Journal of Experimental Botany, Vol. 56, No. 411,
Light Stress in Plants: Mechanisms and Interactions Special Issue, pp. 347–356, January 2005
doi:10.1093/jxb/eri041
Advance Access publication 29 November, 2004
Dynamics of photosystem II: a proteomic approach
to thylakoid protein complexes
E-M. Aro*, M. Suorsa, A. Rokka, Y. Allahverdiyeva, V. Paakkarinen, A. Saleem, N. Battchikova
and E. Rintamäki
Department of Biology, University of Turku, FIN-20014 Turku, Finland
Received 7 April 2004; Accepted 27 September 2004
Abstract
Light-induced damage to PSII proteins
Oxygenic photosynthesis produces various radicals
and active oxygen species with harmful effects on
photosystem II (PSII). Such photodamage occurs at all
light intensities. Damaged PSII centres, however, do
not usually accumulate in the thylakoid membrane due
to a rapid and efficient repair mechanism. The excellent
design of PSII gives protection to most of the protein
components and the damage is most often targeted
only to the reaction centre D1 protein. Repair of PSII via
turnover of the damaged protein subunits is a complex
process involving (i) highly regulated reversible phosphorylation of several PSII core subunits, (ii) monomerization and migration of the PSII core from the
grana to the stroma lamellae, (iii) partial disassembly of
the PSII core monomer, (iv) highly specific proteolysis
of the damaged proteins, and finally (v) a multi-step
replacement of the damaged proteins with de novo
synthesized copies followed by (vi) the reassembly,
dimerization, and photoactivation of the PSII complexes. These processes will shortly be reviewed
paying particular attention to the damage, turnover,
and assembly of the PSII complex in grana and stroma
thylakoids during the photoinhibition–repair cycle of
PSII. Moreover, a two-dimensional Blue-native gel map
of thylakoid membrane protein complexes, and their
modification in the grana and stroma lamellae during
a high-light treatment, is presented.
Photosynthetic water splitting involves highly oxidative
chemistry, which is accomplished by a unique PSII complex. PSII converts light energy into chemical form using
water as a source of electrons and liberating molecular
oxygen as a side product. Light is a harmful substrate
for the water-splitting PSII complex, exposing its protein
subunits to structural damage. A number of protective
systems have evolved in the thylakoid membrane and
surrounding stroma to dissipate excess light energy and to
scavenge various radicals and active oxygen species
(Muller et al., 2001; Ivanov and Khorobrykh, 2003), but
there is always inherent inactivation and damage occurring
in PSII with a very low quantum yield (Tyystjärvi and Aro,
1996; Anderson et al., 1997). Although oxidative damage
occurs in PSII even at low light intensities, irreversible
damage become evident in vivo in intact leaves only at high
light intensities when the repair of PSII cannot keep pace
with the constant oxidative damage (Aro et al., 1993).
It has been known for a long time that the PSII reaction
centre D1 protein, which exhibits the highest turnover rate
of all the thylakoid proteins (Mattoo et al., 1984), is the
major target of oxidative damage in PSII (for reviews see
Prasil et al., 1992; Aro et al., 1993). There is an extensive
literature published on the mechanisms of PSII photoinactivation and damage to the D1 protein both under in vivo
and in vitro conditions (for a review see Melis, 1999), and
therefore these mechanisms will not be considered in detail
here. However, it seems that the D1 protein is not the
only target of photodamage in PSII. Several studies have
suggested that the D2 protein also occasionally becomes
damaged in light (Schuster et al., 1988; Jansen et al., 1996).
More recently, one of the small PSII subunits, the PsbH
protein, was also shown to be frequently replaced in PSII
(Bergantino et al., 2003; E-M Aro et al., unpublished
Key words: Arabidopsis thylakoid membrane proteome, assembly of photosystem II, D1 protein, light stress, photosystem
II photoinhibition, repair of photosystem II.
* To whom correspondence should be addressed. Fax: +358 2 3335549. E-mail: evaaro@utu.fi
Journal of Experimental Botany, Vol. 56, No. 411, ª Society for Experimental Biology 2004; all rights reserved
348 Aro et al.
results). It is thus conceivable that the PsbH and D2
proteins, located in close proximity to the D1 protein
(Zouni et al., 2001), occasionally become targeted for damage and degradation during the photoinhibition-repair cycle
of PSII.
Heterogeneity of the PSII complexes: role in PSII
photoinhibition-repair cycle
The different phases of the PSII photoinhibition-repair
cycle of illuminated leaves have been addressed by first
separating the membrane protein complexes of isolated
thylakoids with Blue-native gel electrophoresis (BNPAGE), followed by the separation of distinct protein
subunits with denaturating SDS–PAGE. After silver staining, the protein subunits were identified with mass spectrometry (MALDI-TOF). A distinct feature of the thylakoid
protein complexes is the diversity of the PSII complexes, as
depicted for Arabidopsis thaliana (L.) Heynh. in Fig. 1.
Table 1 gives an assignment for individual protein subunits
for the recognition of respective protein complexes. In
general, it was found that the BN-SDS-PAGE gel pattern of
thylakoid proteins was very similar between different dicot
species (Arabidopsis, pumpkin, spinach, tobacco) (Fig. 1;
Suorsa et al., 2004; Heinemeyer et al., 2004; E-M Aro
et al., unpublished results). The smallest PSII subassembly
visible in the silver-stained gel is the CP43-less PSII
monomer which is composed of the D1/D2/CP47 proteins,
the cytochrome (Cyt) b559 subunits (PsbE and PsbF), and
the PsbI protein (of these small PSII subunits only PsbE
is faintly seen in the silver-stained gel). The intact PSII
monomer is the second biggest PSII complex and also has
the CP43 internal antenna protein assembled (Fig. 1). The
PSII dimer can be distinguished as a larger complex of c.
600 kDa, migrating in the BN-gel in close proximity to the
PSI complex. PSII supercomplexes of increasing size then
show the association of the LHCII antenna in varying
combinations of the Lhcb1–6 proteins (Heinemeyer et al.,
2004). ATP synthase, the cytochrome (Cyt) b6 f dimer
complex, and various free Lhcb1-6 protein assemblies can
also be distinguished in Fig. 1. It is worth noting that
the gentle solubilization of thylakoid membranes with
n-dodecyl-b-D-maltoside, used for thylakoids in Fig. 1,
preferentially released the complexes from stroma thylakoids and those from grana appressions remained to larger
extent unsolubilized.
For a more detailed understanding of the distribution of
the various forms of PSII complexes in the thylakoid
membrane, the thylakoids were fractionated into the grana
and stroma membranes (Baena-Gonzalez et al., 1999; Kyle
et al., 1984) and the protein complexes were analysed from
both fractions, with results depicted in Fig. 2. Grana
membranes were strongly enriched in PSII dimers and
PSII-LHCII supercomplexes, whereas only a small amount
of PSII core monomers were present and the CP43-less PSII
monomers were completely missing from the grana
appressions. In addition to the PSII complexes, the monomeric and dimeric Cyt b6 f complex, various free subassemblies of LHCII (Lhcb1–6 proteins) complexes, and traces of
PSI complexes were present in the grana fraction. In stroma
thylakoids, on the other hand, the PSII complexes as well as
the various free LHCII subassemblies were present only as
a minority, while the PSI complexes and ATP synthase
were the dominating membrane protein complexes. The
Cyt b6 f complex was also present as a dimer. The PSII
complex in stroma thylakoids was present mainly as a core
monomer, and, in addition, traces of the CP43-less monomer were present in stroma thylakoids. PSII complexes in
grana and stroma thylakoids also show functional heterogeneity and have been assigned a distinct role in the photoinhibition-repair cycle of the complexes (Guenther and
Melis, 1990; Melis, 1999).
Damage occurs to PSII dimers and supercomplexes
in the grana
Functional forms of PSII are located in the grana membranes
where, most probably, oxidative damage also occurs, and so
the PSII core dimers and supercomplexes are predominantly
involved. Two-hour high-light illumination of pumpkin
leaves (1500 lmol photons mÿ2 sÿ1), despite inducing
approximately 30% inhibition of the photochemical efficiency of PSII (Fv/Fm), did not noticeably affect the
distribution of the protein complexes in the grana and stroma
membrane regions (thylakoid fractions only from high-light
illuminated leaves are shown in Fig. 2). When the high-light
illumination of leaves was performed in the presence of the
chloroplast protein translation inhibitor lincomycin to block
the repair process, no disassembly of the PSII complexes
was observed in grana appressions (Fig. 2) despite nearly
60% photoinhibition and photodamage of the PSII centres
(data not shown). Under severe photoinhibition, the integrity
of photodamaged PSII complexes in the grana regions is
apparently guaranteed by phosphorylation of the D1, D2,
and CP43 proteins (Baena-Gonzalez et al., 1999; see last
section). This phosphorylation is likely to prevent a complete
disassembly and a collapse of the photodamaged PSII
complex in the grana region before migration to the repair
site in non-appressed membranes.
Monomerization of PSII complexes is a prerequisite
for repair in stroma thylakoids
In plant chloroplasts, the damaged PSII complex must exit
the grana membrane, as the PSII complexes undergoing
repair have definitively been localized to the stroma thylakoids where the ribosomes have access. Migration of the
damaged PSII to stroma-exposed thylakoids can be regarded
as the first step in the PSII repair cycle (Guenther and Melis,
1990; Melis, 1999). The mechanism and the exact site
for PSII monomerization in the thylakoid membrane, upon
Protein turnover in photosystem II 349
Fig. 1. Chloroplast protein complexes and their subunit composition in Arabidopsis thaliana. Chloroplasts were isolated, solubilized with n-dodecyl-bD-maltoside and subjected to Blue-Native (BN) gel separation of the protein complexes (for experimental details see Suorsa et al., 2004). After the run,
a lane was cut out, solubilized (Laemmli, 1970), laid on the top of SDS-PAGE (15% acrylamide, 6 M urea) and the proteins of each macromolecular
complex were separated. The gel was subsequently stained with silver and the protein subunits were identified by MALDI-TOF mass spectrometry
(Table 1). Identification of the macromolecular protein complexes of thylakoid membranes is given on the top of the gel. For a detailed description of the
BN/SDS-PAGE and MALDI-TOF mass spectrometry see Herranen et al. (2004).
photodamage in the grana or during an early repair phase in
the stroma thylakoid, have been difficult to assess. The
reversible mononer–dimer organization of PSII in the
photoinhibition repair cycle has been more straightforward
to assess in studies of cyanobacterial thylakoid membranes
without any lateral heterogeneity. Recently, it was demonstrated that high-light illumination and damage to PSII
severely reduced the content of PSII dimers and, conversely,
increased the monomer forms of PSII in Synechocystis
thylakoid membranes (Sakurai et al., 2003). Upon recovery
from photoinhibition, the PSII dimers again became the
dominating form in the thylakoid membrane. It is of interest
to note here that the phosphatidylglycerol class of membrane
lipids was shown to be particularly important for the dimer
organization of PSII (Sakurai et al., 2003).
In higher plant chloroplasts, the blocking of the repair
process of PSII by lincomycin, an inhibitor of protein
synthesis in chloroplasts, resulted in a complete disappearance of the PSII core monomer complexes from the stroma
membrane region (illumination of leaves in the presence of
lincomycin, Fig. 2). Thus, in the absence of de novo synthesis and replacement of the damaged proteins in PSII, the
entire PSII core monomer complex collapses upon degradation of the damaged D1 protein. This was accompanied
by the accumulation of free CP47 and, to a minor extent, of
free CP43 in the stroma-exposed thylakoid membranes
(Fig. 2), whereas the other PSII protein subunits were not
found free and were apparently subject to a more efficient
proteolytic degradation than CP47.
Both the degradation of the damaged PSII protein subunits and their replacement by de novo synthesized ones take
place on stroma thylakoids. This process requires at least
a partial disassembly of the PSII complex. Although most
often only the D1 protein is the target for photodamage, the
D2 protein and the PsbH protein are also occasionally
subject to irreversible damage. Considering the 3D structure
of PSII (Ferreira et al., 2004; Zouni et al., 2001) and the
experimental evidence collected so far, it is reasonable to
conclude that at least the CP43 antenna protein, together
with some as yet unidentified low molecular weight (LMW)
subunits, are released from the PSII core monomer complex
in the stroma-exposed thylakoid regions to allow the
degradation of the damaged D1 protein. The CP43-less
PSII monomers present in stroma thylakoid regions (Fig. 2)
are probably an indication of this repair phase. If the PsbH
protein and/or the D2 protein are likewise damaged and need
to be replaced, the disassembly of the PSII core monomer is
likely to be much more extensive.
350 Aro et al.
Table 1. Identification of thylakoid proteins of Arabidopsis thaliana after 2D BN/SDS-PAGE
MALDI-TOF mass spectrometry analysis was performed from protein spots indicated in Fig. 1.
Protein
Gene
ORF
MW (kDa)a
Pep
% Cover
Score
CP47
CP43
D2
D1
Cytochrome b559 a subunit
LHCII
56.2
52.1
39.8
39.0
9.3
28.2
26.6
28.7
28.7
31.2
31.2
30.2
27.5
83.4
82.5
9.5
22.4
12.2
24.3
17.1
15.2
25
27
27
26
37
48
228
179
140
123
69
108
4
6
8
8
9
8
8
7
4
5
6
8
3
6
12
35
30
27
43
24
15
8
51
35
46
37
15
20
50
102
126
132
176
93
109
92
77
93
128
122
56
88
PSI-K
PSI-L
LHCI
LHCI
Cytochrome f
Cytochrome b6
Rieske protein
Subunit IV
ATP synthase, a subunit
ATP synthase, b subunit
ATP synthase, c subunit
ATP synthase, e subunit
NADH dehydrogenase
NADH dehydrogenase
Rubisco, large subunit
Rubisco, small subunit
Fructose-bisphosphate aldolase
psaK
psaL
lhca2
lhca3
petA
petB
petC
petD
atpA
atpB
atpC
atpE
ndhH
ndhK
rbcL
rbcS
–
ArthCp049
ArthCp018
ArthCp017
ArthCp002
ArthCp039
At2g34430
At2g34420
At2g05070
At5g54270
At5g01530
At3g08940
At4g10340
At1g15820
ArthCp022
ArthCp021
ArthCp075
At1g03130
At4g28750
At1g31330
At1g55670
At3g16140
At1g52230
At1g30380
At4g12800
At3g61470
At1g61520
ArthCp035
ArthCp053
At4g03280
ArthCp054
ArthCp007
ArthCp029
At4g04640
ArthCp028
ArthCp080
ArthCp026
ArthCp030
At1g67090
At4g38970
13
10
7
8
5
7
LHCII
LHCII
CP29
CP29
CP26
CP24
PSI-A
PSI-B
PSI-C
PSI-D
PSI-E
PSI-F
PSI-G
PSI-H
psbB
psbC
psbD
psbA
psbE
lhcb1.1
lhcb1.2
lhcb2
lhcb3
lhcb4.1
lhcb4.2
lhcb5
lhcb6
psaA
psaB
psaC
psaD
psaE
psaF
psaG
psaH
13.3
23.1
27.9
29.1
35.4
24.3
22.8
17.5
55.4
54.0
41.2
14.5
45.6
25.6
47.9
20.5
43.0
2
8
5
10
9
4
5
2
20
12
10
3
14
5
14
5
9
7
27
36
26
42
25
34
10
40
37
38
43
30
15
30
28
30
58
80
111
75
187
87
100
80
391
241
204
84
185
78
266
117
158
a
Theoretical MW of the preprotein.
Proteolytic degradation of damaged PSII subunits
There has been much experimental effort to identify the
proteases involved in D1 protein degradation. Proteases of
both the DegP family and the FtsH family have been
assigned a role in D1 protein degradation and both these
proteases have been localized to the stromal region of the
thylakoid membrane (Andersson and Aro, 1997; Lindahl
et al., 1996, 2000; Adam and Clarke, 2002; Kanervo et al.,
2003; Silva et al., 2003). It has been assumed that the
DegP2 protease is responsible for the primary cleavage of
the D1 protein in the stromal loop connecting the transmembrane (TM) helices D and E and thus producing the
23 kDa N-terminal and the 10 kDa C-terminal fragments,
whereas the FtsH protease would have a secondary role
in degrading the primary cleavage fragments of the D1
protein (Haußühl et al., 2001). More recent studies,
however, assign a role for FtsH as a protease capable of
complete degradation of the damaged D1 protein (Bailey
et al., 2002; Silva et al., 2003). It is reasonable to assume
that the proteases of these two families are also involved in
the degradation of the D2 and PsbH proteins upon their
damage during PSII photoinhibition and repair.
Free PSII core proteins, with the exception of the CP43
protein, are very rarely present in the thylakoid membranes
of mature leaves (Zhang et al., 2000; Suorsa et al., 2004)
implying that the degradation of the damaged PSII subunits
must be tightly co-ordinated with the synthesis of new
subunits and their incorporation into the PSII complex.
Only when the synthesis of new PSII subunits is blocked,
for example, by a protein synthesis inhibitor, do the PSII
centres in the stroma thylakoids collapse and those PSII
proteins not being photodamaged become targeted for
proteolytic degradation (Fig. 2).
De novo synthesis, co-translational assembly
and C-terminal processing of the D1 protein
Constant damage and replacement of the D1 protein in
PSII core monomers has made it possible to study the
Protein turnover in photosystem II 351
Fig. 2. Sections of silver-stained BN/SDS–PAGE gels demonstrating the presence of PSII core monomers (M), dimers (D), and supercomplexes (S)
in the grana and stroma fractions of pumpkin (Cucurbita pepo L.) thylakoid membranes. Pumpkin leaves were preilluminated at 1500 lmol photons
mÿ2sÿ1 for 2 h in the absence and presence of the protein synthesis inhibitor lincomycin before isolation and subfractionation of the thylakoid
membranes. Thylakoid isolation and subfractionation into grana and stroma lamellae with the digitonin method were performed as described earlier
(Baena-Gonzalez et al., 1999). The chlorophyll a/b ratios were 2.4 and 8.3 for the grana and stroma membranes in the absence of lincomycin (upper
panels), and 2.2 and 7.6, respectively, in the presence of lincomycin (lower panels). Stroma and grana membranes (65 lg of protein) were then subjected
to BN/SDS-PAGE and stained with silver. PSII monomer (M), dimer (D), and supercomplexes (S) are circled in the upper panel of grana membranes and
the CP43-less monomer (M-CP43) in the upper panel of stroma membranes. Cyt f of the Cyt b6f dimer and monomer complexes is indicated in the lower
left panel of grana membranes and the PSI complex and ATP synthase in the lower panel of stroma membranes. PsaLMW, PSI low-molecular-weight
subunits. Proteins were identified with MALDI-TOF as in Fig. 1. When leaves were illuminated in the presence of lincomycin, the PSII monomers and
CP43-less PSII monomers (M-CP43) disappeared from stroma thylakoids with concomitant accumulation of free CP47 and CP43 proteins, whereas no
disassembly of PSII complexes occurred in grana membranes in the absence of the repair process.
mechanisms of D1 protein replacement in isolated intact
chloroplasts (van Wijk et al., 1997; Zhang et al., 1999, 2000)
as well as in intact leaves (A Rokka et al., unpublished
results). Combination of short 35S-Met pulse-labelling and
chase experiments in intact isolated chloroplasts or in intact
leaves with the subfractionation of PSII subassemblies by
sucrose density centrifugation or 2D BN/SDS–PAGE, has
revealed several individual assembly steps of PSII. The D1
protein was shown to be co-translationally inserted into the
existing PSII complex composed at least of the D2 protein
and the a and b subunits of Cyt b559 (Zhang et al., 1999).
Evidence has been provided that the ribosome nascent D1
chain complex first interacts with the chloroplast signal
recognition particle cpSRP54 (Nilsson et al., 1999), which
targets the nascent D1 chain to the thylakoid membrane.
After being targeted to the thylakoid membrane, the D1
protein elongation and membrane insertion occur concomitantly. The insertion of the elongating D1 protein into the
thylakoid membrane is likely to involve a cpSecY translocon channel, as shown by cross-linking and immunoprecipitation experiments (Zhang et al., 2001). During the D1
elongation and translocation process, the TM segments
of elongating nascent D1 protein exit laterally from
the translocon and start interacting with other PSII
core proteins. The slow rate of translation elongation and
pausing of ribosomes (Kim et al., 1991) are likely to allow
352 Aro et al.
the escape of pairs of D1 TM helices from the cpSecY
translocon channel, thereby making the interaction of D1
with D2 possible during translation elongation (Zhang
et al., 2000; Zhang and Aro, 2002). After two TM segments
of the D1 protein have been translated, the translocon
channel apparently opens laterally and allows an interaction
of the elongating D1 with the receptor complex composed
of D2, Cyt b559, PsbI, and possibly also of CP47, whereas
a strong interaction between D1 and D2 is achieved only
after four TMs of the D1 protein have been translated
(Zhang et al., 2000).
Although the initial assembly of D1 into PSII is a
co-translational process, the formation of functional PSII
complexes also depends on several post-translational assembly steps, including C-terminal processing of the precursor D1 protein and reassociation of the released protein
subunits (see below). The D1 protein is synthesized as
a precursor with a C-terminal extension of 9–16 amino acid
residues (Diner et al., 1988). After termination of translation, the D1 protein is rapidly processed C-terminally by
a lumenal protease CtpA (Bowyer et al., 1992; Anbudurai
et al., 1994). Processing of the D1 protein precedes the
assembly of the CP43 protein (Zhang et al., 2000) and
only thereafter is the ligation of the oxygen evolving complex to the lumenal side of PSII possible (Bowyer et al.,
1992).
The fate and reassembly of other PSII subunits during
the photoinhibition repair cycle
Most of the PSII complexes under the repair process seem
not to release the CP47 antenna pigment protein. However,
the extent of PSII disassembly required for repair depends
on the severity of the damage to the PSII core complex. If
only the D1 protein must be replaced, the least amount
of disassembly is required. However, recently, the small
9 kDa PsbH protein has also been suggested to have a role in
the photoinhibition-repair cycle of PSII (Bergantino et al.,
2003). It has also been noticed that this protein is frequently
replaced with a de novo synthesized PsbH protein copy in
the PSII core monomers during the light exposure of leaves
(A Rokka et al., unpublished results). These experiments
suggest a crucial role for the PsbH protein in the turnover of
the D1 protein and are in line with observations made with
cyanobacterial cells implying a role for PsbH, not only as
a stabilizing subunit for formation of the QB pocket in the
PSII complex but also in the initiation and completion of
the PSII repair cycle (Bergantino et al., 2003).
If the D2 protein has likewise been degraded and requires
a replacement in the PSII core complex, a much more extensive disassembly of the PSII complex can be expected.
In such a case most of the LMW subunits of PSII are
apparently also released from the core monomer. The first
assembly step of PSII, identified in experiments following
the biogenesis of PSII centres, is the D2/Cyt b559/PsbI
complex. It is possible, however, that upon D2 degradation,
the Cyt b559/PsbI/CP47 complex remains attached and
the de novo synthesized D2 protein first associates with
this subassembly of PSII. It has remained unclear whether
the synthesis and assembly of the D2 protein occurs cotranslationally as has been shown for the insertion of the
elongating D1 protein into PSII (Zhang et al., 1999). However, neither the D2 nor the D1 protein has generally been
found free in the thylakoid membrane supporting the cotranslational and co-ordinated synthesis and assembly for
both the D1 and D2 reaction centre proteins (Wollman
et al., 1999; Zerges, 2000).
Recent assembly studies of the PSII centres, using
both pulse-chase experiments (A Rokka et al., unpublished
results) and various psb gene deletion mutants (Suorsa
et al., 2004), have provided evidence that the LMW protein
subunits, PsbH, PsbL, PsbM, PsbTc, PsbR, and probably
also PsbJ, associate with the PSII subassembly composed
of D1/D2/Cyt b559/PsbI/CP47. Apparently these six LMW
proteins are essential for the stabilization of the D1/D2/Cyt
b559/PsbI/CP47 subassembly of PSII and also for providing
the structural integrity for the assembly of yet another set of
proteins to the PSII complex. In the subsequent assembly
step the CP43, PsbK, PsbZ, and PsbW proteins become
distinguished in the PSII core monomer complex. Of the
LMW proteins of PSII, the PsbL subunit is essential for the
stable association of CP43 (Suorsa et al., 2004), the PsbJ
subunit for water splitting and ligation of the oxygenevolving proteins, but also for regulation of the acceptor
side activity of the PSII complex (Hager et al., 2002, Regel
et al., 2001; Swiatek et al., 2003; Suorsa et al., 2004). PsbK
is apparently required for PSII core dimerization (Zheleva
et al., 1998) and the PsbZ and PsbW proteins are essential
for the association of the peripheral light-harvesting Chl a/
b antenna proteins (Swiatek et al., 2001; A Rokka et al.,
unpublished results). It is not yet known whether all these
protein subunits assemble into a PSII core monomer on the
stroma thylakoid membranes. On the other hand, it is clear
that stable PSII dimers with associated minor (CP29, CP26,
CP24) and major (Lhcb1,2,3 proteins) LHCII antenna
proteins are present only in the appressed grana membranes
(Fig. 2).
Photoactivation of PSII and association of the
oxygen-evolving complex (OEC) proteins
To regain the fully functional PSII complexes in the grana,
a proper ligation of the Mn cluster and other inorganic
cofactors, particularly the chloride and calcium ions, as
well as the association of the OEC proteins are required.
Ligation of the Mn cluster can already, in principle, take
place on the stroma thylakoids since the PSII complexes
including all proteins essential for ligation of the Mn cluster
(Ferreira et al., 2004), are present in stroma lamellae (Fig. 2).
However, the quantum efficiency for photoactivation of PSII
in limiting light was found to increase in direct proportion to
Protein turnover in photosystem II 353
the increase in the antenna size of the PSII complex
(Ananyev et al., 2001). This suggests that association of
the LHCII proteins, that is most likely limited to the grana
appressions, also plays a role in the maintenance of the stable
activation of PSII water splitting. This phenomenon is likely
to be related to the in vivo stabilization of the Mn cluster by
binding of the Ca2+ and Clÿ ions and the protein components
of OEC comprising the 33 kDa PsbO protein, the 23 kDa
PsbP protein, and the 17 kDa PsbQ protein. When studying
the association of the OEC proteins with the PSII complex, it
was found that the smallest PSII subassembly harbouring the
PsbO protein was the intact PSII core monomer (Suorsa
et al., 2004). In this respect it is interesting that the CP43
protein was also found to be essential for association of
PsbO, although in structural studies the lumenal loops of
CP47 and the D2 proteins have been emphasized in docking
of the PsbO protein (Nield et al., 2000). Because CP43
associates with the PSII subcomplex under repair in the
stroma exposed membranes (Fig. 2) it is also possible that
the association of PsbO and the photoactivation of PSII are
occurring on stroma thylakoids, before the core monomers
migrate back to the grana. On the other hand, there is
experimental evidence indicating that the final stabilization
of oxygen evolution by ligation of the two other OEC
proteins, PsbP and PsbQ, to the PSII complex occurs mainly
in the grana membrane, possibly after attachment of the
LHCII complex and dimerization of the PSII core monomer
(Hashimoto et al., 1997).
Role of PSII protein phosphorylation in the
photoinhibition-repair cycle
Exposure of leaves to light induces the phosphorylation of
a number of PSII proteins in the thylakoid membrane
(Bennett, 1991), resulting from redox-dependent activation
of several protein kinases. The N-terminal threonine (Thr)
residues are phosphorylated in the D1 and D2 reaction
centre proteins and the CP43 protein of the PSII core
(Bennett, 1977, 1991), while two Thr-phosphorylation sites
were recently reported for the PsbH protein (Vener et al.,
2001). Furthermore, three out of the six pigment-binding
proteins of the PSII light-harvesting Chl a/b antenna
(Lhcb1, Lhcb2, and Lhcb4) become reversibly phosphorylated upon light–dark transitions (Bennett, 1991). The
phosphorylation sites of the Lhcb1 and Lhcb2 proteins
reside in the N-terminal Thr-residues (Michel et al., 1991;
Vener et al., 2001). The Thr-residue number 83 was shown
to become phosphorylated in mature maize Lhcb4 protein
(Testi et al., 1996), whereas recently the Thr-residue
number 6 was demonstrated as a phosphorylation site in
Arabidopsis Lhcb4.2 protein (Hansson and Vener, 2003).
Although the identification of the kinases involved is not
yet clear, several redox-regulated serine/threonine protein
kinases with at least partial substrate specificity for PSII
core and antenna phosphoproteins, have been demonstrated
in the thylakoid membrane, from which the thylakoid
associated kinases (TAK) (Snyders and Kohorn, 2001)
and the Stt7 kinase (Dèpege et al., 2003), being involved
either directly or indirectly in the phosphorylation of the
Lhcb1 and 2 proteins, are the best-characterized ones so far.
Both soluble and membrane-bound phosphatases capable
of dephosphorylating thylakoid phosphoproteins have also
been found in chloroplasts (Sun et al., 1989; Hammer et al.
1997; Vener et al., 1999), but all these enzymes remain
very poorly characterized.
Reduction of free plastoquinone in the thylakoid membrane initiates the activation of the kinases specific for phosphorylation of the PSII core and Lhcb phosphoproteins.
Thus the various cellular processes that impact on the
reduction state of the plastoquinone pool also exert control
over the phosphorylation of both the PSII core and Lhcb
phosphoproteins (Hou et al., 2002; Pursiheimo et al., 2001,
2003) and the steady-state phosphorylation level of proteins
depends on counteracting activities of both the kinases and
phosphatases.
Under unstressed conditions, the full phosphorylation
of PSII core proteins is generally attained at irradiances
corresponding to those experienced by the plant during
growth or at irradiances exceeding those (Rintamäki et al.,
1997). Thus, under the conditions of the highest rate of PSII
photodamage and repair, the core proteins of PSII
are generally phosphorylated, particularly those located in
appressed thylakoid membranes (Baena-Gonzalez et al.,
1999). A number of experimental data suggest that phosphorylation of the PSII core proteins regulates the PSII
photoinhibition-repair cycle by controlling the timing of the
proteolytic degradation of photodamaged D1 protein in
the thylakoid membrane. It is assumed that under severe
photoinhibition, when the ‘repair sites’ in stroma exposed
membranes are fully occupied (Kettunen et al., 1997), the
integrity of photodamaged PSII complexes in the grana
regions is guaranteed by phosphorylation of the PSII core
proteins, the D1, D2, and CP43 proteins. Preventing the
collapse of photodamaged PSII complexes by phosphorylation in the grana region, prior to their migration to a
repair site in non-appressed membranes, allows coordinated protein degradation, repair, and reassembly of
the PSII complexes in the stroma-exposed membranes.
Dephosphorylation of PSII subunits and partial disassembly of the PSII complex only occurs upon the damaged
PSII complex entering the stroma thylakoids. Phospho-D1
and phospho-D2 proteins are not proteolytically degraded
as efficiently as their non-phosphorylated counterparts
(Koivuniemi et al., 1995; Rintamäki et al., 1996). Dephosphorylation indeed is a prerequisite for proteolytic
degradation of damaged PSII subunits. Despite inducing
phosphorylation, light also has an additional regulatory
role, either for the migration of PSII complexes to stroma
thylakoid regions or for dephosphorylation of the damaged
PSII complex, as only in light do the damaged PSII
354 Aro et al.
complexes appear to undergo the repair process in the
stroma exposed thylakoids (Rintamäki et al., 1996).
Finally, it is necessary to emphasize that phosphorylation
of the Lhcb1 and Lhcb2 proteins, the major LHCII antenna
proteins of PSII, occurs under low light and thus does not
play any role in the PSII photoinhibition-repair cycle. In
fact, the kinase involved in Lhcb1 and Lhcb2 protein
phosphorylation becomes inhibited at moderate light intensities by thiol reductants that accumulate with increasing
illumination, initially in the chloroplast stroma and subsequently attaching to the thylakoid membrane reducing
the catalytically active disulphide bond in the kinase into
catalytically inactive dithiol group (Rintamäki et al., 2000;
Martinsuo et al., 2003). Although not connected to phosphorylation, it is evident that the LHCII antenna detaches
from photodamaged PSII in grana appressions, prior to
the monomerization of PSII core dimer and migration of the
PSII core monomer to the stroma thylakoids for repair. The
mechanism inducing this detachment of the LHCII antenna
from the PSII core, however, remains unclear.
Although the phosphorylation of the major LHCII antenna proteins does not seem to play any role in PSII
photodamage or repair, the phosphorylation of the minor
LHCII antenna polypeptide Lhcb4 (CP29) has been assigned a role in the protection of the PSII centres against
cold-induced photoinhibition in C4 plants (Bergantino et al.,
1995). More recently, distinct phosphorylation sites in CP29
were found from C3 plants as well, suggesting a more
widespread role of this phenomenon (Bergantino et al.,
1998). It is intriguing that of all the LHCII proteins only the
Lhcb4 protein phosphorylation requires strong reduction of
the plastoquinone pool occurring at high light intensities
(Pursiheimo et al., 2003) and is thus a relevant candidate
to possess a functional role in the dissipation of excess
excitation energy and in the protection of PSII against
photodamage (Bassi et al., 1997; Mauro et al., 1997).
The molecular mechanisms for such a dissipation of
excitation energy by phospho-CP29 still remain to be
investigated.
Acknowledgements
Research in our laboratory has been supported by the Academy of
Finland, The Finnish Ministry of Agriculture and Forestry, and the
Nordiskt Kontaktorgan för Jurdbruksforskning.
References
Adam Z, Clarke AK. 2002. Cutting edge of chloroplast proteolysis.
Trends in Plant Science 7, 451–456.
Ananyev GM, Zaltsman L, Vasko C, Dismukes GC. 2001. The
inorganic biochemistry of photosynthetic oxygen evolution/water
oxidation. Biochimica et Biophysica Acta 1503, 52–68.
Anbudurai PR, Mor TS, Ohad I, Shestakov SV, Pakrasi HB.
1994. The ctpA gene encodes the C-terminal processing protease
for the D1 protein of the photosystem II reaction center complex.
Proceedings of the National Academy of Sciences, USA 91, 8082–
8086.
Anderson JM, Park YI, Chow WS. 1997. Photoinactivation and
photoprotection of photosystem II in nature. Physiologia Plantarum 100, 314–323.
Andersson B, Aro E-M. 1997. Proteolytic activities and proteases of
plant chloroplasts. Physiologia Plantarum 100, 780–793.
Aro EM, Virgin I, Andersson B. 1993. Photoinhibition of photosystem II: inactivation, protein damage and turnover. Biochimica
et Biophysica Acta 1143, 113–134.
Baena-Gonzalez E, Barbato R, Aro E-M. 1999. Role of PSII
phosphorylation in the PSII repair cycle and oligomeric structure.
Planta 208, 196–204.
Bailey S, Thompson E, Nixon PJ, Horton P, Mullineaux CW,
Robinson C, Mann NH. 2002. A critical role for the Var2 FtsH
homologue of Arabidopsis thaliana in the photosystem II
repair cycle in vivo. Journal of Biological Chemistry 277, 2006–
2011.
Bassi R, Sandona D, Croce R. 1997. Novel aspects of chlorophyll
a/b-binding proteins. Physiologia Plantarum 100, 769–779.
Bennett J. 1977. Phosphorylation of chloroplast membrane polypeptides. Nature 269, 344–346.
Bennett J. 1991. Protein phosphorylation in green plant chloroplasts.
Annual Review of Plant Physiology and Plant Molecular Biology
42, 281–311.
Bergantino E, Brunetta A, Touloupakis E, Segalla A, Szabò I,
Giacometti GM. 2003. Role of the PSII-H subunit in photoprotection. Novel aspects of D1 turnover in Synechocystis 6803.
Journal of Biological Chemistry 278, 41820–41829.
Bergantino E, Dainese P, Cerovic Z, Sechi S, Bassi R. 1995. A
post-translational modification of the photosystem II subunit CP29
protects maize from cold stress. Journal of Biological Chemistry
270, 8474–8481.
Bergantino E, Sandona D, Cugini D, Bassi R. 1998. The
photosystem II subunit CP29 can be phosphorylated in both C3
and C4 plants as suggested by sequence analysis. Plant Molecular
Biology 36, 11–22.
Bowyer JR, Packer JCL, McCormack BA, Whitelegge JP,
Robinson C, Taylor M. 1992. Carboxyl-terminal processing
of the D1 protein and photoactivation of water splitting in
photosystem II. Journal of Biological Chemistry 267, 5424–5433.
Dèpege N, Bellafiore S, Rochaix JD. 2003. Role of chloroplast
protein kinase Stt7 in LHCII phosphorylation and state transition in
Chlamydomonas. Science 299, 1572–1575.
Diner BA, Ries DF, Cohen BN, Metz JG. 1988. COOH-terminal
processing of polypeptide D1 of the photosystem II reaction center
of Scenedesmus obliquus is necessary for the assembly of the
oxygen-evolving complex. Journal of Biological Chemistry 263,
8972–8980.
Ferreira K, Iverson TM, Maghlaoui K, Barber J, Iwata S. 2004.
Architecture of the photosynthetic oxygen-evolving center.
Science 303, 1831–1838.
Guenther J, Melis A. 1990. The physiological significance of
photosystem II heterogeneity in chloroplasts. Photosynthesis
Research 23, 105–109.
Hager M, Hermann M, Biehler K, Krieger-Liszkay A, Bock R.
2002. Lack of the small plastid-encoded PsbJ polypeptide results in
a defective water-splitting apparatus of photosystem II, reduced
photosystem I levels and hypersensitivity to light. Journal of
Biological Chemistry 277, 14031–14039.
Hammer MF, Markwell J, Sarath G. 1997. Purification of
a protein phosphatase from chloroplast stroma capable of dephosphorylating the light-harvesting complex- II. Plant Physiology
113, 227–233.
Protein turnover in photosystem II 355
Hansson M, Vener AV. 2003. Identification of three previously
unknown in vivo phosphorylation sites in thylakoid membranes
of Arabidopsis thaliana. Molecular and Cellular Proteomics 2,
550–559.
Hashimoto A, Ettinger WF, Yamamoto Y, Theg SM. 1997.
Assembly of newly incorporated oxygen-evolving complex subunits in isolated chloroplasts: sites of assembly and mechanism of
binding. The Plant Cell 9, 441–452.
Haußühl K, Andersson B, Adamska, I. 2001. A chloroplast DegP2
protease performs the primary cleavage of the photodamaged D1
protein in plant photosystem II. EMBO Journal 20, 1–10.
Heinemeyer J, Eubel H, Wehmhöner D, Jänsch L, Braun H-P.
2004. Proteomic approach to characterize the supramolecular
organization of photosystems in higher plants. Phytochemistry
65, 1683–1692.
Herranen M, Battchikova N, Zhang P, Graf A, Sirpiö S,
Paakkarinen V, Aro E-M. 2004. Towards functional proteomics
of membrane protein complexes in Synechocystis sp. PCC 6803.
Plant Physiology 134, 470–481.
Hou C-X, Pursiheimo S, Rintamäki E, Aro E-M. 2002. Environmental and metabolic control of LHCII protein phosphorylation:
revealing the mechanisms for dual regulation of the LHCII kinase.
Plant, Cell and Environment 25, 1515–1525.
Ivanov B, Khorobrykh S. 2003. Participation of photosynthetic
electron transport in production and scavenging of reactive oxygen
species. Antioxidant Redox Signaling 5, 43–53.
Jansen MAK, Greenberg BM, Edelman M, Mattoo AK, Gaba V.
1996. Accelerated degradation of the D2 protein of photosystem II
under ultraviolet radiation. Photochemistry and Photobiology 63,
814–817.
Kanervo E, Spetea C, Nishiyama Y, Murata N, Andersson B, Aro
E-M. 2003. Dissecting a cyanobacterial proteolytic system: efficiency in inducing degradation of the D1 protein of photosystem II
in cyanobacteria and plants. Biochimica et Biophysica Acta 1607,
131–140.
Koivuniemi A, Aro E-M, Andersson B. 1995. Degradation of the
D1 and D2 proteins of photosystem II in higher plants is regulated
by reversible phosphorylation. Biochemistry 34, 16022–16029.
Kettunen R, Pursiheimo S, Rintamäki E, van Wijk KJ, Aro E-M.
1997. Transcriptional and translational adjustments of psbA gene
expression in mature chloroplasts during photoinhibition and
subsequent repair of photosystem II. European Journal of Biochemistry 247, 441–448.
Kim J, Klein PG, Mullet JE. 1991. Ribosomes pause at specific
sites during synthesis of membrane-bound chloroplast reaction
centre protein D1. Journal of Biological Chemistry 266, 14931–
14938.
Kyle DJ, Kuang TY, Watson JL, Arntzen CJ. 1984. Movement of
a sub-population of the light-harvesting complez (LHCII) from
grana to stroma lamellae as a consequence of its phosphorylation.
Biochimica et Biophysica Acta 765, 89–96.
Laemmli UK. 1970. Cleavage of structural proteins during the
assembly of head of bacteriophage T4. Nature 227, 680–685.
Lindahl M, Tabak S, Cseke L, Pichersky E, Andersson B, Adam
Z. 1996. Identification, characterization, and molecular cloning of
a homologue of the bacterial FtsH protease in chloroplasts of
higher plants. Journal of Biological Chemistry 271, 29329–29334.
Lindahl M, Spetea C, Hundal T, Oppenheim AB, Adam Z,
Andersson B. 2000. The thylakoid FtsH protease plays a role in
the light-induced turnover of the photosystem II D1 protein. The
Plant Cell 12, 419–431.
Martinsuo P, Pursiheimo S, Aro E-M, Rintamäki E. 2003. Dithiol
oxidant and disulfide reductant dynamically regulate the phosphorylation of light harvesting complex II proteins in thylakoid
membranes. Plant Physiology 133, 37–46.
Mattoo AK, Hoffman-Falk H, Marder JB, Edelman M. 1984.
Regulation of protein metabolism: coupling of photosynthetic
electron transport to in vivo degradation of the rapidly metabolized
32-kilodalton protein of the chloroplast membranes. Proceedings
of the National Academy of Sciences, USA 81, 1380–1384.
Mauro S, Dainese P, Lannoye R, Bassi R. 1997. Cold-resistant and
cold-sensitive maize lines differ in the phosphorylation of the
photosystem II subunit, CP29. Plant Physiology 115, 171–180.
Melis A. 1999. Photosystem-II damage and repair cycle in chloroplasts: what modulates the rate of photodamage in vivo? Trends in
Plant Science 4, 130–135.
Michel H, Griffin PR, Shabanowitz J, Hunt DF, Bennett J.
1991. Tandem mass spectrometry identifies sites of three posttranslational modifications of spinach light-harvesting chlorophyll
protein II. Proteolytic cleavage, acetylation, and phosphorylation.
Journal of Biological Chemistry 266, 17584–17591.
Muller P, Li XP, Niyogi KK. 2001. Non-photochemical quenching.
A response to excess light energy. Plant Physiology 125, 1558–
1566.
Nield J, Kruse O, Ruprecht J, da Fonseca P, Büchel C, Barber J.
2000. Three-dimensional structure of Chlamydomonas reinhardtii
and Synechococcus elongatus photosystem II complexes allows
for comparison of their oxygen-evolving complex organization.
Journal of Biological Chemistry 275, 27940–27946.
Nilsson R, Brunner J, Hoffman NE, van Wijk KJ. 1999.
Interactions of ribosome nascent chain complexes of the chloroplast-encoded D1 thylakoid membrane protein with cpSRP54.
EMBO Journal 18, 733–742.
Prasil O, Adir N, Ohad I. 1992. Dynamics of photosystem II.
Mechanism of photoinhibition and recovery processes. In: Barber
K, ed. Topics in photosynthesis, Vol. II. The Netherlands: Elsevier,
293–348.
Pursiheimo S, Martinsuo P, Rintamäki E, Aro E-M. 2003.
Photosystem II protein phosphorylation follows four distinctly
different regulatory patterns induced by environmental cues. Plant,
Cell and Environment 26, 1995–2003.
Pursiheimo S, Mulo P, Rintamäki E, Aro E-M. 2001. Coregulation
of light-harvesting complex II phosphorylation and lhcb mRNA
accumulation in winter rye. The Plant Journal 26, 317–327.
Regel RE, Ivleva NB, Zer H, Meurer J, Shestakov SV, Herrmann
RG, Pakrasi HB, Ohad I. 2001. Deregulation of electron flow
within photosystem II in the absence of the PsbJ protein. Journal of
Biological Chemistry 276, 41473–41478.
Rintamäki E, Kettunen R, Aro E-M. 1996. Differential D1
dephosphorylation in functional and photodamaged photosystem
II centres. Dephosphorylation is a prerequisite for degradation
of damaged D1*. Journal of Biological Chemistry 271,
14870–14875.
Rintamäki E, Salonen M, Suoranta U-M, Carlberg I, Andersson
B, Aro E-M. 1997. Phosphorylation of light-harvesting complex II
and photosystem II core proteins shows different irradiancedependent regulation in vivo. Application of phosphothreonine
antibodies to analysis of thylakoid phosphoproteins. Journal of
Biological Chemistry 272, 30476–30482.
Rintamäki E, Martinsuo P, Pursiheimo S, Aro E-M. 2000.
Cooperative regulation of light-harvesting complex II phosphorylation via the plastoquinol and the ferredoxin–thioredoxin system
in chloroplasts. Proceedings of the National Academy of Sciences,
USA 97, 11644–11649.
Sakurai I, Hagio M, Gombos Z, Tyystjärvi T, Paakkarinen V,
Aro E-M, Wada H. 2003. Requirement of phosphatidylglycerol
for maintenance of photosynthetic machinery. Plant Physiology
133, 1376–1384.
Silva P, Thompson E, Bailey S, Kruse O, Mullineaux CW,
Robinson C, Mann NH, Nixon PJ. 2003. FtsH is involved in the
356 Aro et al.
early stages of repair of photosystem II in Synechocystis sp PCC
6803. The Plant Cell 15, 2152–2164.
Schuster G, Timberg R, Ohad I. 1988. Turnover of thylakoid
photosystem II proteins during photoinhibition of Chlamydomonas
reinhardtii. European Journal of Biochemistry 177, 403–410.
Snyders S, Kohorn BD. 2001. Disruption of thylakoid-associated
kinase 1 leads to alteration of light harvesting in Arabidopsis.
Journal of Biological Chemistry 276, 32169–32176.
Sun G, Bailey D, Jones MW, Markwell J. 1989. Chloroplast
thylakoid protein phosphatase is a membrane surface-associated
activity. Plant Physiology 89, 238–243.
Suorsa M, Regel R, Paakkarinen V, Battchikova N, Herrmann
RG, Aro E-M. 2004. Protein assembly of photosystem II and
accumulation of subcomplexes in the absence of low molecular
mass subunits PsbL and PsbJ. European Journal of Biochemistry
271, 96–107.
Swiatek M, Kuras R, Sokolenko A, et al. 2001. The chloroplast
gene ycf9 encodes a photosystem II (PSII) core subunit, PsbZ, that
participates in PSII supramolecular architecture. The Plant Cell 13,
347–367.
Swiatek M, Regel R, Meurer J, Wanner G, Pakrasi HB, Ohad I,
Herrmann RG. 2003. Effects of selective inactivation of individual genes for low-molecular-mass subunits on the assembly
of photosystem II, as revealed by chloroplast transformation: the
psbEFLJ operon in Nicotiana tabacum. Molecular and General
Genomics 268, 699–710.
Testi MG, Croce R, Polverino-De Laureto P, Bassi R. 1996. A
CK2 site is reversibly phosphorylated in the photosystem II
subunit CP29. FEBS Letters 399, 245–250.
Tyystjärvi E, Aro E-M. 1996. The rate constant of photoinhibition,
measured in lincomycin-treated leaves, is directly proportional to
light intensity. Proceedings of the National Academy of Sciences,
USA 93, 2213–2218.
van Wijk KJ, Roobol-Boza M, Kettunen R, Andersson B, Aro
E-M. 1997. Synthesis and assembly of the D1 protein into photo-
system II: processing of C-terminus and identification of the initial
assembly partners and complexes during photosystem II repair.
Biochemistry 36, 6178–6186.
Vener AV, Rokka A, Fulgosi H, Andersson B, Herrmann RG.
1999. A cyclophilin-regulated PP2A-like protein phosphatase in
thylakoid membranes of plant chloroplasts. Biochemistry 38,
14955–14965.
Vener AV, Harms A, Sussman R, Vierstra RD. 2001. Mass
spectrometric resolution of reversible protein phosphorylation in
photosynthetic membranes of Arabidopsis thaliana. Journal of
Biological Chemistry 276, 6959–6966.
Wollman F-A, Minai L, Nechushtai R. 1999. The biogenesis and
assembly of photosynthetic proteins in thylakoid membranes.
Biochimica et Biophysica Acta 1411, 21–85.
Zerges W. 2000. Translation in chloroplasts. Biochimie 82, 583–601.
Zhang L, Aro E-M. 2002. Synthesis, membrane insertion and
assembly of the chloroplast-encoded D1 protein into photosystem
II. FEBS Letters 512, 13–18.
Zhang L, Paakkarinen V, Suorsa M, Aro E-M. 2001. A SecY
homologue is involved in chloroplast-encoded D1 protein biogenesis. Journal of Biological Chemistry 276, 37809–37814.
Zhang L, Paakkarinen V, van Wijk KJ, Aro E-M. 1999. Cotranslational assembly of the D1 protein into photosystem II.
Journal of Biological Chemistry 274, 16062–16067.
Zhang L, Paakkarinen V, van Wijk KJ, Aro E-M. 2000. Biogenesis of the chloroplast-encoded D1 protein: regulation of
translation elongation, insertion, and assembly into photosystem
II. The Plant Cell 12, 1769–1781.
Zheleva D, Sharma J, Panico M, Morris HR, Barber J. 1998.
Isolation and characterization of monomeric and dimeric CP47reaction center photosystem II complexes. Journal of Biological
Chemistry 273, 16122–16127.
Zouni A, Witt HT, Kern J, Fromme P, Krauss N, Saenger W,
Orth P. 2001. Crystal structure of photosystem II from Synechococcus elongatus at 3.8 Å resolution. Nature 409, 739–743.