Isolation and Characterization of a Light-Harvesting Chlorophyll a/b Protein Complex Associated with Photosystem I

A chlorophyll a/b protein complex has been isolated from a resolved native photosystem I complex by mildly dissociating sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The chlorophyll a/b protein contains a single polypeptide of molecular weight 20 kilodaltons, and has a chlorophyll a/b ratio of 3.5 to 4.0. The visible absorbance spectrum of the chlorophyll a/b protein complex showed a maximum at 667 nanometers in the red region and a 77 K fluorescence emission maximum at 681 nanometers. Alternatively, by treatment of the native photosystem I complex with lithium dodecyl sulfate and Triton, the chlorophyll a/b protein complex could be isolated by chromatography on Sephadex G-75. Immunological assays using antibodies to the P₇₀₀-chlorophyll a-protein and the photosystem II light-harvesting chlorophyll a/b protein show no cross-reaction between the photosystem I chlorophyll a/b protein and the other two chlorophyll-containing protein complexes.     

Correspondence of Apoproteins of Light-Harvesting Chlorophyll a/b Complexes Associated with Photosystem I to cab Genes: Evidence for a Novel Type IV Apoprotein

Apoproteins of spinach and pea light-harvesting chlorophylla/b complexes associated with photosystem I (LHCI) were identifiedby their chlorophyll fluorescence spectra and protein sequences.Spinach LHCI holocomplex consisted of four apoproteins of 25kDa, 23 kDa, 21 kDa and 20.5 kDa. LHCI subcomplex isolated bysucrose density gradient centrifugation fluoresced at 680 nmwith a shoulder around 700–710 nm at 77 K. It containedthe 23 kDa protein of which the N-terminal sequence correspondedto Type II gene of LHCI. Another LHCI subcomplex isolated bygel electrophoresis emitted at 679 nm and contained the 25 kDaprotein, of which the N-terminus was blocked. Its internal sequenceswere determined after protease treatment and found to be homologousto Type III gene of LHCI. An oligomeric subcomplex of LHCI isolatedby gel electrophoresis emitted at 726 nm and consisted of the21 kDa and 20.5 kDa apoproteins. N-terminal sequence of the20.5 kDa component corresponded to the Type I gene of LHCI.The 21 kDa component did not have any clear homologue, but itsN-terminal sequence was weakly but significantly homologousto all LHC components particularly to Type I LHCI among others.It was, thus, concluded that the 21 kDa protein is the fourthtype of LHCI apoprotein. Similar sequence homology was foundfor pea LHCI apoproteins. (Received September 10, 1990; Accepted November 22, 1990)     

Redox Regulation of Light-Harvesting Complex II and cab mRNA Abundance in Dunaliella salina

We demonstrate that photosynthetic adjustment at the level of the light-harvesting complex associated with photosystem II (LCHII) in Dunaliella salina is a response to changes in the redox state of intersystem electron transport as estimated by photosystem II (PSII) excitation pressure. To elucidate the molecular basis of this phenomenon, LHCII apoprotein accumulation and cab mRNA abundance were examined. Growth regimes that induced low, but equivalent, excitation pressures (either 13[deg]C/20 [mu]mol m-2 s-1 or 30[deg]C/150 ([mu]mol m-2 s-1) resulted in increased LHCII apoprotein and cab mRNA accumulation relative to algal cultures grown under high excitation pressures (either 13[deg]C/150 [mu]mol m-2 s-1 or 30[deg]C/2500 [mu]mol m-2 s-1). Thermodynamic relaxation of high excitation pressures, accomplished by shifting cultures from a 13 to a 30[deg]C growth regime at constant irradiance for 12 h, resulted in a 6- and 8-fold increase in LHCII apoprotein and cab mRNA abundance, respectively. Similarly, photodynamic relaxation of high excitation pressure, accomplished by a shift from a light to a dark growth regime at constant temperature, resulted in a 2.4- to 4-fold increase in LHCII apoprotein and cab mRNA levels, respectively. We conclude that photosynthetic adjustment to temperature mimics adjustment to high irradiance through a common redox sensing/signaling mechanism. Both temperature and light modulate the redox state of the first, stable quinone electron acceptor of PSII, which reflects the redox poise of intersystem electron transport. Changes in redox poise signal the nucleus to regulate cab mRNA abundance, which, in turn, determines the accumulation of light-harvesting apoprotein. This redox mechanism may represent a general acclimation mechanism for photosynthetic adjustment to environmental stimuli.     

Organization and Stability of Polypeptides Associated with the Chlorophyll a-b Light-Harvesting Complex of Photosystem-II

In the oxygen-evolving photosystem-II (PSII) of higher plantchioroplasts and green algae, most of the light-harvesting functionis performed by the chlorophyll (Chl) a-b-protein complex (LHC-II).On the average, the LHC-II contains about 210 Chl (a+b) moleculesper PSII reaction center. The polypeptide composition, copynumber and organization of assembly in the LHC-II complex arenot fully understood at present. This work utilized the chlorinaf2 mutant of barley (lacking Chl b and having a LHC-II antennaof only 13 Chl a molecules) to determine the organization andstability of assembly of proteins in the LHC-II. High-resolutionSDS-PAGE and immunoblot analysis showed the presence of fourmain constitutive polypeptides in the wild-type LHC-II (termedhere subunits a, b, c and d) with molecular masses in the range30–25 kDa. Of those, only subunit d (a 25 kDa polypeptide)was found to occur at an equal copy number per PSII reactioncenter in both wild-type and in the Chl b-less chlorina f2 mutant.All other subunits were either absent or existed in much loweramounts in the mutant. Subunit d is a polypeptide constituentof the major Chl-protein subcomplex (CPII) of the LHC-II. Itis stably incorporated in the thylakoid membrane in the absenceof Chl b and probably binds the 13 Chl a molecules in the residualLHC-II antenna of the chlorina f2 mutant. We propose that, ofall LHC-II polypeptides, subunit d is most proximal to the PSIIcore and may serve as a linker in the process of excitationenergy transfer from the bulk LHC-II to the PSII reaction centerin chloroplasts. (Received February 25, 1992; Accepted May 12, 1992)     

Light Intensity-Induced Changes in cab mRNA and Light Harvesting Complex II Apoprotein Levels in the Unicellular Chlorophyte Dunaliella tertiolecta

During a transition from high growth irradiance (700 micromoles quanta per square meter per second) to low growth irradiance (70 micromoles quanta per square meter per second), the unicellular marine chlorophyte Dunaliella tertiolecta Butcher increases the cellular pool size of the light-harvesting complex of photosystem II (LHC II). We showed that the increase in LHC II apoproteins and in chlorophyll content per cell is preceded by an approximately fourfold increase in cab mRNA. The increase in cab mRNA is detectable within 1.5 hours following a shift from high to low light intensity. An increase in the relative abundance of cab mRNA was also found following a shift from high light to darkness and from high light to low light in the presence of gabaculine, a chlorophyll synthesis inhibitor. However, the LHC II apoproteins did not accumulate in the latter experiments, suggesting that LHC II apoprotein synthesis is coupled to chlorophyll synthesis at or beyond translation. We propose that changes in energy balance brought about by a change in light intensity may control a regulatory factor acting to repress cab mRNA expression in high light.     

Isolation of Lamellar Aggregates of the Light-Harvesting Chlorophyll a/b Protein Complex of Photosystem II with Long-Range Chiral Order and Structural Flexibility

Isolation of LHCII, the light-harvesting chlorophyll a/b complex of photosystem II, based on the procedure described by Krupaet al.(1987,Plant Physiol.84, 19–24), was optimized for obtaining purified lamellar aggregates with long-range chiral order and structural flexibility (the capability of undergoing light-induced reversible structural changes). By varying the concentration of the detergent Triton X-100 for the solubilization of thylakoid membranes, we obtained four types of LHCII aggregates: (i) With low detergent concentration, ≤0.6% (v/v), the aggregates contained lipids in high amount. These preparations with Chl a/b ratios of about 1.4 contained minor antenna complexes with a fingerprint of an additional CD band at (+) 505 nm; they formed disordered lamellae and exhibited no or weak psi-type CD bands (psi, polymerization- or salt-induced), which did not possess the ability to undergo light-induced changes (ΔCD). (ii) At the optimal concentration, around 0.7 ± 0.1% (v/v), the detergent removed some lipids and most of the minor complexes, and the Chl a/b ratio dropped to 1.0–1.1. LHCII formed loosely stacked two-dimensional lamellae which exhibited psi-type CD bands and large light-induced reversible structural changes (ΔCD). (iii) At detergent concentration above the optimum, around 0.8–1% (v/v), the lipid content of LHCII decreased and minor complexes could not be detected. LHCII formed disordered aggregates and showed neither psi-type CD nor ΔCD. (iv) High concentrations (≥1.1% (v/v)) Triton X-100 led to very pure but largely delipidated samples assembled into tightly stacked three-dimensional lamellar structures with intense psi-type CD but no ΔCD.     

Induction of the De Novo Formation of the Photosystem I-Related Light-Harvesting Chlorophyll Protein Complex LHCPa by Photoheterotrophic Nutrition

The green alga Chlamydobotrys stellata contains in addition to the normal light-harvesting chlorophyll protein complex LHCPb a special LHCPa which is free of chlorophyll b and connected only to photosystem I (Brandt, Zufall, Wiessner 1983 Plant Physiol 71: 128-131). The kinetics of these two LHCP forms were analyzed during the transition in nutrition of the alga from autotrophy to photoheterotrophy, e.g. the replacement of CO₂ by acetate as carbon source. As shown by incorporation experiments with [¹⁴C]acetate, this change in nutrition leads to an increased synthesis of LHCPa, whereas the synthesis of the photosystem II-related LHCPb decreases. The increase of the LHCPa synthesis starts immediately after the onset of photoheterotrophic nutrition together with the synthesis of the chlorophyll protein complex CPI. There is no interchange of complex components between LHCPa and LHCPb during the depletion of the latter. The formation of LHCPa is discussed with respect to the regulation of gene expression.     

Photo and Nutritional Regulation of the Light-Harvesting Chlorophyll a/b-Binding Protein of Photosystem II mRNA Levels in Euglena

In Euglena gracillis var bacillaris, light exposure increases the level of mRNA encoding the light-harvesting chlorophyll a/b-binding protein of photosystem II (LHCPII) approximately twofold. LHCPII mRNA levels increased in the dark upon either malate or ethanol addition. LHCPII mRNA is present but LHCPII is not synthesized in the bleached mutants W₃BUL and W₁₀BSmL, which lack protochlorophyll(ide) and most if not all of the chloroplast genome. Light exposure increased LHCPII mRNA levels in W₃BUL but not in W₁₀BSmL. Carbon availability and light acting through a nonchloroplast photoreceptor appear to regulate LHCPII mRNA levels. A chloroplast photoreceptor and/or a product produced by the chloroplast appear to regulate LHCPII mRNA translation.     

The Photosystem II Light-Harvesting Protein Lhcb3 Affects the Macrostructure of Photosystem II and the Rate of State Transitions in Arabidopsis

The main trimeric light-harvesting complex of higher plants (LHCII) consists of three different Lhcb proteins (Lhcb1-3). We show that Arabidopsis thaliana T-DNA knockout plants lacking Lhcb3 (koLhcb3) compensate for the lack of Lhcb3 by producing increased amounts of Lhcb1 and Lhcb2. As in wild-type plants, LHCII-photosystem II (PSII) supercomplexes were present in Lhcb3 knockout plants (koLhcb3), and preservation of the LHCII trimers (M trimers) indicates that the Lhcb3 in M trimers has been replaced by Lhcb1 and/or Lhcb2. However, the rotational position of the M LHCII trimer was altered, suggesting that the Lhcb3 subunit affects the macrostructural arrangement of the LHCII antenna. The absence of Lhcb3 did not result in any significant alteration in PSII efficiency or qE type of nonphotochemical quenching, but the rate of transition from State 1 to State 2 was increased in koLhcb3, although the final extent of state transition was unchanged. The level of phosphorylation of LHCII was increased in the koLhcb3 plants compared with wild-type plants in both State 1 and State 2. The relative increase in phosphorylation upon transition from State 1 to State 2 was also significantly higher in koLhcb3. It is suggested that the main function of Lhcb3 is to modulate the rate of state transitions.     

Functional Analyses of the Plant Photosystem I-Light-Harvesting Complex II Supercomplex Reveal That Light-Harvesting Complex II Loosely Bound to Photosystem II Is a Very Efficient Antenna for Photosystem I in State II

State transitions are an important photosynthetic short-term response that allows energy distribution balancing between photosystems I (PSI) and II (PSII). In plants when PSII is preferentially excited compared with PSI (State II), part of the major light-harvesting complex LHCII migrates to PSI to form a PSI-LHCII supercomplex. So far, little is known about this complex, mainly due to purification problems. Here, a stable PSI-LHCII supercomplex is purified from Arabidopsis thaliana and maize (Zea mays) plants. It is demonstrated that LHCIIs loosely bound to PSII in State I are the trimers mainly involved in state transitions and become strongly bound to PSI in State II. Specific Lhcb1-3 isoforms are differently represented in the mobile LHCII compared with S and M trimers. Fluorescence analyses indicate that excitation energy migration from mobile LHCII to PSI is rapid and efficient, and the quantum yield of photochemical conversion of PSI-LHCII is substantially unaffected with respect to PSI, despite a sizable increase of the antenna size. An updated PSI-LHCII structural model suggests that the low-energy chlorophylls 611 and 612 in LHCII interact with the chlorophyll 11145 at the interface of PSI. In contrast with the common opinion, we suggest that the mobile pool of LHCII may be considered an intimate part of the PSI antenna system that is displaced to PSII in State I.     

Photosystem II Excitation Pressure and Development of Resistance to Photoinhibition (I. Light-Harvesting Complex II Abundance and Zeaxanthin Content in Chlorella vulgaris)

The basis of the increased resistance to photoinhibition upon growth at low temperature was investigated. Photosystem II (PSII) excitation pressure was estimated in vivo as 1 - qp (photochemical quenching). We established that Chlorella vulgaris exposed to either 5[deg]C/150 [mu]mol m-2 s-1 or 27[deg]C/2200 [mu]mol m-2 s-1 experienced a high PSII excitation pressure of 0.70 to 0.75. In contrast, Chlorella exposed to either 27[deg]C/150 [mu]mol m-2 s-1 or 5[deg]C/20 [mu]mol m-2 s-1 experienced a low PSII excitation pressure of 0.10 to 0.20. Chlorella grown under either regime at high PSII excitation pressure exhibited: (a) 3-fold higher light-saturated rates of O2 evolution; (b) the complete conversion of PSII[alpha] centers to PSII[beta] centers; (c) a 3-fold lower epoxidation state of the xanthophyll cycle intermediates; (d) a 2.4-fold higher ratio of chlorophyll a/b; and (e) a lower abundance of light-harvesting polypeptides than Chlorella grown at either regime at low PSII excitation pressure. In addition, cells grown at 5[deg]C/150 [mu]mol m-2 s-1 exhibited resistance to photoinhibition comparable to that of cells grown at 27[deg]C/2200 [mu]mol m-2 s-1 and were 3- to 4-fold more resistant to photoinhibition than cells grown at either regime at low excitation pressure. We conclude that increased resistance to photoinhibition upon growth at low temperature reflects photosynthetic adjustment to high excitation pressure, which results in an increased capacity for nonradiative dissipation of excess light through zeaxanthin coupled with a lower probability of light absorption due to reduced chlorophyll per cell and decreased abundance of light-harvesting polypeptides.     

Thermostability and Photostability of Photosystem II in Leaves of the Chlorina-f2 Barley Mutant Deficient in Light-Harvesting Chlorophyll a/b Protein Complexes

The chlorophyll-b-less chlorina-f2 barley mutant is deficient in the major as well as some minor light-harvesting chlorophyll-protein complexes of photosystem II (LHCII). Although the LHCII deficiency had relatively minor repercussions on the leaf photosynthetic performances, the responses of photosystem II (PSII) to elevated temperatures and to bright light were markedly modified. The chlorina-f2 mutation noticeably reduced the thermostability of PSII, with thermal denaturation of PSII starting at about 35[deg]C and 38.5[deg]C in chlorina-f2 and in the wild type, respectively. The increased susceptibility of PSII to heat stress in chlorina-f2 leaves was due to the weakness of its electron donor side, with moderate heat stress causing detachment of the 33-kD extrinsic PSII protein from the oxygen-evolving complex. Prolonged dark adaptation of chlorina-f2 leaves was also observed to inhibit the PSII donor side. However, weak illumination slowly reversed the dark-induced inhibition of PSII in chlorina-f2 and cancelled the difference in PSII thermostability observed between chlorina-f2 and wild-type leaves. The mutant was more sensitive to photoinhibition than the wild type, with strong light stress impairing the PSII donor side in chlorina-f2 but not in the wild type. This difference was not observed in anaerobiosis or in the presence of 3-(3,4-dichlorophenyl)- 1,1-dimethylurea, diuron. The acceptor side of PSII was only slightly affected by the mutation and/or the aforementioned stress conditions. Taken together, our results indicate that LHCII stabilize the PSII complexes and maintain the water-oxidizing system in a functional state under varying environmental conditions.     

Discovery of a Chlorophyll Binding Protein Complex Involved in the Early Steps of Photosystem II Assembly in Synechocystis

Efficient assembly and repair of the oxygen-evolving photosystem II (PSII) complex is vital for maintaining photosynthetic activity in plants, algae, and cyanobacteria. How chlorophyll is delivered to PSII during assembly and how vulnerable assembly complexes are protected from photodamage are unknown. Here, we identify a chlorophyll and β-carotene binding protein complex in the cyanobacterium Synechocystis PCC 6803 important for formation of the D1/D2 reaction center assembly complex. It is composed of putative short-chain dehydrogenase/reductase Ycf39, encoded by the slr0399 gene, and two members of the high-light-inducible protein (Hlip) family, HliC and HliD, which are small membrane proteins related to the light-harvesting chlorophyll binding complexes found in plants. Perturbed chlorophyll recycling in a Ycf39-null mutant and copurification of chlorophyll synthase and unassembled D1 with the Ycf39-Hlip complex indicate a role in the delivery of chlorophyll to newly synthesized D1. Sequence similarities suggest the presence of a related complex in chloroplasts.     

Reorganization of the Photosystem II Unit in Developing Thylakoids of Higher Plants after Transfer to Darkness : Changes in Chlorophyll b, Light-Harvesting Chlorophyll Protein Content, and Grana Stacking

A light-dependent reversible grana stacking-unstacking process, paralleled by a reorganization of thylakoid components, has been noticed in greening etiolated bean (Phaseolus vulgaris, var. red kidney) leaves upon transfer to darkness. The reorganization, based on biochemical and biophysical criteria, involves mainly the photosystem II (PSII) unit components: upon transfer to darkness, the light-harvesting chlorophyll protein (LHCP), its 25 kilodalton polypeptide and chlorophyll b are decreased, while the CPa and its 42 kilodalton polypeptide are increased and new PSII units of smaller size are formed. This reorganization of components occurs only in thylakoids still in the process of development and not in those present in steady state conditions.

It is proposed that this process does not reflect the turnover of the LHCP component per se, but a regulatory process operating during development, by which the ratio of light-harvesting to PSII reaction center components, determined by the environmental conditions, controls the photosynthetic rate.

     

Organization of Chlorophyll a in the Light-Harvesting Chlorophyll a/b Protein Complex as Shown by Circular Dichroism : Liquid Crystal-Like Domains

The development of thylakoid stacking, accumulation of the light-harvesting chlorophyll a/b protein complex (LHCP), and the changes of circular dichroism (CD) which reflect the organization of chlorophyll molecules in greening thylakoids of bean Phaseolus vulgaris cv Red Kidney leaves were investigated.

Chloroplasts formed under intermittent light contained large double sheets of membrane with extensive appression in addition to separate lamellae. Thylakoids of such chloroplasts were devoid of LHCP and exhibited a relatively small CD in the chlorophyll absorption region. Upon continuous illumination, the rearrangement of membranes to characteristic grana and the accumulation of the LHCP was accompanied by the gradual appearance of the very intense CD signal with peaks at 682 to 684 (+) and 665 to 672 nanometers (−). The magnitude of differential absorption was approximately 100 times larger than that of the chlorophyll a in solution. This suggests a superhelical liquid crystal-like organization for LHCP, a texture which can be altered by changes of the electric field in the photosynthetic membranes.

     

Phytochrome Regulation of Greening in Pisum: Chlorophyll Accumulation and Abundance of mRNA for the Light-Harvesting Chlorophyll a/b Binding Proteins

A brief pulse of red light eliminates or reduces the lag in chlorophyll accumulation that occurs when dark-grown pea seedlings are transferred to continuous white light. The red light pulse also induces the accumulation of specific mRNAs. We compared time courses, escape from reversal by far-red light, and fluence-response behavior for induction of mRNA for the light-harvesting chlorophyll a/b binding proteins (Cab mRNA) with those for induction of rapid chlorophyll accumulation in seedlings of Pisum sativum cv Alaska. In both cases the time courses of low fluence and very low fluence responses diverged from each other in a similar fashion: the low fluence responses continued to increase for at least 24 hours, while the very low fluence responses reached saturation by 8 to 16 hours. Both responses escaped from reversibility by far-red slowly, approaching the red control level after 16 hours. The fluence-response curve for the Cab mRNA increase, on the other hand, showed threshold and saturation at fluences 10-fold lower than threshold and saturation values for the greening response. Therefore, the level of Cab mRNA, as measured by the presence of sequences hybridizing to a cDNA probe, does not limit the rate of chlorophyll accumulation after transfer of pea seedlings to white light. The Cab mRNA level in the buds of seedlings grown under continuous red light remained high even when the red fluence rate was too low to allow significant greening. In this case also, abundance of Cab mRNA cannot be what limits chlorophyll accumulation.     

An Oxygen-Evolving Photosystem II Complex Associated with the Core Substructure of the Phycobilisome from Synechococcus elongatus

Oxygen-evolving photosystem II (PS II) particles isolated fromthe thermophilic cyanobacterium Synechococcus elongatus consistedof about twenty polypeptides. Six polypeptides were identifiedby reaction with specific antisera as constituent subunit polypeptidesof oxygen-evolving PS II reaction center complexes. The mostabundant polypeptides were the and ß subunits ofallophycocyanin. Comparison with the polypeptide profile ofisolated phycobilisomes, as well as immunoblotting with an antiserumagainst the large linker polypeptide, showed that the largelinker polypeptide or some proteolytic fragments of it werepresent in the preparation. Thus, each PS II particle is, inessence, an oxygen-evolving PS II complex that is associatedwith the core substructure of the phycobilisome. Cross-linkingexperiments indicated that fragments of the large linker polypeptidesare closely associated with one another and that the Chl-carrying47- kDa polypeptide is located in close proximity to the D2protein and the extrinsic 33-kDa protein. (Received November 12, 1991; Accepted January 23, 1992)     

Evidence from Crosslinking for Nearest-Neighbor Relationships among the Three Extrinsic Proteins of Spinach Photosystem II Complexes that are Associated with Oxygen Evolution

Treatment of oxygen-evolving Photosystem II complexes, whichlack light-harvesting chlorophyll a/b proteins, with a seriesof disuccinimidyl esters with different chain lengths yieldeda crosslinked product which consisted of one molecule each ofthe extrinsic 33 kDa and 23 kDa proteins. In addition, crosslinkingbetween the 33 kDa protein and the chlorophyll-carrying 47 kDaprotein and between the 23 kDa and 17 kDa proteins was confirmed.Thus, the three extrinsic proteins are closely associated witheach other to form a complex which is attached to the PS IIreaction center complexes. (Received December 1, 1989; Accepted May 2, 1990)     

Differential Detergent Stability of the Major Light-Harvesting Complex II in Thylakoids Isolated from Monocotyledonous and Dicotyledonous Plants

A survey of isolated thylakoids from 11 different higher plant species (Spinacia oleracea L., Pisum sativum L., Vicia faba L., Brassica napus L., Vigna sinensis L., Vinca minor L., Secale cereale L., Triticum aestivum L., Triticosecale Wittn., Hordeum vulgare L., Zea mays L.) indicated that the ratio of the oligomeric:monomeric form of the light-harvesting complex II was twofold higher for the dicots (3.16 ± 0.35) than the monocots (1.64 ± 0.25) examined under identical separation procedures. Under conditions specifically designed to stabilize the oligomeric form in vitro, we show that the oligomeric form of dicot light-harvesting complex II is twice as stable to solubilization in the presence of sodium dodecyl sulfate (SDS) than that observed for monocots. This decreased stability of monocot light-harvesting complex II is associated with a twofold increase in the trienoic fatty acid level of thylakoid phosphatidylglycerol but with no significant changes in the trienoic fatty acid levels in the major galactolipids. In addition, SDS polyacrylamide gel electrophoresis and western blot analyses with monoclonal antibodies indicated that monocots exhibited greater heterogeneity in the polypeptide complements associated with subfractions of light-harvesting complex II than the dicots examined. The data indicate that the oligomeric form of the light-harvesting complex II is not the result of a simple oligomerization of a common monomeric unit. We suggest that the difference in stability of the oligomeric form of light-harvesting complex II in isolated thylakoids of monocots and dicots is probably due to a differential accessibility to SDS. The differential SDS accessibility may be due to differences in thylakoid protein-protein and/or protein-lipid interactions.