Mass spectrometry footprinting reveals the structural rearrangements of cyanobacterial orange carotenoid protein upon light activation

The orange carotenoid protein (OCP), a member of the family of blue light photoactive proteins, is required for efficient photoprotection in many cyanobacteria. Photoexcitation of the carotenoid in the OCP results in structural changes within the chromophore and the protein to give an active red form of OCP that is required for phycobilisome binding and consequent fluorescence quenching. We characterized the light-dependent structural changes by mass spectrometry-based carboxyl footprinting and found that an α helix in the N-terminal extension of OCP plays a key role in this photoactivation process. Although this helix is located on and associates with the outside of the β-sheet core in the C-terminal domain of OCP in the dark, photoinduced changes in the domain structure disrupt this interaction. We propose that this mechanism couples light-dependent carotenoid conformational changes to global protein conformational dynamics in favor of functional phycobilisome binding, and is an essential part of the OCP photocycle.     

Photoactivation and relaxation studies on the cyanobacterial orange carotenoid protein in the presence of copper ion

Photosynthesis starts with absorption of light energy by light-harvesting antenna complexes with subsequent production of energy-rich organic compounds. However, all photosynthetic organisms face the challenge of excess photochemical conversion capacity. In cyanobacteria, non-photochemical quenching (NPQ) performed by the orange carotenoid protein (OCP) is one of the most important mechanisms to regulate the light energy captured by light-harvesting antennas. This regulation permits the cell to meet its cellular energy requirements and at the same time protects the photosynthetic apparatus under fluctuating light conditions. Several reports have revealed that thermal dissipation increases under excess copper in plants. To explore the effects and mechanisms of copper on cyanobacteria NPQ, photoactivation and relaxation of OCP in the presence of copper were examined in this communication. When OCP^o (OCP at orange state) is converted into OCP^r(OCP at red state), copper ion has no effect on the photoactivation kinetics. Relaxation of OCP^r to OCP^o, however, is largely delayed—almost completely blocked, in the presence of copper. Even the addition of the fluorescence recovery protein (FRP) cannot activate the relaxation process. Native polyacrylamide gel electrophoresis (PAGE) analysis result indicates the heterogeneous population of Cu²⁺-locked OCP^r. The Cu²⁺-OCP binding constant was estimated using a hyperbolic binding curve. Functional roles of copper-binding OCP in vivo are discussed.     

Photoprotection in cyanobacteria: the orange carotenoid protein (OCP)-related non-photochemical-quenching mechanism

Plants and algae have developed multiple protective mechanisms to survive under high light conditions. Thermal dissipation of excitation energy in the membrane-bound chlorophyll-antenna of photosystem II (PSII) decreases the energy arriving at the reaction center and thus reduces the generation of toxic photo-oxidative species. This process results in a decrease of PSII-related fluorescence emission, known as non-photochemical quenching (NPQ). It has always been assumed that cyanobacteria, the progenitor of the chloroplast, lacked an equivalent photoprotective mechanism. Recently, however, evidence has been presented for the existence of at least three distinct mechanisms for dissipating excess absorbed energy in cyanobacteria. One of these mechanisms, characterized by a blue-light-induced fluorescence quenching, is related to the phycobilisomes, the extramembranal antenna of cyanobacterial PSII. In this photoprotective mechanism the soluble carotenoid-binding protein (OCP) encoded by the slr1963 gene in Synechocystis sp. PCC 6803, of previously unknown function, plays an essential role. The amount of energy transferred from the phycobilisomes to the photosystems is reduced and the OCP acts as the photoreceptor and as the mediator of this antenna-related process. These are novel roles for a soluble carotenoid protein.     

Biophysical modeling of in vitro and in vivo processes underlying regulated photoprotective mechanism in cyanobacteria

Non-photochemical quenching (NPQ) is a mechanism responsible for high light tolerance in photosynthetic organisms. In cyanobacteria, NPQ is realized by the interplay between light-harvesting complexes, phycobilisomes (PBs), a light sensor and effector of NPQ, the photoactive orange carotenoid protein (OCP), and the fluorescence recovery protein (FRP). Here, we introduced a biophysical model, which takes into account the whole spectrum of interactions between PBs, OCP, and FRP and describes the experimental PBs fluorescence kinetics, unraveling interaction rate constants between the components involved and their relative concentrations in the cell. We took benefit from the possibility to reconstruct the photoprotection mechanism and its parts in vitro, where most of the parameters could be varied, to develop the model and then applied it to describe the NPQ kinetics in the Synechocystis sp. PCC 6803 mutant lacking photosystems. Our analyses revealed  that while an excess of the OCP over PBs is required to obtain substantial PBs fluorescence quenching in vitro, in vivo the OCP/PBs ratio is less than unity, due to higher local concentration of PBs, which was estimated as ~10^?5 M, compared to in vitro experiments. The analysis of PBs fluorescence recovery on the basis of the generalized model of enzymatic catalysis resulted in determination of the FRP concentration in vivo close to 10% of the OCP concentration. Finally, the possible role of the FRP oligomeric state alteration in the kinetics of PBs fluorescence was shown. This paper provides the most comprehensive model of the OCP-induced PBs fluorescence quenching to date and the results are important for better understanding of the regulatory molecular mechanisms underlying NPQ in cyanobacteria.

     

Interaction of the signaling state analog and the apoprotein form of the orange carotenoid protein with the fluorescence recovery protein

Photoprotection in cyanobacteria relies on the interplay between the orange carotenoid protein (OCP) and the fluorescence recovery protein (FRP) in a process termed non-photochemical quenching, NPQ. Illumination with blue-green light converts OCP from the basic orange state (OCP^O) into the red-shifted, active state (OCP^R) that quenches phycobilisome (PBs) fluorescence to avoid excessive energy flow to the photosynthetic reaction centers. Upon binding of FRP, OCP^R is converted to OCP^O and dissociates from PBs; however, the mode and site of OCP^R/FRP interactions remain elusive. Recently, we have introduced the purple OCP^W288A mutant as a competent model for the signaling state OCP^R (Sluchanko et al., Biochim Biophys Acta 1858:1–11, 2017). Here, we have utilized fluorescence labeling of OCP at its native cysteine residues to generate fluorescent OCP proteins for fluorescence correlation spectroscopy (FCS). Our results show that OCP^W288A has a 1.6(±0.4)-fold larger hydrodynamic radius than OCP^O, supporting the hypothesis of domain separation upon OCP photoactivation. Whereas the addition of FRP did not change the diffusion behavior of OCP^O, a substantial compaction of the OCP^W288A mutant and of the OCP apoprotein was observed. These results show that sufficiently stable complexes between FRP and OCP^W288A or the OCP apoprotein are formed to be detected by FCS. 1:1 complex formation with a micromolar apparent dissociation constant between OCP apoprotein and FRP was confirmed by size-exclusion chromatography. Beyond the established OCP/FRP interaction underlying NPQ cessation, the OCP apoprotein/FRP interaction suggests a more general role of FRP as a scaffold protein for OCP maturation.     

Influence of zeaxanthin and echinenone binding on the activity of the Orange Carotenoid Protein

In most cyanobacteria high irradiance induces a photoprotective mechanism that downregulates photosynthesis by increasing thermal dissipation of the energy absorbed by the phycobilisome, the water-soluble antenna. The light activation of a soluble carotenoid protein, the Orange-Carotenoid-Protein (OCP), binding hydroxyechinenone, a keto carotenoid, is the key inducer of this mechanism. Light causes structural changes within the carotenoid and the protein, leading to the conversion of a dark orange form into a red active form. Here, we tested whether echinenone or zeaxanthin can replace hydroxyechinenone in a study in which the nature of the carotenoid bound to the OCP was genetically changed. In a mutant lacking hydroxyechinenone and echinenone, the OCP was found to bind zeaxanthin but the stability of the binding appeared to be lower and light was unable to photoconvert the dark form into a red active form. Moreover, in the strains containing zeaxanthin-OCP, blue-green light did not induce the photoprotective mechanism. In contrast, in mutants in which echinenone is bound to the OCP, the protein is photoactivated and photoprotection is induced. Our results strongly suggest that the presence of the carotenoid carbonyl group that distinguishes echinenone and hydroxyechinenone from zeaxanthin is essential for the OCP activity.     

Light-induced energy dissipation in iron-starved cyanobacteria: roles of OCP and IsiA proteins

In response to iron deficiency, cyanobacteria synthesize the iron stress-induced chlorophyll binding protein IsiA. This protein protects cyanobacterial cells against iron stress. It has been proposed that the protective role of IsiA is related to a blue light-induced nonphotochemical fluorescence quenching (NPQ) mechanism. In iron-replete cyanobacterial cell cultures, strong blue light is known to induce a mechanism that dissipates excess absorbed energy in the phycobilisome, the extramembranal antenna of cyanobacteria. In this photoprotective mechanism, the soluble Orange Carotenoid Protein (OCP) plays an essential role. Here, we demonstrate that in iron-starved cells, blue light is unable to quench fluorescence in the absence of the phycobilisomes or the OCP. By contrast, the absence of IsiA does not affect the induction of fluorescence quenching or its recovery. We conclude that in cyanobacteria grown under iron starvation conditions, the blue light-induced nonphotochemical quenching involves the phycobilisome OCP-related energy dissipation mechanism and not IsiA. IsiA, however, does seem to protect the cells from the stress generated by iron starvation, initially by increasing the size of the photosystem I antenna. Subsequently, the IsiA converts the excess energy absorbed by the phycobilisomes into heat through a mechanism different from the dynamic and reversible light-induced NPQ processes.     

Site of non-photochemical quenching of the phycobilisome by orange carotenoid protein in the cyanobacterium Synechocystis sp. PCC 6803

In cyanobacteria, the thermal dissipation of excess absorbed energy at the level of the phycobilisome (PBS)-antenna is triggered by absorption of strong blue-green light by the photoactive orange carotenoid protein (OCP). This process known as non-photochemical quenching, whose molecular mechanism remains in many respects unclear, is revealed in vivo as a decrease in phycobilisome fluorescence. In vitro reconstituted system on the interaction of the OCP and the PBS isolated from the cyanobacterium Synechocystis sp. PCC 6803 presents evidence that the OCP is not only a photosensor, but also an effecter that makes direct contacts with the PBS and causes dissipation of absorbed energy. To localize the site(s) of quenching, we have analyzed the role of chromophorylated polypeptides of the PBS using PBS-deficient mutants in conjunction with in vitro systems of assembled PBS and of isolated components of the PBS core. The results demonstrated that L(CM), the core-membrane linker protein and terminal emitter of the PBS, could act as the docking site for OCP in vitro. The ApcD and ApcF terminal emitters of the PBS core are not directly subjected to quenching. The data suggests that there could be close contact between the phycocyanobilin chromophore of L(CM) and the 3'-hydroxyechinenone chromophore present in OCP and that L(CM) could be involved in OCP-induced quenching. According to the reduced average life-time of the PBS-fluorescence and linear dependence of fluorescence intensity of the PBS on OCP concentration, the quenching has mostly dynamic character. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.     

Essential role of two tyrosines and two tryptophans on the photoprotection activity of the Orange Carotenoid Protein

Photosynthetic organisms have developed photoprotective mechanisms to protect themselves from lethal high light intensities. One of these mechanisms involves the dissipation of excess absorbed light energy into heat. In cyanobacteria, light activation of a soluble carotenoid protein, the Orange Carotenoid Protein (OCP), binding a keto carotenoid, is the key inducer of this mechanism. Blue-green light absorption triggers structural changes within the carotenoid and the protein, leading to the conversion of a dark orange form into a red active form. Here we report the role in photoconversion and photoprotection of individual conserved tyrosines and tryptophans surrounding the rings of the carotenoid. Our results demonstrate that the interaction between the keto group of the carotenoid and Tyr201 and Trp288 is essential for OCP photoactivity. In addition, these amino acids are responsible for carotenoid affinity and specificity. We have already demonstrated that the aromatic character of Tyr44 and Trp110 interacting with the hydroxyl ring is critical. Here we show that the replacement of Tyr44 by Ser affects the stability of the red form avoiding its accumulation at any temperature, while Trp110Ser is affected in the energy necessary to the orange to red conversion and in the interaction with the antenna. Collectively our data support the idea that the red form is essential for photoprotection but not sufficient. Specific conformational changes occurring in the protein seem to be critical to the events leading to energy dissipation.     

Structural Determinants Underlying Photoprotection in the Photoactive Orange Carotenoid Protein of Cyanobacteria

The photoprotective processes of photosynthetic organisms involve the dissipation of excess absorbed light energy as heat. Photoprotection in cyanobacteria is mechanistically distinct from that in plants; it involves the orange carotenoid protein (OCP), a water-soluble protein containing a single carotenoid. The OCP is a new member of the family of blue light-photoactive proteins; blue-green light triggers the OCP-mediated photoprotective response. Here we report structural and functional characterization of the wild type and two mutant forms of the OCP, from the model organism Synechocystis PCC6803. The structural analysis provides high resolution detail of the carotenoid-protein interactions that underlie the optical properties of the OCP, unique among carotenoid-proteins in binding a single pigment per polypeptide chain. Collectively, these data implicate several key amino acids in the function of the OCP and reveal that the photoconversion and photoprotective responses of the OCP to blue-green light can be decoupled.     

The time course of non-photochemical quenching in phycobilisomes of Synechocystis sp. PCC6803 as revealed by picosecond time-resolved fluorimetry

As high-intensity solar radiation can lead to extensive damage of the photosynthetic apparatus, cyanobacteria have developed various protection mechanisms to reduce the effective excitation energy transfer (EET) from the antenna complexes to the reaction center. One of them is non-photochemical quenching (NPQ) of the phycobilisome (PB) fluorescence. In Synechocystis sp. PCC6803 this role is carried by the orange carotenoid protein (OCP), which reacts to high-intensity light by a series of conformational changes, enabling the binding of OCP to the PBs reducing the flow of energy into the photosystems. In this paper the mechanisms of energy migration in two mutant PB complexes of Synechocystis sp. were investigated and compared. The mutant CK is lacking phycocyanin in the PBs while the mutant ΔPSI/PSII does not contain both photosystems. Fluorescence decay spectra with picosecond time resolution were registered using a single photon counting technique. The studies were performed in a wide range of temperatures — from 4 to 300 K. The time course of NPQ and fluorescence recovery in darkness was studied at room temperature using both steady-state and time-resolved fluorescence measurements. The OCP induced NPQ has been shown to be due to EET from PB cores to the red form of OCP under photon flux densities up to 1000 μmol photons m^− 2 s^− 1. The gradual changes of the energy transfer rate from allophycocyanin to OCP were observed during the irradiation of the sample with blue light and consequent adaptation to darkness. This fact was interpreted as the revelation of intermolecular interaction between OCP and PB binding site. At low temperatures a significantly enhanced EET from allophycocyanin to terminal emitters has been shown, due to the decreased back transfer from terminal emitter to APC. The activation of OCP not only leads to fluorescence quenching, but also affects the rate constants of energy transfer as shown by model based analysis of the decay associated spectra. The results indicate that the ability of OCP to quench the fluorescence is strongly temperature dependent. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.     

The role of far-red spectral states in the energy regulation of phycobilisomes

The main light-harvesting pigment-protein complex of cyanobacteria and certain algae is the phycobilisome, which harvests sunlight and regulates the flow of absorbed energy to provide the photochemical reaction centres with a constant energy throughput. At least two light-driven mechanisms of excited energy quenching in phycobilisomes have been identified: the dominant mechanism in many strains of cyanobacteria depends on the orange carotenoid protein (OCP), while the second mechanism is intrinsically available to a phycobilisome and is possibly activated faster than the former. Recent single molecule spectroscopy studies have shown that far-red (FR) emission states are related to the OCP-dependent mechanism and it was proposed that the second mechanism may involve similar states. In this study, we examined the dynamics of simultaneously measured emission spectra and intensities from a large set of individual phycobilisome complexes from Synechocystis PCC 6803. Our results suggest a direct relationship between FR spectral states and thermal energy dissipating states and can be explained by a single phycobilin pigment in the phycobilisome core acting as the site of both quenching and FR emission likely due to the presence of a charge-transfer state. Our experimental method provides a means to accurately resolve the fluorescence lifetimes and spectra of the FR states, which enabled us to quantify a kinetic model that reproduces most of the experimentally determined properties of the FR states.     

Role of inter-domain cavity in the attachment of the orange carotenoid protein to the phycobilisome core and to the fluorescence recovery protein

Using molecular modeling and known spatial structure of proteins, we have derived a universal 3D model of the orange carotenoid protein (OCP) and phycobilisome (PBS) interaction in the process of non-photochemical PBS quenching. The characteristic tip of the phycobilin domain of the core-membrane linker polypeptide (L_CM) forms the attachment site on the PBS core surface for interaction with the central inter-domain cavity of the OCP molecule. This spatial arrangement has to be the most advantageous one because the L_CM, as the major terminal PBS-fluorescence emitter, accumulates energy from the most other phycobiliproteins within the PBS before quenching by OCP. In agreement with the constructed model, in cyanobacteria, the small fluorescence recovery protein is wedged in the OCP’s central cavity, weakening the PBS and OCP interaction. The presence of another one protein, the red carotenoid protein, in some cyanobacterial species, which also can interact with the PBS, also corresponds to this model.     

Oligomerization processes limit photoactivation and recovery of the orange carotenoid protein

The orange carotenoid protein (OCP) is a photoactive protein involved in cyanobacterial photoprotection by quenching of the excess of light-harvested energy. The photoactivation mechanism remains elusive, in part due to absence of data pertaining to the timescales over which protein structural changes take place. It also remains unclear whether or not oligomerization of the dark-adapted and light-adapted OCP could play a role in the regulation of its energy-quenching activity. Here, we probed photoinduced structural changes in OCP by a combination of static and time-resolved X-ray scattering and steady-state and transient optical spectroscopy in the visible range. Our results suggest that oligomerization partakes in regulation of the OCP photocycle, with different oligomers slowing down the overall thermal recovery of the dark-adapted state of OCP. They furthermore reveal that upon non-photoproductive excitation a numbed state forms, which remains in a non-photoexcitable structural state for at least ≈0.5 μs after absorption of a first photon.     

Synthetic OCP heterodimers are photoactive and recapitulate the fusion of two primitive carotenoproteins in the evolution of cyanobacterial photoprotection

The orange carotenoid protein (OCP) governs photoprotection in the majority of cyanobacteria. It is structurally and functionally modular, comprised of a C‐terminal regulatory domain (CTD), an N‐terminal effector domain (NTD) and a ketocarotenoid; the chromophore spans the two domains in the ground state and translocates fully into the NTD upon illumination. Using both the canonical OCP1 from Fremyella diplosiphon and the presumably more primitive OCP2 paralog from the same organism, we show that an NTD‐CTD heterodimer forms when the domains are expressed as separate polypeptides. The carotenoid is required for the heterodimeric association, assembling an orange complex which is stable in the dark. Both OCP1 and OCP2 heterodimers are photoactive, undergoing light‐driven heterodimer dissociation, but differ in their ability to reassociate in darkness, setting the stage for bioengineering photoprotection in cyanobacteria as well as for developing new photoswitches for biotechnology. Additionally, we reveal that homodimeric CTD can bind carotenoid in the absence of NTD, and name this truncated variant the C‐terminal domain‐like carotenoid protein (CCP). This finding supports the hypothesis that the OCP evolved from an ancient fusion event between genes for two different carotenoid‐binding proteins ancestral to the NTD and CTD. We suggest that the CCP and its homologs constitute a new family of carotenoproteins within the NTF2‐like superfamily found across all kingdoms of life.     

Non-photochemical quenching of fluorescence in cyanobacteria

The pathways of energy dissipation of excessive absorbed energy in cyanobacteria in comparison with that in higher plants are discussed. Two mechanisms of non-photochemical quenching in cyanobacteria are described. In one case this quenching occurs as light-induced decrease of the fluorescence yield of long-wavelength chlorophylls of the photosystem I trimers induced by inactive reaction centers: P700 cation-radical or P700 in triplet state. In the other case, non-photochemical quenching in cyanobacteria takes place with contribution of water-soluble protein OCP (containing 3′-hydroxyechinenone) that induces reversible quenching of allophycocyanin fluorescence in phycobilisomes. The possible evolutionary pathways of the involvement of carotenoid-binding proteins in non-photochemical quenching are discussed comparing the cyanobacterial OCP and plant PsbS protein. Published in Russian in Biokhimiya, 2007, Vol. 72, No. 10, pp. 1385–1395.     

Restoration of rapidly reversible photoprotective energy dissipation in the absence of PsbS protein by enhanced DeltapH

Variations in the light environment require higher plants to regulate the light harvesting process. Under high light a mechanism known as non-photochemical quenching (NPQ) is triggered to dissipate excess absorbed light energy within the photosystem II (PSII) antenna as heat, preventing photodamage to the reaction center. The major component of NPQ, known as qE, is rapidly reversible in the dark and dependent upon the transmembrane proton gradient (ΔpH), formed as a result of photosynthetic electron transport. Using diaminodurene and phenazine metasulfate, mediators of cyclic electron flow around photosystem I, to enhance ΔpH, it is demonstrated that rapidly reversible qE-type quenching can be observed in intact chloroplasts from Arabidopsis plants lacking the PsbS protein, previously believed to be indispensible for the process. The qE in chloroplasts lacking PsbS significantly quenched the level of fluorescence when all PSII reaction centers were in the open state (F(o) state), protected PSII reaction centers from photoinhibition, was modulated by zeaxanthin and was accompanied by the qE-typical absorption spectral changes, known as ΔA(535). Titrations of the ΔpH dependence of qE in the absence of PsbS reveal that this protein affects the cooperativity and sensitivity of the photoprotective process to protons. The roles of PsbS and zeaxanthin are discussed in light of their involvement in the control of the proton-antenna association constant, pK, via regulation of the interconnected phenomena of PSII antenna reorganization/aggregation and hydrophobicity.     

The orange carotenoid protein in photoprotection of photosystem II in cyanobacteria

Photoprotective mechanisms have evolved in photosynthetic organisms to cope with fluctuating light conditions. Under high irradiance, the production of dangerous oxygen species is stimulated and causes photo-oxidative stress. One of these photoprotective mechanisms, non photochemical quenching (qE), decreases the excess absorbed energy arriving at the reaction centers by increasing thermal dissipation at the level of the antenna. In this review we describe results leading to the discovery of this process in cyanobacteria (qE(cya)), which is mechanistically distinct from its counterpart in plants, and recent progress in the elucidation of this mechanism. The cyanobacterial photoactive soluble orange carotenoid protein is essential for the triggering of this photoprotective mechanism. Light induces structural changes in the carotenoid and the protein leading to the formation of a red active form. The activated red form interacts with the phycobilisome, the cyanobacterial light-harvesting antenna, and induces a decrease of the phycobilisome fluorescence emission and of the energy arriving to the reaction centers. The orange carotenoid protein is the first photoactive protein to be identified that contains a carotenoid as the chromophore. Moreover, its photocycle is completely different from those of other photoactive proteins. A second protein, called the Fluorescence Recovery Protein encoded by the slr1964 gene in Synechocystis PCC 6803, plays a key role in dislodging the red orange carotenoid protein from the phycobilisome and in the conversion of the free red orange carotenoid protein to the orange, inactive, form. This protein is essential to recover the full antenna capacity under low light conditions after exposure to high irradiance. This article is part of a Special Issue entitled: Photosystem II.     

Co-regulation of photosynthetic processes under potassium deficiency across CO<Subscript>2</Subscript> levels in soybean: mechanisms of limitations and adaptations

The slow kinetic phases of the chlorophyll a fluorescence transient (induction) are valuable tools in studying dynamic regulation of light harvesting, light energy distribution between photosystems, and heat dissipation in photosynthetic organisms. However, the origin of these phases are not yet fully understood. This is especially true in the case of prokaryotic oxygenic photoautotrophs, the cyanobacteria. To understand the origin of the slowest (tens of minutes) kinetic phase, the M–T fluorescence decline, in the context of light acclimation of these globally important microorganisms, we have compared spectrally resolved fluorescence induction data from the wild type Synechocystis sp. PCC 6803 cells, using orange (λ?=?593 nm) actinic light, with those of mutants, ΔapcD and ΔOCP, that are unable to perform either state transition or fluorescence quenching by orange carotenoid protein (OCP), respectively. Our results suggest a multiple origin of the M–T decline and reveal a complex interplay of various known regulatory processes in maintaining the redox homeostasis of a cyanobacterial cell. In addition, they lead us to suggest that a new type of regulatory process, operating on the timescale of minutes to hours, is involved in dissipating excess light energy in cyanobacteria.     

Features of protein−protein interactions in the cyanobacterial photoprotection mechanism

Photoprotective mechanisms of cyanobacteria are characterized by several features associated with the structure of their water-soluble antenna complexes–the phycobilisomes (PBs). During energy transfer from PBs to chlorophyll of photosystem reaction centers, the “energy funnel” principle is realized, which regulates energy flux due to the specialized interaction of the PBs core with a quenching molecule capable of effectively dissipating electron excitation energy into heat. The role of the quencher is performed by ketocarotenoid within the photoactive orange carotenoid protein (OCP), which is also a sensor for light flux. At a high level of insolation, OCP is reversibly photoactivated, and this is accompanied by a sig- nificant change in its structure and spectral characteristics. Such conformational changes open the possibility for pro- tein–protein interactions between OCP and the PBs core (i.e., activation of photoprotection mechanisms) or the fluores- cence recovery protein. Even though OCP was discovered in 1981, little was known about the conformation of its active form until recently, as well as about the properties of homologs of its N and C domains. Studies carried out during recent years have made a breakthrough in understanding of the structural-functional organization of OCP and have enabled discovery of new aspects of the regulation of photoprotection processes in cyanobacteria. This review focuses on aspects of protein–pro- tein interactions between the main participants of photoprotection reactions and on certain properties of representatives of newly discovered families of OCP homologs.