting at 200 mM NaCl. Protein
fractions were analyzed for purity by SDS-PAGE, and the aggregation state was
determined by size-exclusion HPLC (PL-GFC 1000 Å, Polymer Laboratories
Ltd.). The final PC612 fractions were dialyzed against Buffer A and
concentrated to a protein concentration of 20 mg/ml.
PC612 crystals were obtained by mixing 6.7% polyethylene glycol
4000 with 6.3 mg/ml purified PC612 in the presence of 70
mM bis-Tris (pH 7.0). 6 µl hanging drops were equilibrated
against a 1-ml reservoir containing 10% polyethylene glycol 4000 in 100
mM bis-Tris (pH 7.0).
Data Collection and Structure DeterminationPC612
crystallized in the P63 space group with cell dimensions of
a = b = 153 Å = 39 Å and
=
120o and diffracted maximally to 2.7 Å. A data set was
collected using a single crystal on beamline X11 of the EMBL-Hamburg
outstation at DESY using an MAR CCD detector
(Table I). The data was scaled
and merged using the DENZO/SCALEPACK suite
(30). The final data was 84.7%
complete to 2.7 Å and was used for structure determination by molecular
replacement with CNS (31)
using the 1.6 Å Tv-PC structure (Protein Data Bank code 1KTP
[PDB]
;
see Ref. 25) as the search
model (see "Results" for further details).
RefinementThe structure was refined using CNS
(31). Following simulated
annealing, B-factor refinement, and water molecule addition, the
structure was inspected against electron density maps calculated in CNS and
examined visually using Quanta (Accelrys). Extensive use of calculated omit
maps were used to manually adjust and confirm the positions of all residues
and co-factors. The final model had a crystallographic R-factor of
20.9% and a Rfree of 27.8%. The coordinates and structure
factors were deposited in the Protein Data Bank under the code 1ON7.
 |
RESULTS
|
---|
Isolation, Characterization, Crystallization, and Structure
Determination
PC612 Isolation and
CharacterizationWe have previously determined the structure of
Tv-PC in a rhombohedral crystal space group. Protein for
crystallization was isolated in trimeric form from the thylakoid membranes
following detergent treatment and anion-exchange chromatography
(23). The isolated protein had
an absorption maxima at 620 nm, as has been reported previously
(4) for PC. This form of
Tv-PC resulted in two crystal structures, the first from data
collected at room temperature (Protein Data Bank code 1I7Y
[PDB]
, 2.5 Å
resolution; see Ref. 23) and
the second from data collected at 100K from frozen crystals (Protein Data Bank
code 1KTP
[PDB]
, 1.6 Å resolution; see Ref.
25).
Following our successful determination of the Tv-PC structure at
high resolution, we decided to obtain a more complete picture of this species
phycobilisomes by determination of the structure of the APC component of the
phycobilisome core (5). To
obtain APC, we modified the isolation procedure to separate the APC fraction
from the bulk PC that is in large excess (see "Experimental
Procedures" for details). In the course of isolation of APC, we obtained
three fractions: i) APC, with an absorption at
= 651 nm; ii) a
fraction of PC that had an absorption blue-shifted to
= 612 nm (hence
referred to as PC612); and iii), a fraction that appeared to be a
mixture of the other two fractions (Fig.
1). All three fractions appeared to have less than 5%
non-pigmented protein contaminants by SDS-PAGE analysis (data not shown). Both
PC and APC fractions exclusively contained (
)3 trimers
as identified by size-exclusion HPLC (data not shown).
PC612 Crystallization and Structure
DeterminationCrystallization trials of all three fractions were
performed using various modifications of the conditions optimized previously
for Tv-PC. Very thin plate-like crystals of purified APC were
obtained that were not amenable to structure determination. However, the
PC612 fraction crystallized readily, and large blue hexagonal
crystals were obtained. X-ray diffraction analysis, performed on an R-Axis IIc
diffractometer, showed that these crystals belonged to a hexagonal space group
and were unlike the previously described crystal forms of Tv-PC. We
thus undertook the task of high-resolution data collection at cryogenic
temperature using synchrotron radiation (the EMBL PX beam-line (X11) at the
DORIS storage ring, DESY, Hamburg, Germany). The crystals did not diffract as
well as the rhombohedral form
(25); however, a complete data
set to 2.7 Å was obtained (Table
I). Analysis of the diffraction pattern showed that the crystal
unit cell dimensions were about 153 x 153 x 39 Å. Following
data processing, using the DENZO/SCALEPACK suite, it became apparent that
reflections of the type h = 0, k = 0 had not been collected,
and so the exact space group could not be determined by identification of
systematic absences. However, processing of the data to higher symmetry
hexagonal space groups (P622, P6122, etc.) resulted in
high Rsym values (> 0.12), whereas those of lower
symmetry had Rmerge values of about 0.06.
Molecular replacement (CNS, Ref.
31) was thus performed using
the 1KTP
[PDB]
structure in all six hexagonal space groups of type P6,
P61, etc. The only space group for which a translation
function solution and correct proper packing could be obtained was
P63. The crystallographic R-factor for the molecular
replacement solution after rigid-body refinement was 0.39. Calculation of the
Matthews coefficient (32)
indicated the possibility of either one or two (
) monomers in the
asymmetric unit (Vm = 3.55 or 1.78,
respectively). Thus at this stage, CNS was used to search for a possible
second monomer in the asymmetric unit. No such solution could be obtained, and
calculated electron density maps using the unrefined monomer as the source of
phases showed clear density for the monomer, without additional protein. Thus
the hexagonal crystal form of PC612 has a single monomer in the
asymmetric unit, with two (
)3 trimers in the unit cell
(Fig. 2). Interestingly, the
PC612 form of Tv-PC is similar to the first PC structure
determined that from Mastigocladus laminosus (this structure has not
been deposited in the Protein Data Bank; see Ref.
10).
RefinementThe PC612 structure was refined using
maximum likelihood simulated annealing
(31), followed by multiple
rounds of B-factor, coordinate minimization, and manual fitting. The
locations of 36 water molecules were identified, and the final refined
structure (Table II) had a
crystallographic R-factor of 20.9% (Rfree =
27.8%). All geometric constraints were within or better than the mean values
as determined by the Protein Data Bank ADIT validation server. Only the highly
conserved Thr
77 residue has non-typical peptide geometry
(
= 76.7°,
= 128.3°), as has been found for this residue in
all previously determined PC structures
(10,
23).
Overall Quality of the Structure
The PC612 structure shows the typical globin-like fold
identified in the past (8). The
overall structure is similar to the previously determined high-resolution
structure 1KTP
[PDB]
with a root mean square deviation coordinate difference of 0.82
and 1.02 Å over all
carbon and all atoms respectively.
However, a number of characteristics make the PC612 structure
unique. The (
) monomers are organized into
(
)3 trimers, which were also the basic unit in
solution prior to crystallization. The packing of the unit cell
(Fig. 2) shows that the trimers
are not associated further into (
)6 hexamers, as was
previously shown in the 1KTP
[PDB]
structure, as well as for many other PC
structures
(89,
1125).
The immediate result of this form of crystal packing is a high solvent content
(65%) as opposed to only 42% for the 1KTP
[PDB]
structure. One consequence of the
high solvent content is the relatively high B-factors, especially
those of certain stretches of the
subunit. In crystal structures made
up of (
)6 hexamers (i.e. 1KTP
[PDB]
), the
subunit forms most of the trimer-trimer contacts, whereas the
subunits
are involved in the formation of both the (
) monomer-monomer and
in (
)6 hexamer-(
)6 hexamer
interactions. In the PC612 structure, the
subunits have few
intermolecular contacts (Fig.
3), whereas the
subunits link one another and thus are
stabilized (Fig. 3). Each
trimer is linked to an adjacent trimer by a very limited contact region formed
by residues near the
155 co-factor
(Fig. 2). In the 1KTP
[PDB]
structure, the mean B-factor is 19.9 Å2 for all
protein atoms. In the PC612 structure, the overall
B-factor is considerably higher, 50.7 Å2. The major
source of this higher value is due to residues of the
subunit that has
an overall B-factor of 59.8 Å2, whereas the mean
value of the
subunit is 38.8Å2. These values indicate
a high degree of disorder in the
subunit, and the calculated electron
density is rather weak for side chains of residues located on
helices B and E (residues Ala50-Ala84) and
helix G (residues 128140). These residues are intimately involved in
the binding of the
84 phycocyanobilin co-factor, and indeed the
B-factors of this co-factor are significantly higher (>30
Å2) than those of the other two co-factors.
The trimeric PC612 structure has two very different molecular
faces (Fig. 3), as opposed to
the 1KTP
[PDB]
and other hexamerforming PC types. In the present structure one face
is made up of
subunits enclosing a narrow triangular opening
(Fig. 3a, blue
surface), whereas the opposite face
(Fig. 3b, orange
surface) is made up of
subunits, which surround a rather large
open volume. The
84 co-factors (Fig.
3, red surface) are more accessible from this side than
from the
subunit side.
Only 36 solvent molecules could be identified in the PC612
structure, as opposed to 377 molecules in the 1KTP
[PDB]
structure and 88 in the
1I7Y
[PDB]
structure. Although this can certainly be a result of the lower
resolution, we also attribute it to the elevated B-factors of the
side chains.
Asparagine 72 of the
Subunit Is Unmethylated in
PC612
Another novelty of the PC612 structure is found in aspargine 72
of the
subunit. This residue has been found to undergo a
post-translational modification by methylation of its
-nitrogen atom.
-N-methylasparagine has been found in many PB-containing
species of cyanobacteria and red-algae, both by biochemical and structural
analysis
(3335).
In the previously determined high resolution Tv-PC structure
(25) there was obvious
electron density identifying the methyl group as seen in
Fig. 4, a and
b. Similar density could be identified in the lower
resolution 1I7Y
[PDB]
Tv-PC structure (Ref.
23 and data not shown). When
electron density maps surrounding the Asn
72 residue in the
PC612 structure were examined, we could not identify any additional
density that could be associated with a methyl group. Simulated annealing omit
maps (both
2FoFc and
FoFc) were
calculated using CNS (31), and
although electron density surrounding the residue position itself is very
clear, no additional density signifying the presence of the methyl group could
be identified (Fig.
4c). The B-factors of the Asn
72
residue and its surroundings are quite low, so it does not appear that the
lack of electron density is due to flexibility in the site. We thus propose
that the unmethylated PC612 is a novel form of Tv-PC. This
result concurs with absorption spectra measurements of unmethylated or
undermethylated PC species that showed a blue-shift in their absorption
spectra (35).
 |
DISCUSSION
|
---|
The PB is an antenna complex finely tuned to transfer energy to the
reaction center of Photosystem II with an efficiency of over 95%
(36). It performs this
function with a high degree of directionality, even though there are large
numbers of similar pigment molecules within each complex
(3). A number of possible
energy transfer pathways have been described in great detail on the basis of
crystal structures and Förster energy transfer theory
(10,
12,
20). The initiation point of
energy transfer takes into account that each of the different phycobilin
pigments has modified absorption/emission properties as a result of the unique
chemical background formed by the protein matrix. In PC, the three
phycocyanobilin co-factors have slightly different absorption maxima, and in
combination with their relative geometric position it has been proposed that
the
155 co-factor is a sensitizing type pigment (absorbing to the blue),
the
84 co-factor is a fluorescing type pigment (absorbing further to the
red), and the
84 co-factor is an intermediate pigment type
(4). It has previously been
proposed that methylation of Asn
72 is required to isolate the
84 co-factor from solvent interactions, thereby altering its absorption
and emission properties (34,
35). Indeed, in a
cyanobacterial strain deficient in the phycobiliprotein methylation
(
) monomers had a slightly blue-shifted absorption spectra and
showed less PC-APC energy transfer in vivo
(35). In studies of a variety
of species, different relative amounts of methylated versus
unmethylated phycobiliproteins were identified, indicating that at least some
fraction of the proteins may be unmethylated
(35). One could envision
changes in the primary sequence of the
subunit that could bring about
the same result achieved by methylation. Because the machinery required for
post-translation methylation is ubiquitous in phycobiliproteins, it would
appear that there is an evolutionary advantage to the presence or lack of the
methyl group. The state of methylation could represent a site for the
fine-tuning of the properties of the
84 co-factor with the existence of
non-methylated protein serving a special role.
The PC612 identified in this study was isolated as a fraction of
PC that co-purified with the APC core proteins. The copurification might be an
indication of the proximity between the two protein types. If so, what could
the possible role of the PC612 be?
To form a complete PB, a number of architectural problems must be overcome.
If the entire PB is made up of disks (hexamers of phycoerythrin, PC, or APC),
these disks must fit one onto another in some ordered fashion, as has been
seen in electron micrographic studies
(4,
5) and at least appears to be
simulated in most PB protein crystal structures
(20,
21,
24,
25). Interpretation of these
experimental observations suggests that two rods on either side of the APC
core lie in a parallel fashion, with a third rod doublet perpendicular to the
other rods (3) or at an angle
(24). The first problem with
this arrangement lies in matching the sizes of the PC rods and APC core
(Fig. 5). The first layer is
composed of two APC hexamers (Fig.
5, dashed disks), whereas the second layer contains a
single APC hexamer, which fits into the "saddle" in the first
layer interface. The total height of the core is thus less than twice the
diameter of a single disk. If each disk is about 110 Å in diameter, then
the total core height (Fig. 5)
is only about 190 Å. When the first layers of rods
(Fig. 5, R1, horizontal lines) attach to the core
(parallel to the membrane surface) the circumference of the APC disk can
attach to the triangular aperture of the terminal PC hexamer and bring the PC
84 co-factors into the direction of the APC
84 co-factors.
However, the second layer of PC hexamers
(Fig. 5,
R2) cannot now fit in a manner similar to that of
the first layer because the total height of the two layers is about 220
Å, and the circumference of the top APC hexamer will not be centered in
the PC hexamer orifice. Because both R2 rods must contact the
single APC hexamer, they are indented in relation to R1 by the
radius of the APC disk, which is about 55 Å or less than a complete
hexamer (about 60 Å). The last PC rod types
(Fig. 5,
R3) are layered perpendicular to the
R1 and R2 rods
(3), and the contact with the
top APC hexamer must be different, because both orifices cannot simultaneously
meet the disk circumference (if they lie in a parallel fashion). A recent
suggestion by Jiang et al.
(16) shows the R3
rods as separate, and thus four rods contact a single APC hexamer. However,
this would necessitate interpenetration of the rods, because one cannot fit
four 110 Å rigid disks around the available circumference of a single
APC hexamer (whose total available circumference is less than 350 Å).
Examination of the circumference of the PC hexamer does not show any cavities
that might allow the interpenetration of the rods, and thus a different
solution is needed. One possibility is that the four rods of type
R2 and R3 are positioned further away from the APS core,
thus allowing the four rods to be arranged around a single hexamer. This large
separation might be performed by as yet unidentified linker proteins
(1,
5,
26,
28,
37,
38). This arrangement would,
however, separate the terminal PC cofactors from the core co-factors and
significantly lower the energy transfer efficiency. Another possibility is
that a single, flexible trimer may be situated in between the rods and core.
PC612 may serve in this interstitial role
(Fig. 5, concave
disks). Each P612 trimer has two different faces (unlike PC
hexamers in which both outer faces are identical). One face has mostly
subunit residues facing out (Fig.
3a), which could associate with the terminal rod hexamer
in a
subunit-
subunit interaction (similar to the interactions
used in rod formation). The second face contains mostly
subunit
residues, which enclose a larger empty void, with the
84 co-factors
pointing out toward the APC core (Fig.
3b). This larger void could accommodate the circumference
of APC core hexamers in different orientations. The flexibility of the
subunits may also enable the PC612 to bind the APC core in a tight
manner, with a smaller diameter, thus bringing the APC co-factors into close
contact with the PC612
84 co-factors. In addition, the
circumference of the trimer has indentations leading to two different
diameters, one of which is only 90 Å
(Fig. 2). These indentations
could serve as the contact points between R2- and
R3-type rods, allowing the arrangement of all four rods around a
single APC disk.

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|
FIG. 5. Model and molecular dimensions of the phycobilisomes structure as
described under "Discussion." The tricylindrical APC core
(dashed lines) is surrounded by three pairs of PC rods
(R1, horizontal lines; R2,
vertical lines; and R3, diagonal lines). The
PC612 connector subunit is depicted as a concave disk
between the rods and core. The model shows the different contacts between PC
rods of different types that could be connected by a flexible PC612
subunit. Linker protein are not shown but may be needed for PB assembly.
|
|
A question that remains open is the lack of Asn
72
methylation in the PC612. Because the absorption spectrum is
blue-shifted, it would be less optimal for directed energy transfer from the
PC620 to APC (35).
However, if the tight binding of the PC612 to the APC brings the
APC external
84 co-factors in close contact with the PC612
internal
84 co-factors, solvent could be ejected and the chemical
environment of the nonmethylated residue may be made hydrophobic, inducing a
significant change in absorption further to the red than in PC620.
The presence of linker proteins could have an additional modifying effect on
absorption and energy transfer from the
84 co-factor to APC. Thus the
PC612 may serve as both a structural and functional linker between
the PC rods and APC cores. Structural studies on larger complexes containing
both APC and PC components are under way to visualize the molecular details of
PB assembly.
 |
FOOTNOTES
|
---|
The atomic coordinates and structure factors (code 10N7) have been
deposited in the Protein Data Bank, Research Collaboratory for Structural
Bioinformatics, Rutgers University, New Brunswick, NJ
(http://www.rcsb.org/).
* This work was supported in part by the Israel Science Foundation founded by
the Israel Academy of Sciences and Humanities (438/02) and the Technion Fund
for the Promotion of Research. 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.: 972-4-8292141; Fax:
972-4-8233735; E-mail:
nadir{at}tx.technion.ac.il.
1 The abbreviations used are: PB, phycobilisomes; APC, allophycocyanin;
bis-Tris, 2-[bis(2-hydroxyethyl)amino]-2-(hydroxymethyl)propane-1,3-diol; PC,
C-phycocyanin; PC612, phycocyanin absorbing at 612 nm;
PC620, phycocyanin absorbing at 620 nm; Tv-PC, phycocyanin
from T. vulcanus; HPLC, high pressure liquid chromatography; CNS,
crystallography NMR software. 
 |
ACKNOWLEDGMENTS
|
---|
We thank the staff at the EMBL-Hamburg for their help in data collection.
We thank Itzhak Ohad for his helpful comments.
 |
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