PsaL subunit is required for the formation of photosystem I trimers in the cyanobacterium Synechocystis sp. PCC 6803.
ABSTRACT When membranes of the wild type strain of the cyanobacterium Synechocystis sp. PCC 6803 were solubilized with detergents and fractionated by sucrose-gradient ultracentrifugation, photosystem I could be obtained as trimers and monomers. We could not obtain trimers from the membranes of any mutant strain that lacked PsaL subunit. In contrast, absence of PsaE, PsaD, PsaF, or PsaJ did not completely abolish the ability of photosystem I to form trimers. Furthermore, PsaL is accessible to digestion by thermolysin in the monomers but not in the trimers of photosystem I purified from wild type membranes. Therefore, PsaL is necessary for trimerization of photosystem I and may constitute the trimer-forming domain in the structure of photosystem I.
Volume 336, number 2, 330-334
0 1993 Federation of European Biochemical Societies 00145793/93/$6.00
PsaL subunit is required for the formation of photosystem I trimers
in the cyanobacterium
Synechocystis sp. PCC 6803
Vaishali P. Chitnis*, Parag R. Chitnis
Division of Biology, Kansas State University, Manhattan, KS 66506-4901, USA
Received 25 October 1993
When membranes of the wild type strain of the cyanobacterium Synechocystis sp. PCC 6803 were solubilized with detergents and fractionated by
sucrose-gradient ultracentrifugation, photosystem I could be obtained as trimers and monomers. We could not obtain trimers from the membranes
of any mutant strain that lacked PsaL subunit. In contrast, absence of PsaE, PsaD, PsaF, or PsaJ did not completely abolish the ability of
photosystem I to form trimers. Furthermore, PsaL is accessible to digestion by thermolysin in the monomers but not in the trimers of photosystem
I purified from wild type membranes. Therefore, PsaL is necessary for trimerization of photosystem I and may constitute the trimer-forming domain
in the structure of photosystem I.
Photosystem I; Photosynthesis; Organization; Cyanobacteria; Synechocystis sp. PCC 6803
Photosystem I is one of the membrane-protein
plexes of the photosynthetic apparatus of cyanobacteria
and plants. It accepts electrons from plastocyanin or
cytochrome c553 on the p-side (lumenal) of the mem-
brane and donates them to ferredoxin or flavodoxin on
the n-side (cytoplasmic or stromal). PSI has been iso-
lated from higher plants, algae, and cyanobacteria and
is remarkably conserved regarding its function, struc-
ture, and subunit composition [1,2]. Purified PSI typi-
cally contains at least eleven polypeptides,
mately 100 chlorophyll a molecules, a pair of phylloqui-
nones and three 4Fe-4S clusters [ 11. Subunits PsaA and
PsaB are two homologous
polypeptides that contain the electron transfer centers
P700, A,, and A,, and the 4Fe4S center Fx [1,3]. PsaC
contains the remaining iron-sulfur centers, F, and F,
. The remaining subunits of PSI do not contain any
electron transfer components and are accessory in their
Organization of proteins in PSI has been inferred
from their hydropathy profiles, the nature of their tran-
sit peptides, biochemical experiments involving pro-
tease accessibility, antibody-epitope
cross-linking and reconstitution [1,2]. Some domains of
the PsaD, PsaE and PsaL subunits of PSI from higher
plants are exposed to proteases [5-71. The large subunits
(PsaA and PsaB) and some low molecular weight sub-
units (PsaL, PsaK, PsaI, PsaJ, and PsaM) are integral
membrane proteins with hydrophobic
domains . PsaF is exposed on the p-side of the mem-
brane. PsaD, PsaC and PsaE are on the n-side of photo-
synthetic membrane. Cross-linking and in vitro recon-
stitution experiments reveal that PsaD, PsaE and PsaC
are in contact with each other and a considerable part
of PsaC is probably buried under PsaD and PsaE [6,9-
111. Electron microscopy and electron diffraction stud-
ies suggest that these three proteins form a protruding
cap on the stromal surface of the PSI complex [12,13].
The cyanobacterium Synechocystis sp. PCC 6803
provides an attractive system for studying the struc-
ture-function relationship in PSI. Simpler genetic or-
ganization, natural transformability
carry out homologous recombination, and the ability to
grow under heterotrophic
mutagenesis of photosynthetic proteins possible in this
species . We are studying the functions of the acces-
sory subunits of PSI through targeted mutagenesis of
the genes that encode these subunits in Synechocystis sp.
PCC 6803. Previously we cloned and characterized
psaD, psaE, psaF, psaJ and psaL and then generated
mutant strains in which these genes have been inac-
tivated [15-l 91. In the present study, we investigated the
presence of trimeric quaternary structure of PSI in the
membranes of these mutants. We report the absence of
PSI trimers in the detergent-solubilized
PsaL-less strains and show that PsaL is susceptible to
proteolysis in monomers but not in trimers of in wild
the capacity to
conditions make targeted
*Corresponding author. Fax: (1) (913) 532-6653.
This is contribution no. 94-175-J from the Kansas Agricultural Exper-
2. MATERIALS AND METHODS
Radioactive chemicals were purchased from NEN Corp. All en-
zymes and reagents used for molecular cloning were obtained from
Published by Elsevier Science Publishers B. K
Volume 336, number 2
FEBS LETTERS December 1993
Promega Biotech (Madison, WI). Detergents were from Calbiochem
(La Jolla, CA). The other chemicals and antibiotics were purchased
from Sigma or Fisher Biotech. The antibody specific to the carboxyl
terminus of PsaB was kindly provided by Dr. James Guikema, Kansas
State University . The cyanobacterial strains used in this study are
described in Table I.
Synechocystb sp. PCC 6803 cultures of both the wild-type and
mutant strains were grown in BGl 1 medium containing 5 mM glucose
and appropriate antibiotics . Cells were harvested during the late
exponential phase of growth and suspended in 0.4 M sucrose, 10 mM
NaCl, 200 PM phenylmethyl sulfonyltluoride, and 5 mM benzamid-
ine, and 50 mM Tris, pH 8.0. Equal volume of 150-2OOpm glass beads
was added, and the cells were broken with a bead beater (Biospec).
Photosynthetic membranes were separated from unbroken cells,
washed with, and then resuspended in 0.4 M sucrose, 10 mM NaCl,
and 50 mM Tris, pH 8.0. The membranes were incubated typically
with dodecyl-/I-o-maltoside (w/w chlorophyll:detergent
for 15 min on ice and then centrifuged for 15 min at 20,000 x g at 4°C.
The solubilized membranes were layered on a step-gradient made up
of lo%, 15%, 20%, 25% and 30% sucrose containing 10 mM MOPS
(pH 7.0) and 0.01% dodecyl-/?-o-maltoside. The samples were centri-
fuged at 200,000 x g for 16 h at 4°C. Chlorophyll concentrations in
membranes and PSI fractions were determined in 80% (v/v) acetone
according to . The photosynthetic membranes or PSI fractions
were stored at -20°C until use.
For protease accessibility studies, monomers and trimers of PSI
from wild-type membranes (100 pg chlorophyll/ml) were incubated
with thermolysin at a concentration
chlorophyll in the presence of 1 mM CaCl, at 37°C for different
durations. The reactions were terminated with 20 mM EDTA and the
samples were solubilized for 1 h at room temperature in the presence
of 1% SDS and 0.1% /I-mercaptoethanol.
Tricine-urea-SDS-PAGE according to  on slab gels containing
14% acrylamide and 6 M urea. To determine relative levels of different
subunits of PSI complexes, the gels were stained with Coomassie blue,
destained, and scanned with a personal laser densitometer (Molecular
Dynamics). For Western blotting, proteins were transferred to nitro-
cellulose membranes, probed with an antibody against the C-terminus
of PsaB, and immunodetected using enhanced chemiluminescence kit
ratio of 1: 15)
of 50 pg protease per mg of
Proteins were separated by
3. RESULTS AND DISCUSSION
The quaternary structure of PSI may involve the for-
mation of trimers. Several groups have isolated and
characterized trimers of PSI from cyanobacteria, plants
and a prochlorophyte
light spectroscopy of the monomers and trimers of PSI
has revealed remarkable similarity between the two, ex-
cept for the amplitude of a spectral component at long
wavelength. Despite some supporting data, the exis-
tence of trimers in vivo is difficult to demonstrate .
We optimized conditions for obtaining monomers and
trimers of PSI by sucrose-gradient centrifugation.
choice of detergent used for solubilization
branes and the ratio of detergent to membranes influ-
enced the yield of the trimeric form of PSI. We tested
Triton X-100, hexyl-, heptyl-, octyl-, nonyl-, and decyl-
gents 3-08, 3-10, 3-12, and 3-16, and dodecyl-/3-n-mal-
toside for their ability to resolve PSI from membranes
into monomeric and trimeric forms. Dodecyl-/3-n-mal-
toside was the most suitable detergent to dissolve mem-
branes with minimal disturbance
structure of PSI. The ratio (w/w) of chlorophyll to deter-
gent during solubilization of membranes affected the
proportion of PSI that could be isolated as trimers; the
optimal ratio for the photosynthetic
Synechocystis sp. PCC 6803 was found to be 1: 15. After
incubating with the detergent for 15 min on ice, the
membranes (in 5% sucrose, 50 mM Tricine, 10 mM
NaCl) were centrifuged at 20,000 x g for 15 min and the
supernatant was layered on a lO--30% step-gradient of
sucrose. Centrifugation in 13 ml tubes at 200,000 x g for
16 h resolved the pigmented complexes of the photosyn-
thetic membranes into distinct bands (Fig. 1A). Absorp-
tion spectra of the pigmented bands indicated that the
upper orange band contained carotenoid-proteins.
middle and lower green bands contained chlorophyll.
The gradients were fractionated
their chlorophyll contents were determined. The middle
green band contained approximately 50% of total chlo-
rophyll that was layered on the gradient, while the heav-
ier green band contained the remaining chlorophyll
(Fig. 1B). All fractions containing chlorophyll
[24-321. Steady-state polarized
to the quaternary
into 1 ml parts and
Strains of Synechocysris sp. PCC 6803 that were used in this study
EF The psaF and psaE genes replaced by genes for kanamycin and chloramphenicol resistance, respectively.
gene transcriptionally inactivated
The psaE and psaL genes replaced by genes for kanamycin and chloramphenicol resistance, respectively EL
Glucose-tolerant wild-type strain
The psaE gene replaced by a gene for kanamycin resistance
The psaD gene replaced by a gene for chloramphenicol resistance
The psaF gene replaced by a gene for kanamycin resistance. The psd gene transcriptionally inactivated
The psaL gene replaced by a gene for chloramphenicol resistance
The psaE and psaD genes replaced by genes for kanamycin and chloramphenicol resistance, respectively
The psaF and psaD genes replaced by genes for kanamycin and chloramphenicol resistance, respectively.
The psaJ gene transcriptionally inactivated
FL The psaF and psaL genes replaced by genes for kanamycin and chloramphenicol resistance, respectively.
The psd gene transcriptionally inactivated
Xu, Chitnis, Chitnis,
Xu, Chitnis, Chitnis,
Xu, Chitnis, Chitnis,
Volume 336, number 2 FEBSLETTERS
-PS I (Monomers) + PS II
Fig. 1. Resolution of monomeric and trimeric forms of photosystem
I by sucrose-gradient centrifugation. Photosynthetic membranes con-
taining 200 pg chlorophyll in 5% sucrose were incubated on ice for
15 min with dodecyl#-o-maltoside
centrifuged at 20,000 x g for 15 min at 4’C. The solubilized mem-
branes were layered on a lo-30% sucrose gradient containing 20 mM
MOPS and 0.01% dodecyl-/3-n-maltoside. The samples were centri-
fuged for 16 h at 200,000 x g (panel A). The gradients were fraction-
ated into 1 ml parts and chlorophyll content of each fraction was
determined (panel B). The values are an average of three independent
experiments and the bars represent standard deviation. The presence
of PSI in the chlorophyll-containing fractions was confirmed by West-
em blot analysis using an antibody against the PsaB subunit of PSI
(panel C). The antibody-antigen reaction was detected using a horse-
radish-peroxidase conjugated antibody and enhanced chemilumines-
cence substrates for horseradish peroxidase.
(1: 15 chlorophyll/detergent ratio),
tained PSI, as evident from SDS-PAGE analysis (data
not shown) and Western blotting (Fig. 1C). The upper
green fraction also contained photosystem II (data not
shown). When a mixture of molecular weight markers
was layered on the top of gradients and centrifuged, the
median positions of the markers were: bovine serum
albumin (66 kDa) in fraction #4, alcohol dehydroge-
nase (150 kDa) in fraction #5, /3-amylase (200 kDa) in
fraction #6, bovine thyroglobulin (669 kDa) in fraction
#9 and blue dextran (2000 kDa) at the bottom. The
estimated molecular masses of monomers and trimers
of PSI are 235-300 kDa and 670-750 kDa, respectively
[27,28,31]. Thus solubilization of the wild-type mem-
branes by dodecyl-j?-o-maltoside
and heavier forms of PS I which presumably repre-
sented monomers and trimers of PSI. Similar treatment
of membranes from ALC7-3, the PsaL-less mutant
strain, resulted in only one green band on sucrose-gradi-
ents that corresponded to the upper green band in the
tubes containing the wild-type PSI. When ALC 7-3
membranes were incubated with the detergent and cen-
trifuged on sucrose gradients, the fractions that nor-
mally contained the trimers of PSI in wild-type mem-
resulted into lighter
branes did not contain any chlorophyll or PsaB protein
(Fig. 1). Therefore trimers of PSI could not be obtained
by the procedures that yielded maximal amounts of
trimers from the wild type membranes. We varied the
detergent to chlorophyll ratio during solubilization of
membranes and used other detergents listed before, but
failed to obtain PSI trimers from ALC7-3 membranes.
Therefore, PSI complexes in ALC7-3 membranes may
not be organized in stable trimeric quaternary
tures or are unable to form trimers after solubilization
by a detergent.
The absence of PSI trimers in detergent-solubilized
ALC7-3 membranes raised a possibility that the acces-
sory subunits of PSI in general may be required for the
formation of PSI trimers. To test this hypothesis we
isolated photosynthetic membranes from several strains
of Synechocystis sp. PCC6803 (Table I) and incubated
them with dodecyl-/3-D-maltoside
chlorophyll ratio). Resolution of monomers and trimers
of PSI from these membranes revealed that trimers
could be obtained from the wild type. AEK2, ADC4,
AFK6, EF, DF and DE strains (Fig. 2). All these strains
contain a functional psuL gene and the PsaL protein is
present in the PS I complexes purified from them [ 15-
19]. The proportion of trimers was drastically reduced
in detergent-solubilized membranes of ADC4, DF and
DE. These membranes lacked PsaD, which is required
for stable assembly of many other subunits into PSI
[lO,l 11. Chaotropic agents and detergents remove the
small molecular weight subunits including PsaL from
PSI of strains that lack PsaD . Therefore it is likely
that removal of other subunits, especially PsaL, may be
responsible for the decreased yield of trimers from the
membranes of PsaD-less mutants. PsaD may interact
with PsaL and stabilizes its assembly in PSI. Trimers of
PSI could not be obtained from the membranes of three
independently generated mutant strains, ALC7-3, EL,
and FL (Fig. 2). These strains lack a functional psaL
gene and thus its product, but contain assembled PSI
complexes. Therefore, PsaD, PsaE, PsaF or PsaJ may
influence yield of trimers in the detergent-solubilized
membranes but are not essential for the formation of
PSI trimers. In contrast, the absence of PsaL led to the
(15: 1 detergent to
Fig. 2. Trimers of PSI cannot be obtained in PsaL-less mutants.
Trimers and monomers from the membranes of different mutant
strains (Table I) were resolved on sucrose-density gradients as de-
scribed in Fig. 1.
Volume 336, number 2
lack of trimers in the detergent-solubilized
indicating that PsaL is involved in trimerization of PSI.
The requirement of PsaL for the formation of PSI
trimers could be due to a direct structural or an indirect
regulatory role. Crystals
Synechococcus elongatus have been obtained and used
to decipher a three-dimensional structure at 6 A resolu-
tion . Each monomer consists of a catalytic domain
and a smaller domain that connects the monomers to
form a trimer. It is likely that PsaL is a structural com-
ponent of the trimer-forming
this possibility, we studied surface exposure of protein
subunits in monomers and trimers of PSI by investigat-
ing their accessibility to proteolysis (Fig. 3). Monomers
and trimers of PSI were incubated with thermolysin for
different duration and polypeptide compositions were
analyzed. PsaA-PsaB, PsaD, PsaF, PsaE, and PsaC sub-
units were relatively resistant to proteolysis for several
hours in both monomeric and trimeric PSI. For exam-
ple, - 90% of PsaA-PsaB remained undegraded in both
forms after 4 h of protease digestion (Fig. 3, and data
not shown). In contrast, PsaL was degraded in the mon-
omeric PSI, but were resistant to proteolysis in the
trimeric PSI (Fig. 3). It took approximately 4 h to com-
pletely degrade PsaL in PSI monomers.
same time, only 15% PsaL was degraded from the
trimeric PSI. Incubation of trimers with protease for
4 h at 37°C resulted in conversion of approximately
10% trimers into monomers. Therefore, the PsaL that
was digested by thermolysin in trimers of PSI might be,
at least partly, due to the conversion of trimers into
monomers. Alternatively, degradation
have caused dissociation of trimers. All subunits of PSI
remained intact for 24 h when thermolysin was omitted
during incubation (data not shown). When 0.1% SDS
was added during proteolysis, all proteins in the mon-
omeric and trimeric fractions of PSI were completely
degraded by thermolysin (data not shown). Therefore,
the protection of PsaL in trimers and other subunits in
both forms of PSI was due to their conformation rather
than due to lack of protease recognition sites. Our re-
sults demonstrate that PsaL is accessible to thermolysin
in monomers and is protected in trimers of PSI, indicat-
ing that it may form at least a part of the central domain
that keeps units of a trimer together.
The analysis of crystals of trimeric PSI from the cya-
Synechococcus elongatus has revealed lo-
cations of the 4Fe-4S clusters F,, F, and F,, 28 01-
helices and 45 chlorophyll a molecules . Identities of
the helices are not unequivocally established. Helices j
and k, which have been named arbitrarily, were pre-
sumed to be part of the PsaK subunit. They are close
to the 3-fold axis in a tight bundle and therefore are
proposed to be responsible for the organization of the
monomers into the trimeric form . We believe that
the helices j and k in the trimer-forming central domain
of PSI may be parts of PsaL instead of PsaK on the
of trimeric PSI from
domain. To investigate
of PsaL might
MOnOmMC PS I Trimeric PS I
0 1 2 3 4 0 1 2 3 4 Time(h)
Fig. 3. Protease accessibility of PSI subunits in monomeric and
trimeric complexes. The monomers and trimers of PSI were diluted to
100 pg chlorophyll/ml and incubated with thermolysin (50 ,@mg
chlorophyll) and 1 mM CaCl, for 0, 1, 2, 3, 4 h. Subsequently, the
reactions were terminated by addition of 25 mM EDTA and the
proteins were solubilized by incubating samples with 1% SDS and
0.1% jI-mercaptoethanol for 1 h at room temperature and were sepa-
rated using Tricine-urea-SDS-PAGE. The proteins were stained with
Coomassie blue. The experiment was repeated six times, using differ-
ent periods of proteolysis. Polypeptide profiles of PSI after proteolysis
in a representative experiment are shown.
basis of following reasons. First, PsaL was required for
the formation of PSI trimers (Figs. 1,2). Second, PsaL
was protected from proteolysis in trimers but not in
monomers of PSI (Fig. 3), suggesting that PsaL is ex-
posed on the surface of monomers that is involved in
trimer-formation. Third, the ALC7-3
which trimers cannot be obtained, lacks PsaL but not
PsaK . PsaK was present in the PSI purified from
all PsaL-less mutants. Thus the presence of PsaK was
not sufficient to allow trimerization of PSI. Fourth, the
suggestion that the helicesj and k are parts of PsaK was
based on the presence of electron density linking these
helices. Hydropathy analysis of PsaL indicates the pres-
ence of a hydrophilic N-terminal region followed by
three hydrophobic regions . The first hydrophobic
region is only approximately
and therefore may not serve as a transmembrane helix.
Therefore, PsaL may contain two transmembrane
lices, with a rather large N-terminal domain. The loop
joining the putative transmembrane
contains three charged residues in Synechocystis sp.
PCC 6803 and four charged residues in Synechococcus
elongatus. In contrast, the putative interhelical loop in
PsaK of Synechococcus elongatus contains five charged
residues. Therefore both PsaL and PsaK are likely can-
didates to be responsible for the electron densities con-
necting helices j and k. Thus, the helices in the central
domain that were identified by X-ray diffraction analy-
sis of PSI crystals may belong to PsaL. Moreover PsaL
may also connect the central domain to the catalytic
domain of PSI by a rather large amino-terminal
domain. It is likely that PsaK may have a trimer-spe-
13 amino acids in length
helices of PsaL
Volume 336, number 2
cific, non-essential role in the organization of PSI, since
it is protected in trimers but not in monomers from
degradation by thermolysin (Fig. 3).
Results of the present study demonstrate
detergent-solubilized membranes of PsaL-less mutants
of Synechocystis sp. PCC 6803 do not contain trimers
of PSI. It is, however, difficult to test if PSI in these
mutants is solely in monomeric form in vivo. If the
absence of trimers in vitro is also true in the cells of
these mutants, it would mean that trimeric form of PSI
is not essential for the function of PSI. The mutant
strains of Synechocystis sp. PCC 6803 that lack PsaL
alone  or in combination with other non-essential
subunits of PSI (Xu, Chitnis and Chitnis, unpublished
results) do not significantly differ in their photoautotro-
phic growth and photosynthetic activities from the wild
type strain. It is possible that trimerization may play a
role by modulating light-harvesting efficiency of PSI.
Acknowledgements: We thank Qiang Xu and W.R. Odom for critically
reading the manuscript. This work was supported in part by research
grants from the NSF (MCB#9202751) and USDA-CSRS (92-37306-
7661) and an equipment grant from USDA-CSRS (93-04084).
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