Assembly of photoactive orange carotenoid protein from its domains unravels a carotenoid shuttle mechanism

Elements of the C-terminal tail of a C-terminal domain homolog of the Orange Carotenoid Protein determining xanthophyll uptake from liposomes

Carotenoids perform multifaceted roles in life ranging from coloration over light harvesting to photoprotection. The Orange Carotenoid Protein (OCP), a light-driven photoswitch involved in cyanobacterial photoprotection, accommodates a ketocarotenoid vital for its function. OCP extracts its ketocarotenoid directly from membranes, or accepts it from homologs of its C-terminal domain (CTDH). The CTDH from Anabaena (AnaCTDH) was shown to be important for carotenoid transfer and delivery from/to membranes. The C-terminal tail of AnaCTDH is a critical structural element likely serving as a gatekeeper and facilitator of carotenoid uptake from membranes. We investigated the impact of amino acid substitutions within the AnaCTDH-CTT on echinenone and canthaxanthin uptake from DOPC and DMPG liposomes. The transfer rate was uniformly reduced for substitutions of Arg-137 and Arg-138 to Gln or Ala, and depended on the lipid type, indicating a weaker interaction particularly with the lipid head group. Our results further suggest that Glu-132 has a membrane-anchoring effect on the PC lipids, specifically at the choline motif as inferred from the strongly different effects of the CTT variants on the extraction from the two liposome types. The substitution of Pro-130 by Gly suggests that the CTT is perpendicular to both the membrane and the main AnaCTDH protein during carotenoid extraction. Finally, the simultaneous mutation of Leu-133, Leu-134 and Leu-136 for alanines showed that the hydrophobicity of the CTT is crucial for carotenoid uptake. Since some substitutions accelerated carotenoid transfer into AnaCTDH while others slowed it down, carotenoprotein properties can be engineered toward the requirements of applications.

Orange carotenoid proteins: structural understanding of evolution and function

Cyanobacteria uniquely contain a primitive water-soluble carotenoprotein, the orange carotenoid protein (OCP). Nearly all extant cyanobacterial genomes contain genes for the OCP or its homologs, implying an evolutionary constraint for cyanobacteria to conserve its function. Genes encoding the OCP and its two constituent structural domains, the N-terminal domain, helical carotenoid proteins (HCPs), and its C-terminal domain, are found in the most basal lineages of extant cyanobacteria. These three carotenoproteins exemplify the importance of the protein for carotenoid properties, including protein dynamics, in response to environmental changes in facilitating a photoresponse and energy quenching. Here, we review new structural insights for these carotenoproteins and situate the role of the protein in what is currently understood about their functions.

Insights into the molecular mechanism of yellow cuticle coloration by a chitin-binding carotenoprotein in gregarious locusts

Carotenoids are hydrophobic pigments binding to diverse carotenoproteins, many of which remain unexplored. Focusing on yellow gregarious locusts accumulating cuticular carotenoids, here we use engineered Escherichia coli cells to reconstitute a functional water-soluble β-carotene-binding protein, BBP. HPLC and Raman spectroscopy confirmed that recombinant BBP avidly binds β-carotene, inducing the unusual vibronic structure of its absorbance spectrum, just like native BBP extracted from the locust cuticles. Bound to recombinant BBP, β-carotene exhibits pronounced circular dichroism and allows BBP to withstand heating (T0.5 = 68 °C), detergents and pH variations. Using bacteria producing distinct xanthophylls we demonstrate that, while β-carotene is the preferred carotenoid, BBP can also extract from membranes ketocarotenoids and, very poorly, hydroxycarotenoids. We show that BBP-carotenoid complex reversibly binds to chitin, but not to chitosan, implying the role for chitin acetyl groups in cuticular BBP deposition. Reconstructing such locust coloration mechanism in vitro paves the way for structural studies and BBP applications.

Insights into energy quenching mechanisms and carotenoid uptake by orange carotenoid protein homologs: HCP4 and CTDH

Photodamage to the photosynthetic apparatus by excessive light radiation has led to the evolution of a variety of energy dissipation mechanisms. A mechanism that exists in some cyanobacterial species, enables non-photochemical quenching of excitation energy within the phycobilisome (PBS) antenna complex by the Orange Carotenoid Protein (OCP). The OCP contains an active N-terminal domain (NTD) and a regulatory C-terminal domain (CTD). Some cyanobacteria also have genes encoding for homologs to both the CTD (CTDH) and the NTD (referred to as helical carotenoid proteins, HCP). The CTDH facilitates uptake of carotenoids from the thylakoid membranes to be transferred to the HCPs. Holo-HCPs exhibit diverse functionalities such as carotenoid carriers, singlet oxygen quenchers, and in the case of HCP4, constitutive OCP-like energy quenching. Here, we present the first crystal structure of the holo-HCP4 binding canthaxanthin molecule and an improved structure of the apo-CTDH from Anabaena sp. PCC 7120. We propose here models of the binding of the HCP4 to the PBS and the associated energy quenching mechanism. Our results show that the presence of the carotenoid is essential for fluorescence quenching. We also examined interactions within OCP-like species, including HCP4 and CTDH, providing the basis for mechanisms of carotenoid transfer from CTDH to HCPs.

Structural framework for the understanding spectroscopic and functional signatures of the cyanobacterial Orange Carotenoid Protein families

The Orange Carotenoid Protein (OCP) is a unique photoreceptor crucial for cyanobacterial photoprotection. Best studied Synechocystis sp. PCC 6803 OCP belongs to the large OCP1 family. Downregulated by the Fluorescence Recovery Protein (FRP) in low-light, high-light-activated OCP1 binds to the phycobilisomes and performs non-photochemical quenching. Recently discovered families OCP2 and OCP3 remain structurally and functionally underexplored, and no systematic comparative studies have ever been conducted. Here we present two first crystal structures of OCP2 from morphoecophysiologically different cyanobacteria and provide their comprehensive structural, spectroscopic and functional comparison with OCP1, the recently described OCP3 and all-OCP ancestor. Structures enable correlation of spectroscopic signatures with the effective number of hydrogen and discovered here chalcogen bonds anchoring the ketocarotenoid in OCP, as well as with the rotation of the echinenone's β-ionone ring in the CTD. Structural data also helped rationalize the observed differences in OCP/FRP and OCP/phycobilisome functional interactions. These data are expected to foster OCP research and applications in optogenetics, targeted carotenoid delivery and cyanobacterial biomass engineering.

Lipid composition and properties affect protein-mediated carotenoid uptake efficiency from membranes

Carotenoids are pigments of diverse functions ranging from coloration over light-harvesting to photoprotection. Yet, the number of carotenoid-binding proteins, which mobilize these pigments in physiological media, is limited, and the mechanisms of carotenoid mobilization are still not well understood. The same applies for the determinants of carotenoid uptake from membranes into carotenoproteins, especially regarding the dependence on the chemical properties of membrane lipids. Here, we investigate xanthophyll uptake capacity and kinetics of a paradigmatic carotenoid-binding protein, the homolog of the Orange Carotenoid Protein's C-terminal domain from Anabaena sp. PCC 7120 (AnaCTDH), using liposomes formed from defined lipid species and loaded with canthaxanthin (CAN) and echinenone (ECN), respectively. Phospholipids with different chain length and degree of saturation were investigated. The composition of carotenoid-loaded liposomes directly affected the incorporation yield and storage ratio of CAN and ECN as well as the rate of carotenoid uptake by AnaCTDH. Generally, saturated PC lipids were identified as unsuitable, and a high phase transition temperature of the lipids negatively affected the carotenoid incorporation and storage yield. For efficient carotenoid transfer, the velocity increases with increasing chain length or membrane thickness. An average transfer yield of 93 % and 43 % were obtained for the formation of AnaCTDH(CAN) and AnaCTDH(ECN) holoproteins, respectively. In summary, the most suitable lipids for the formation of AnaCTDH(CAN/ECN) holoproteins by carotenoid transfer from artificial liposomes are phosphatidylcholine (18:1) and phosphatidylglycerol (14:0). Thus, these two lipids provide the best conditions for further investigation of lipid-protein interaction and the carotenoid uptake process.

Color-Tuning Mechanism of the Lit Form of Orange Carotenoid Protein

Orange carotenoid protein (OCP) of photosynthetic cyanobacteria binds to ketocarotenoids noncovalently and absorbs excess light to protect the host organism from light-induced oxidative damage. Herein, we found that mutating valine 40 in the α3 helix of Gloeocapsa sp. PCC 7513 (GlOCP1) resulted in blue- or red-shifts of 6-20 nm in the absorption maxima of the lit forms. We analyzed the origins of absorption maxima shifts by integrating X-ray crystallography, homology modeling, molecular dynamics simulations, and hybrid quantum mechanics/molecular mechanics calculations. Our analysis suggested that the single residue mutations alter the polar environment surrounding the bound canthaxanthin, thereby modulating the degree of charge transfer in the photoexcited state of the chromophore. Our integrated investigations reveal the mechanism of color adaptation specific to OCPs and suggest a design principle for color-specific photoswitches.

Structural basis for the ligand promiscuity of the neofunctionalized, carotenoid-binding fasciclin domain protein AstaP

Fasciclins (FAS1) are ancient adhesion protein domains with no common small ligand binding reported. A unique microalgal FAS1-containing astaxanthin (AXT)-binding protein (AstaP) binds a broad repertoire of carotenoids by a largely unknown mechanism. Here, we explain the ligand promiscuity of AstaP-orange1 (AstaPo1) by determining its NMR structure in complex with AXT and validating this structure by SAXS, calorimetry, optical spectroscopy and mutagenesis. α1-α2 helices of the AstaPo1 FAS1 domain embrace the carotenoid polyene like a jaw, forming a hydrophobic tunnel, too short to cap the AXT β-ionone rings and dictate specificity. AXT-contacting AstaPo1 residues exhibit different conservation in AstaPs with the tentative carotenoid-binding function and in FAS1 proteins generally, which supports the idea of AstaP neofunctionalization within green algae. Intriguingly, a cyanobacterial homolog with a similar domain structure cannot bind carotenoids under identical conditions. These structure-activity relationships provide the first step towards the sequence-based prediction of the carotenoid-binding FAS1 members.

Functional specialization of expanded orange carotenoid protein paralogs in subaerial Nostoc species

Orange carotenoid protein (OCP) is a photoactive protein that participates in the photoprotection of cyanobacteria. There are two full-length OCP proteins, four N-terminal paralogs (helical carotenoid protein, HCP), and one C-terminal domain-like carotenoid protein (CCP) found in Nostoc flagelliforme, a desert cyanobacterium. All HCPs (HCP1-3 and HCP6) from N. flagelliforme demonstrated their excellent singlet oxygen quenching activities, in which HCP2 was the strongest singlet oxygen quencher compared with others. Two OCPs, OCPx1 and OCPx2, were not involved in singlet oxygen scavenging; instead, they functioned as phycobilisome fluorescence quenchers. The fast-acting OCPx1 showed more effective photoactivation and stronger phycobilisome fluorescence quenching compared to OCPx2, which behaved differently from all reported OCP paralogs. The resolved crystal structure and mutant analysis revealed that Trp111 and Met125 play essential roles in OCPx2, which is dominant and long-acting. The resolved crystal structure of OCPx2 is maintained in a monomer state and showed more flexible regulation in energy quenching activities compared with the packed oligomer of OCPx1. The recombinant apo-CCP obtained the carotenoid pigment from holo-HCPs and holo-OCPx1 of N. flagelliforme. No such carotenoid transferring processes were observed between apo-CCP and holo-OCPx2. The close phylogenetic relationship of OCP paralogs from subaerial Nostoc species indicates an adaptive evolution toward development of photoprotection: protecting cellular metabolism against singlet oxygen damage using HCPs and against excess energy captured by active phycobilisomes using two different working modes of OCPx.

Stages of OCP-FRP Interactions in the Regulation of Photoprotection in Cyanobacteria, Part 2: Small-Angle Neutron Scattering with Partial Deuteration

We used small-angle neutron scattering partially coupled with size-exclusion chromatography to unravel the solution structures of two variants of the Orange Carotenoid Protein (OCP) lacking the N-terminal extension (OCP-ΔNTE) and its complex formation with the Fluorescence Recovery Protein (FRP). The dark-adapted, orange form OCP-ΔNTE^O is fully photoswitchable and preferentially binds the pigment echinenone. Its complex with FRP consists of a monomeric OCP component, which closely resembles the compact structure expected for the OCP ground state, OCP^O. In contrast, the pink form OCP-ΔNTE^P, preferentially binding the pigment canthaxanthin, is mostly nonswitchable. The pink OCP form appears to occur as a dimer and is characterized by a separation of the N- and C-terminal domains, with the canthaxanthin embedded only into the N-terminal domain. Therefore, OCP-ΔNTE^P can be viewed as a prototypical model system for the active, spectrally red-shifted state of OCP, OCP^R. The dimeric structure of OCP-ΔNTE^P is retained in its complex with FRP. Small-angle neutron scattering using partially deuterated OCP-FRP complexes reveals that FRP undergoes significant structural changes upon complex formation with OCP. The observed structures are assigned to individual intermediates of the OCP photocycle in the presence of FRP.

Protein-Mediated Carotenoid Delivery Suppresses the Photoinducible Oxidation of Lipofuscin in Retinal Pigment Epithelial Cells

Lipofuscin of retinal pigment epithelium (RPE) cells is a complex heterogeneous system of chromophores which accumulates as granules during the cell's lifespan. Lipofuscin serves as a source of various cytotoxic effects linked with oxidative stress. Several age-related eye diseases such as macular degeneration of the retina, as well as some severe inherited eye pathologies, are accompanied by a significant increase in lipofuscin granule concentration. The accumulation of carotenoids in the RPE could provide an effective antioxidant protection against lipofuscin cytotoxic manifestations. Given the highly lipophilic nature of carotenoids, their targeted delivery to the vulnerable tissues can potentially be assisted by special proteins. In this study, we demonstrate how protein-mediated delivery of zeaxanthin using water-soluble Bombyx mori carotenoid-binding protein (BmCBP-ZEA) suppresses the photoinducible oxidative stress in RPE cells caused by irradiation of lipofuscin with intense white light. We implemented fluorescence lifetime imaging of the RPE cell culture ARPE-19 fed with lipofuscin granules and then irradiated by white light with and without the addition of BmCBP-ZEA. We demonstrate that after irradiation the mean fluorescence lifetime of lipofuscin significantly increases, while the presence of BmCBP-ZEA at 200 nM concentration suppresses the increase in the average lifetime of lipofuscin fluorescence, indicating an approx. 35% inhibition of the oxidative stress. This phenomenon serves as indirect yet important evidence of the efficiency of the protein-mediated carotenoid delivery into pigment epithelium cells.

Parameterization of a single H-bond in Orange Carotenoid Protein by atomic mutation reveals principles of evolutionary design of complex chemical photosystems

Introduction: Dissecting the intricate networks of covalent and non-covalent interactions that stabilize complex protein structures is notoriously difficult and requires subtle atomic-level exchanges to precisely affect local chemical functionality. The function of the Orange Carotenoid Protein (OCP), a light-driven photoswitch involved in cyanobacterial photoprotection, depends strongly on two H-bonds between the 4-ketolated xanthophyll cofactor and two highly conserved residues in the C-terminal domain (Trp288 and Tyr201). Method: By orthogonal translation, we replaced Trp288 in Synechocystis OCP with 3-benzothienyl-L-alanine (BTA), thereby exchanging the imino nitrogen for a sulphur atom. Results: Although the high-resolution (1.8 Å) crystal structure of the fully photoactive OCP-W288_BTA protein showed perfect isomorphism to the native structure, the spectroscopic and kinetic properties changed distinctly. We accurately parameterized the effects of the absence of a single H-bond on the spectroscopic and thermodynamic properties of OCP photoconversion and reveal general principles underlying the design of photoreceptors by natural evolution. Discussion: Such "molecular surgery" is superior over trial-and-error methods in hypothesis-driven research of complex chemical systems.

Silkworm carotenoprotein as an efficient carotenoid extractor, solubilizer and transporter

Found in many organisms, water-soluble carotenoproteins are prospective antioxidant nanocarriers for biomedical applications. Yet, the toolkit of characterized carotenoproteins is rather limited: such proteins are either too specific binders of only few different carotenoids, or their ability to transfer carotenoids to various acceptor systems is unknown. Here, by focusing on a recently characterized recombinant ~27-kDa Carotenoid-Binding Protein from Bombyx mori (BmCBP) [Slonimskiy et al., International Journal of Biological Macromolecules 214 (2022): 664-671], we analyze its carotenoid-binding repertoire and potential as a carotenoid delivery module. We show that BmCBP forms productive complexes with both hydroxyl- and ketocarotenoids - lutein, zeaxanthin, astaxanthin, canthaxanthin and a smaller antioxidant, aporhodoxanthinone, but not with β-carotene or retinal, which defines its broad ligand specificity toward xanthophylls valuable to human health. Moreover, the His-tagged BmCBP apoform is capable of cost-efficient and scalable enrichment of xanthophylls from various crude methanolic herbal extracts. Upon carotenoid binding, BmCBP remains monomeric and shows a remarkable ability to dynamically shuttle carotenoids to biological membrane models and to unrelated carotenoproteins, which in particular makes from the cyanobacterial Orange Carotenoid Protein a blue-light controlled photoswitch. Furthermore, administration of BmCBP loaded by zeaxanthin stimulates fibroblast growth, which is attractive for cell- and tissue-based assays.

A primordial Orange Carotenoid Protein: Structure, photoswitching activity and evolutionary aspects

Cyanobacteria are photosynthesizing prokaryotes responsible for the Great Oxygenation Event on Earth ~2.5 Ga years ago. They use a specific photoprotective mechanism based on the 35-kDa photoactive Orange Carotenoid Protein (OCP), a promising target for developing novel optogenetic tools and for biomass engineering. The two-domain OCP presumably stems from domain fusion, yet the primitive thylakoid-less cyanobacteria Gloeobacter encodes a complete OCP. Its photosynthesis regulation lacks the so-called Fluorescence Recovery Protein (FRP), which in Synechocystis inhibits OCP-mediated phycobilisome fluorescence quenching, and Gloeobacter OCP belongs to the recently defined, heterogeneous clade OCPX (GlOCPX), the least characterized compared to OCP2 and especially OCP1 clades. Here, we describe the first crystal structure of OCPX, which explains unique functional adaptations of Gloeobacter OCPX compared to OCP1 from Synechocystis. We show that monomeric GlOCPX exploits a remarkable intramolecular locking mechanism stabilizing its dark-adapted state and exhibits drastically accelerated, less temperature-dependent recovery after photoactivation. While GlOCPX quenches Synechocystis phycobilisomes similar to Synechocystis OCP1, it evades interaction with and regulation by FRP from other species and likely uses alternative mechanisms for fluorescence recovery. This analysis of a primordial OCPX sheds light on its evolution, rationalizing renaming and subdivision of the OCPX clade into subclades - OCP3a, OCP3b, OCP3c.

Modulation of Membrane Microviscosity by Protein-Mediated Carotenoid Delivery as Revealed by Time-Resolved Fluorescence Anisotropy

Carotenoids are potent antioxidants with a wide range of biomedical applications. However, their delivery into human cells is challenging and relatively inefficient. While the use of natural water-soluble carotenoproteins capable to reversibly bind carotenoids and transfer them into membranes is promising, the quantitative estimation of the delivery remains unclear. In the present work, we studied echinenone (ECN) delivery by cyanobacterial carotenoprotein AnaCTDH (C-terminal domain homolog of the Orange Carotenoid Protein from Anabaena), into liposome membranes labelled with BODIPY fluorescent probe. We observed that addition of AnaCTDH-ECN to liposomes led to the significant changes in the fast-kinetic component of the fluorescence decay curve, pointing on the dipole-dipole interactions between the probe and ECN within the membrane. It may serve as an indirect evidence of ECN delivery into membrane. To study the delivery in detail, we carried out molecular dynamics modeling of the localization of ECN within the lipid bilayer and calculate its orientation factor. Next, we exploited FRET to assess concentration of ECN delivered by AnaCTDH. Finally, we used time-resolved fluorescence anisotropy to assess changes in microviscosity of liposomal membranes. Incorporation of liposomes with β-carotene increased membrane microviscosity while the effect of astaxanthin and its mono- and diester forms was less pronounced. At temperatures below 30 °C addition of AnaCTDH-ECN increased membrane microviscosity in a concentration-dependent manner, supporting the protein-mediated carotenoid delivery mechanism. Combining all data, we propose FRET-based analysis and assessment of membrane microviscosity as potent approaches to characterize the efficiency of carotenoids delivery into membranes.

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 photo-induced 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.

Reconstitution of the functional carotenoid-binding protein from silkworm in E. coli

Natural water-soluble carotenoproteins are promising antioxidant nanocarriers for biomedical applications. The Carotenoid-Binding Protein from silkworm Bombyx mori (BmCBP) is responsible for depositing carotenoids in cocoons. This determines the silk coloration, which is relevant for sericulture for four thousand years. While BmCBP function is well-characterized by molecular genetics, its structure and carotenoid-binding mechanism remain to be studied. To facilitate this, here we report on successful production of soluble BmCBP in Escherichia coli, its purification and characterization. According to CD spectroscopy and SEC-MALS, this protein folds into a ~ 27-kDa monomer capable of dose-dependent binding of lutein, a natural BmCBP ligand, in vitro. Binding leads to a >10 nm red-shift of the carotenoid absorbance and quenches tryptophan fluorescence of BmCBP. Using zeaxanthin, a close lutein isomer that can be stably produced in engineered E.coli strains, we successfully reconstitute the BmCBP holoform and characterize its properties. While BmCBP successfully matures into the holoform, BmCBP-zeaxanthin complexes are contaminated by the apoform. We demonstrate that the yield of the holoform can be increased by adding bovine serum albumin during cell lysis and that the remaining BmCBP apoform is efficiently removed using hydroxyapatite chromatography. Bacterial production of BmCBP paves the way for its structural studies and applications.

A synthetic switch based on orange carotenoid protein to control blue-green light responses in chloroplasts

Synthetic biology approaches to engineer light-responsive systems are widely used, but their applications in plants are still limited due to the interference with endogenous photoreceptors and the intrinsic requirement of light for photosynthesis. Cyanobacteria possess a family of soluble carotenoid-associated proteins named orange carotenoid proteins (OCPs) that, when activated by blue-green light, undergo a reversible conformational change that enables the photoprotection mechanism that occurs on the phycobilisome. Exploiting this system, we developed a chloroplast-localized synthetic photoswitch based on a protein complementation assay where two nanoluciferase fragments were fused to separate polypeptides corresponding to the OCP2 domains. Since Arabidopsis (Arabidopsis thaliana) does not possess the prosthetic group needed for the assembly of the OCP2 complex, we first implemented the carotenoid biosynthetic pathway with a bacterial β-carotene ketolase enzyme (crtW) to generate keto-carotenoid-producing plants. The photoswitch was tested and characterized in Arabidopsis protoplasts and stably transformed plants with experiments aimed to uncover its regulation by a range of light intensities, wavelengths, and its conversion dynamics. Finally, we applied the OCP-based photoswitch to control transcriptional responses in chloroplasts in response to green light illumination by fusing the two OCP fragments with the plastidial SIGMA FACTOR 2 and bacteriophage T4 anti-sigma factor AsiA. This pioneering study establishes the basis for future implementation of plastid optogenetics to regulate organelle responses upon exposure to specific light spectra.

Development of a General PAT Strategy for Online Monitoring of Complex Mixtures—On the Example of Natural Product Extracts from Bearberry Leaf (Arctostaphylos uva-ursi)

For the first time, a universally applicable and methodical approach from characterization to a PAT concept for complex mixtures is conducted—exemplified on natural products extraction processes. Bearberry leaf (Arctostaphylos uva-ursi) extract is chosen as an example of a typical complex mixture of natural plant origin and generalizable in its composition. Within the quality by design (QbD) based process development the development and implementation of a concept for process analytical technology (PAT), a key enabling technology, is the next necessary step in risk and quality-based process development and operation. To obtain and provide an overview of the broad field of PAT, the development process is shown on the example of a complex multi-component plant extract. This study researches the potential of different process analytical technologies for online monitoring of different component groups and classifies their possible applications within the framework of a QbD-based process. Offline and online analytics are established on the basis of two extraction runs. Based on this data set, PLS models are created for the spectral data, and correlations are conducted for univariate data. In a third run, the prediction potential is researched. Conclusively, the results of this study are arranged in the concept of a holistic quality and risk-based process design and operation concept.

Microalgal protein AstaP is a potent carotenoid solubilizer and delivery module with a broad carotenoid binding repertoire

Carotenoids are lipophilic substances with many biological functions, from coloration to photoprotection. Being potent antioxidants, carotenoids have multiple biomedical applications, including the treatment of neurodegenerative disorders and retina degeneration. Nevertheless, the delivery of carotenoids is substantially limited by their poor solubility in the aqueous phase. Natural water-soluble carotenoproteins can facilitate this task, necessitating studies on their ability to uptake and deliver carotenoids. One such promising carotenoprotein, AstaP (astaxanthin-binding protein), was recently identified in eukaryotic microalgae, but its structure and functional properties remained largely uncharacterized. By using a correctly folded recombinant protein, here we show that AstaP is an efficient carotenoid solubilizer that can stably bind not only astaxanthin but also zeaxanthin, canthaxanthin, and, to a lesser extent, β-carotene, that is, carotenoids especially valuable to human health. AstaP accepts carotenoids provided as acetone solutions or embedded in membranes, forming carotenoid-protein complexes with an apparent stoichiometry of 1:1. We successfully produced AstaP holoproteins in specific carotenoid-producing strains of Escherichia coli, proving it is amenable to cost-efficient biotechnology processes. Regardless of the carotenoid type, AstaP remains monomeric in both apo- and holoform, while its rather minimalistic mass (~ 20 kDa) makes it an especially attractive antioxidant delivery module. In vitro, AstaP transfers different carotenoids to liposomes and to unrelated proteins from cyanobacteria, which can modulate their photoactivity and/or oligomerization. These findings expand the toolkit of the characterized carotenoid binding proteins and outline the perspective of the use of AstaP as a unique monomeric antioxidant nanocarrier with an extensive carotenoid binding repertoire.

Role of hydrogen bond alternation and charge transfer states in photoactivation of the Orange Carotenoid Protein

Here, we propose a possible photoactivation mechanism of a 35-kDa blue light-triggered photoreceptor, the Orange Carotenoid Protein (OCP), suggesting that the reaction involves the transient formation of a protonated ketocarotenoid (oxocarbenium cation) state. Taking advantage of engineering an OCP variant carrying the Y201W mutation, which shows superior spectroscopic and structural properties, it is shown that the presence of Trp201 augments the impact of one critical H-bond between the ketocarotenoid and the protein. This confers an unprecedented homogeneity of the dark-adapted OCP state and substantially increases the yield of the excited photoproduct S*, which is important for the productive photocycle to proceed. A 1.37 Å crystal structure of OCP Y201W combined with femtosecond time-resolved absorption spectroscopy, kinetic analysis, and deconvolution of the spectral intermediates, as well as extensive quantum chemical calculations incorporating the effect of the local electric field, highlighted the role of charge-transfer states during OCP photoconversion.

Yaroshevich et al. present a chemical reaction mechanism of a 35-kDa blue light-triggered photoreceptor, the Orange Carotenoid Protein (OCP). They find that photoactivation critically involves the transient formation of a protonated ketocarotenoid (oxocarbenium cation) state. This study suggests the role of charge-transfer states during OCP photoconversion.

Integrated Structural Studies for Elucidating Carotenoid-Protein Interactions

Carotenoids are ancient pigment molecules that, when associated with proteins, have a tremendous range of functional properties. Unlike most protein prosthetic groups, there are no recognizable primary structure motifs that predict carotenoid binding, hence the structural details of their amino acid interactions in proteins must be worked out empirically. Here we describe our recent efforts to combine complementary biophysical methods to elucidate the precise details of protein-carotenoid interactions in the Orange Carotenoid Protein and its evolutionary antecedents, the Helical Carotenoid Proteins (HCPs), CTD-like carotenoid proteins (CCPs).

Sub-lithic photosynthesis in hot desert habitats

In hyper-arid soil environments, photosynthetic microorganisms are largely restricted to hypolithic (sub-lithic) habitats: i.e., on the ventral surfaces of translucent pebbles in desert pavements. Here, we combined fluorometric, spectroscopic, biochemical and metagenomic approaches to investigate in situ the light transmission properties of quartz stones in the Namib Desert, and assess the photosynthetic activity of the underlying hypolithic cyanobacterial biofilms. Quartz pebbles greatly reduced the total photon flux to the ventral surface biofilms and filtered out primarily the short wavelength portion of the solar spectrum. Chlorophylls d and f were not detected in biofilm pigment extracts; however, hypolithic cyanobacterial communities showed some evidence of adaptation to sub-lithic conditions, including the prevalence of genes encoding Helical Carotenoid Proteins, which are associated with desiccation stress. Under water-saturated conditions, hypolithic communities showed no evidence of light stress, even when the quartz stones were exposed to full midday sunlight. This initial study creates a foundation for future in-situ and laboratory exploration of various adaptation mechanisms employed by photosynthetic organisms forming hypolithic microbial communities.

NMR resonance assignment and backbone dynamics of a C-terminal domain homolog of orange carotenoid protein

Photoprotection in cyanobacteria is mediated by the Orange Carotenoid Protein (OCP), a two-domain photoswitch which has multiple natural homologs of its N- and C-terminal domains. Recently, it was demonstrated that C-terminal domain homologs (CTDHs) of OCP are standalone carotenoproteins participating in multidirectional carotenoid transfer between membranes and proteins. Non-covalent embedment of a ketocarotenoid causes dimerization of the small 16-kDa water-soluble CTDH protein; however, dynamic interactions of CTDH with membranes and other proteins apparently require the monomeric state. Although crystallography recently provided static snapshots of the Anabaena CTDH (AnaCTDH) spatial structure in the apo-form, which predicted mobility of some putative functional segments, no crystallographic information on the holo-form of CTDH is presently available. In order to use NMR techniques to cope with the dynamics of the AnaCTDH protein, it was necessary to obtain 1H, 13C and 15N resonance assignments. AnaCTDH samples enriched with 13C and 15N isotopes were prepared using recombinant protein expression, and NMR resonance assignment was achieved for more than 90% of the residues. The obtained results revealed that the structure of AnaCTDH in solution and in the crystal are largely equivalent. Together with 15N NMR relaxation experiments, our data shed light on the AnaCTDH dynamics and provide the platform for the subsequent analysis of the holo-CTDH structure in solution, for the better understanding of light-triggered protein-protein interactions and the development of antioxidant nanocarriers for biomedical applications in the future.

Structural analysis of a new carotenoid-binding protein: the C-terminal domain homolog of the OCP

The Orange Carotenoid Protein (OCP) is a water-soluble protein that governs photoprotection in many cyanobacteria. The 35 kDa OCP is structurally and functionally modular, consisting of an N-terminal effector domain (NTD) and a C-terminal regulatory domain (CTD); a carotenoid spans the two domains. The CTD is a member of the ubiquitous Nuclear Transport Factor-2 (NTF2) superfamily (pfam02136). With the increasing availability of cyanobacterial genomes, bioinformatic analysis has revealed the existence of a new family of proteins, homologs to the CTD, the C-terminal domain-like carotenoid proteins (CCPs). Here we purify holo-CCP2 directly from cyanobacteria and establish that it natively binds canthaxanthin (CAN). We use small-angle X-ray scattering (SAXS) to characterize the structure of this carotenoprotein in two distinct oligomeric states. A single carotenoid molecule spans the two CCPs in the dimer. Our analysis with X-ray footprinting-mass spectrometry (XFMS) identifies critical residues for carotenoid binding that likely contribute to the extreme red shift (ca. 80 nm) of the absorption maximum of the carotenoid bound by the CCP2 dimer and a further 10 nm shift in the tetramer form. These data provide the first structural description of carotenoid binding by a protein consisting of only an NTF2 domain.

Soluble Cyanobacterial Carotenoprotein as a Robust Antioxidant Nanocarrier and Delivery Module

To counteract oxidative stress, antioxidants including carotenoids are highly promising, yet their exploitation is drastically limited by the poor bioavailability and fast photodestruction, whereas current delivery systems are far from being efficient. Here we demonstrate that the recently discovered nanometer-sized water-soluble carotenoprotein from Anabaena sp. PCC 7120 (termed AnaCTDH) transiently interacts with liposomes to efficiently extract carotenoids via carotenoid-mediated homodimerization, yielding violet–purple protein samples. We characterize the spectroscopic properties of the obtained pigment–protein complexes and the thermodynamics of liposome–protein carotenoid transfer and demonstrate the delivery of carotenoid echinenone from AnaCTDH into liposomes with an efficiency of up to 70 ± 3%. Most importantly, we show efficient carotenoid delivery to membranes of mammalian cells, which provides protection from reactive oxygen species (ROS). Incubation of neuroblastoma cell line Tet21N in the presence of 1 μM AnaCTDH binding echinenone decreased antimycin A ROS production by 25% (p < 0.05). The described carotenoprotein may be considered as part of modular systems for the targeted antioxidant delivery.

Probing of carotenoid-tryptophan hydrogen bonding dynamics in the single-tryptophan photoactive Orange Carotenoid Protein

The photoactive Orange Carotenoid Protein (OCP) plays a key role in cyanobacterial photoprotection. In OCP, a single non-covalently bound keto-carotenoid molecule acts as a light intensity sensor, while the protein is responsible for forming molecular contacts with the light-harvesting antenna, the fluorescence of which is quenched by OCP. Activation of this physiological interaction requires signal transduction from the photoexcited carotenoid to the protein matrix. Recent works revealed an asynchrony between conformational transitions of the carotenoid and the protein. Intrinsic tryptophan (Trp) fluorescence has provided valuable information about the protein part of OCP during its photocycle. However, wild-type OCP contains five Trp residues, which makes extraction of site-specific information impossible. In this work, we overcame this problem by characterizing the photocycle of a fully photoactive OCP variant (OCP-3FH) with only the most critical tryptophan residue (Trp-288) in place. Trp-288 is of special interest because it forms a hydrogen bond to the carotenoid's keto-oxygen to keep OCP in its dark-adapted state. Using femtosecond pump-probe fluorescence spectroscopy we analyzed the photocycle of OCP-3FH and determined the formation rate of the very first intermediate suggesting that generation of the recently discovered S* state of the carotenoid in OCP precedes the breakage of the hydrogen bonds. Therefore, following Trp fluorescence of the unique photoactive OCP-3FH variant, we identified the rate of the H-bond breakage and provided novel insights into early events accompanying photoactivation of wild-type OCP.

Fluorescence recovery protein: a powerful yet underexplored regulator of photoprotection in cyanobacteria†

Cyanobacteria utilize an elegant photoprotection mechanism mediated by the photoactive Orange Carotenoid Protein (OCP), which upon binding dissipates excess energy from light-harvesting complexes, phycobilisomes. The OCP activity is efficiently regulated by its partner, the Fluorescence Recovery Protein (FRP). FRP accelerates OCP conversion to the resting state, thus counteracting the OCP-mediated photoprotection. Behind the deceptive simplicity of such regulation is hidden a multistep process involving dramatic conformational rearrangements in OCP and FRP, the details of which became clearer only a decade after the FRP discovery. Yet many questions regarding the functioning of FRP have remained controversial. In this review, we summarize the current knowledge and understanding of the FRP role in cyanobacterial photoprotection as well as its evolutionary history that presumably lies far beyond cyanobacteria.

Engineering the photoactive orange carotenoid protein with redox-controllable structural dynamics and photoprotective function

Photosynthesis requires various photoprotective mechanisms for survival of organisms in high light. In cyanobacteria exposed to high light, the Orange Carotenoid Protein (OCP) is reversibly photoswitched from the orange (OCP^O) to the red (OCP^R) form, the latter binds to the antenna (phycobilisomes, PBs) and quenches its overexcitation. OCP^R accumulation implicates restructuring of a compact dark-adapted OCP^O state including detachment of the N-terminal extension (NTE) and separation of protein domains, which is reversed by interaction with the Fluorescence Recovery Protein (FRP). OCP phototransformation supposedly occurs via an intermediate characterized by an OCP^R-like absorption spectrum and an OCP^O-like protein structure, but the hierarchy of steps remains debatable. Here, we devise and analyze an OCP variant with the NTE trapped on the C-terminal domain (CTD) via an engineered disulfide bridge (OCP_CC). NTE trapping preserves OCP photocycling within the compact protein structure but precludes functional interaction with PBs and especially FRP, which is completely restored upon reduction of the disulfide bridge. Non-interacting with the dark-adapted oxidized OCP_CC, FRP binds reduced OCP_CC nearly as efficiently as OCP^O devoid of the NTE, suggesting that the low-affinity FRP binding to OCP^O is realized via NTE displacement. The low efficiency of excitation energy transfer in complexes between PBs and oxidized OCP_CC indicates that OCP_CC binds to PBs in an orientation suboptimal for quenching PBs fluorescence. Our approach supports the presence of the OCP^R-like intermediate in the OCP photocycle and shows effective uncoupling of spectral changes from functional OCP photoactivation, enabling redox control of its structural dynamics and function.

Comparative ultrafast spectroscopy and structural analysis of OCP1 and OCP2 from Tolypothrix

The orange carotenoid protein (OCP) is a structurally and functionally modular photoactive protein involved in cyanobacterial photoprotection. Recently, based on bioinformatic analysis and phylogenetic relationships, new families of OCP have been described, OCP2 and OCPx. The first characterization of the OCP2 showed both faster photoconversion and back-conversion, and lower fluorescence quenching of phycobilisomes relative to the well-characterized OCP1. Moreover, OCP2 is not regulated by the fluorescence recovery protein (FRP). In this work, we present a comprehensive study combining ultrafast spectroscopy and structural analysis to compare the photoactivation mechanisms of OCP1 and OCP2 from Tolypothrix PCC 7601. We show that despite significant differences in their functional characteristics, the spectroscopic properties of OCP1 and OCP2 are comparable. This indicates that the OCP functionality is not directly related to the spectroscopic properties of the bound carotenoid. In addition, the structural analysis by X-ray footprinting reveals that, overall, OCP1 and OCP2 have grossly the same photoactivation mechanism. However, the OCP2 is less reactive to radiolytic labeling, suggesting that the protein is less flexible than OCP1. This observation could explain fast photoconversion of OCP2.

Changing Color for Photoprotection: The Orange Carotenoid Protein

Under high irradiance, light becomes dangerous for photosynthetic organisms and they must protect themselves. Cyanobacteria have developed a simple mechanism, involving a photoactive soluble carotenoid protein, the orange carotenoid protein (OCP), which increases thermal dissipation of excess energy by interacting with the cyanobacterial antenna, the phycobilisome. Here, we summarize our knowledge of the OCP-related photoprotective mechanism, including the remarkable progress that has been achieved in recent years on OCP photoactivation and interaction with phycobilisomes, as well as with the fluorescence recovery protein, which is necessary to end photoprotection. A recently discovered unique mechanism of carotenoid transfer between soluble proteins related to OCP is also described.

Photosynthetic characterization of transgenic Synechocystis expressing a plant thiol/disulfide-modulating protein

A previous study showed that introducing an Arabidopsis thaliana thiol/disulfide-modulating protein, Low Quantum Yield of Photosystem II 1 (LQY1), into the cyanobacterium Synechocystis sp. PCC6803 increased the efficiency of Photosystem II (PSII) photochemistry. In the present study, the authors provided additional evidence for the role of AtLQY1 in improving PSII photochemical efficiency and cell growth. Light response curve analysis showed that AtLQY1-expressing Synechocystis grown at a moderate growth light intensity (50 µmol photons m^-2 s^-1) had higher minimal, maximal, and variable fluorescence than the empty-vector control, under a wide range of actinic light intensities. Light induction and dark recovery curves demonstrated that AtLQY1-expressing Synechocystis grown at the moderate growth light intensity had higher effective PSII quantum yield, higher photochemical quenching, lower regulated heat dissipation (non-photochemical quenching), low amounts of reduced plastoquinone, and higher amounts of oxidized plastoquinone than the empty-vector control. Furthermore, growth curve analysis indicated that AtLQY1-expressing Synechocystis grew faster than the empty-vector control at the moderate growth light intensity. These results suggest that transgenic expression of AtLQY1 in Synechocystis significantly improves PSII photochemical efficiency and overall cell growth.

Structural peculiarities of keto-carotenoids in water-soluble proteins revealed by simulation of linear absorption

To prevent irreversible damage caused by an excess of incident light, the photosynthetic machinery of many cyanobacteria uniquely utilizes the water-soluble orange carotenoid protein (OCP) containing a single keto-carotenoid molecule. This molecule is non-covalently embedded into the two OCP domains which are interconnected by a flexible linker. The phenomenon of OCP photoactivation, causing significant changes in carotenoid absorption in the orange and red form of OCP, is currently being thoroughly studied. Numerous additional spectral forms of natural and synthetic OCP-like proteins have been unearthed. The optical properties of carotenoids are strongly determined by the interaction of their electronic states with vibrational modes, the surrounding protein matrix, and the solvent. In this work, the effects of the pigment-protein interaction and vibrational relaxation in OCP were studied by computational simulation of linear absorption. Taking into account Raman spectroscopy data and applying the multimode Brownian oscillator model as well as the cumulant expansion technique, we have calculated a set of characteristic microparameters sufficient to demarcate different carotenoid states in OCP forms, using the model carotenoids spheroidene and spheroidenone in methanol/acetone solution as benchmarks. The most crucial microparameters, which determine the effect of solvent and protein environment, are the Huang-Rhys factors and the frequencies of C[double bond, length as m-dash]C and C-C stretching modes, the low-frequency mode and the FWHM due to inhomogeneous line broadening. Considering the difference of linear absorption between spheroidene and spheroidenone, which remarkably resembles the photoinduced changes of OCP absorption, and applying quantum chemical calculations, we discuss structural and functional determinants of carotenoid binding proteins.

Introducing an Arabidopsis thaliana Thylakoid Thiol/Disulfide-Modulating Protein Into Synechocystis Increases the Efficiency of Photosystem II Photochemistry

Photosynthetic species are subjected to a variety of environmental stresses, including suboptimal irradiance. In oxygenic photosynthetic organisms, a major effect of high light exposure is damage to the Photosystem II (PSII) reaction-center protein D1. This process even happens under low or moderate light. To cope with photodamage to D1, photosynthetic organisms evolved an intricate PSII repair and reassembly cycle, which requires the participation of different auxiliary proteins, including thiol/disulfide-modulating proteins. Most of these auxiliary proteins exist ubiquitously in oxygenic photosynthetic organisms. Due to differences in mobility and environmental conditions, land plants are subject to more extensive high light stress than algae and cyanobacteria. Therefore, land plants evolved additional thiol/disulfide-modulating proteins, such as Low Quantum Yield of PSII 1 (LQY1), to aid in the repair and reassembly cycle of PSII. In this study, we introduced an Arabidopsis thaliana homolog of LQY1 (AtLQY1) into the cyanobacterium Synechocystis sp. PCC6803 and performed a series of biochemical and physiological assays on AtLQY1-expressing Synechocystis. At a moderate growth light intensity (50 µmol photons m-2 s-1), AtLQY1-expressing Synechocystis was found to have significantly higher F v /F m , and lower nonphotochemical quenching and reactive oxygen species levels than the empty-vector control, which is opposite from the loss-of-function Atlqy1 mutant phenotype. Light response curve analysis of PSII operating efficiency and electron transport rate showed that AtLQY1-expressing Synechocystis also outperform the empty-vector control under higher light intensities. The increases in F v /F m , PSII operating efficiency, and PSII electron transport rate in AtLQY1-expressing Synechocystis under such growth conditions most likely come from an increased amount of PSII, because the level of D1 protein was found to be higher in AtLQY1-expressing Synechocystis. These results suggest that introducing AtLQY1 is beneficial to Synechocystis.

Interdomain interactions reveal the molecular evolution of the orange carotenoid protein

The photoactive orange carotenoid protein (OCP) is a blue-light intensity sensor involved in cyanobacterial photoprotection. Three OCP families co-exist (OCPX, OCP1 and OCP2), having originated from the fusion of ancestral domain genes. Here, we report the characterization of an OCPX and the evolutionary characterization of OCP paralogues focusing on the role of the linker connecting the domains. The addition of the linker with specific amino acids enabled the photocycle of the OCP ancestor. OCPX is the paralogue closest to this ancestor. A second diversification gave rise to OCP1 and OCP2. OCPX and OCP2 present fast deactivation and weak antenna interaction. In OCP1, OCP deactivation became slower and interaction with the antenna became stronger, requiring a further protein to detach OCP from the antenna and accelerate its deactivation. OCP2 lost the tendency to dimerize, unlike OCPX and OCP1, and the role of its linker is slightly different, giving less controlled photoactivation.

A genetically encoded fluorescent temperature sensor derived from the photoactive Orange Carotenoid Protein

The heterogeneity of metabolic reactions leads to a non-uniform distribution of temperature in different parts of the living cell. The demand to study normal functioning and pathological abnormalities of cellular processes requires the development of new visualization methods. Previously, we have shown that the 35-kDa photoswitchable Orange Carotenoid Protein (OCP) has a strong temperature dependency of photoconversion rates, and its tertiary structure undergoes significant structural rearrangements upon photoactivation, which makes this protein a nano-sized temperature sensor. However, the determination of OCP conversion rates requires measurements of carotenoid absorption, which is not suitable for microscopy. In order to solve this problem, we fused green and red fluorescent proteins (TagGFP and TagRFP) to the structure of OCP, producing photoactive chimeras. In such chimeras, electronic excitation of the fluorescent protein is effectively quenched by the carotenoid in OCP. Photoactivation of OCP-based chimeras triggers rearrangements of complex geometry, permitting measurements of the conversion rates by monitoring changes of fluorescence intensity. This approach allowed us to determine the local temperature of the microenvironment. Future directions to improve the OCP-based sensor are discussed.

OCP-FRP protein complex topologies suggest a mechanism for controlling high light tolerance in cyanobacteria

In cyanobacteria, high light photoactivates the orange carotenoid protein (OCP) that binds to antennae complexes, dissipating energy and preventing the destruction of the photosynthetic apparatus. At low light, OCP is efficiently deactivated by a poorly understood action of the dimeric fluorescence recovery protein (FRP). Here, we engineer FRP variants with defined oligomeric states and scrutinize their functional interaction with OCP. Complemented by disulfide trapping and chemical crosslinking, structural analysis in solution reveals the topology of metastable complexes of OCP and the FRP scaffold with different stoichiometries. Unable to tightly bind monomeric FRP, photoactivated OCP recruits dimeric FRP, which subsequently monomerizes giving 1:1 complexes. This could be facilitated by a transient OCP–2FRP–OCP complex formed via the two FRP head domains, significantly improving FRP efficiency at elevated OCP levels. By identifying key molecular interfaces, our findings may inspire the design of optically triggered systems transducing light signals into protein–protein interactions.

Cyanobacterial photoprotection is controlled by OCP and FRP proteins, but their dynamic interplay is not fully understood. Here, the authors combine protein engineering, disulfide trapping and structural analyses to provide mechanistic insights into the transient OCP-FRP interaction.

Structural rearrangements in the C-terminal domain homolog of Orange Carotenoid Protein are crucial for carotenoid transfer

A recently reported family of soluble cyanobacterial carotenoproteins, homologs of the C-terminal domain (CTDH) of the photoprotective Orange Carotenoid Protein, is suggested to mediate carotenoid transfer from the thylakoid membrane to the Helical Carotenoid Proteins, which are paralogs of the N-terminal domain of the OCP. Here we present the three-dimensional structure of a carotenoid-free CTDH variant from Anabaena (Nostoc) PCC 7120. This CTDH contains a cysteine residue at position 103. Two dimer-forming interfaces were identified, one stabilized by a disulfide bond between monomers and the second between each monomer's β-sheets, both compatible with small-angle X-ray scattering data and likely representing intermediates of carotenoid transfer processes. The crystal structure revealed a major positional change of the C-terminal tail. Further mutational analysis revealed the importance of the C-terminal tail in both carotenoid uptake and delivery. These results have allowed us to suggest a detailed model for carotenoid transfer via these soluble proteins.

Site, trigger, quenching mechanism and recovery of non-photochemical quenching in cyanobacteria: recent updates

Cyanobacteria exhibit a novel form of non-photochemical quenching (NPQ) at the level of the phycobilisome. NPQ is a process that protects photosystem II (PSII) from possible highlight-induced photo-damage. Although significant advancement has been made in understanding the NPQ, there are still some missing details. This critical review focuses on how the orange carotenoid protein (OCP) and its partner fluorescence recovery protein (FRP) control the extent of quenching. What is and what is not known about the NPQ is discussed under four subtitles; where does exactly the site of quenching lie? (site), how is the quenching being triggered? (trigger), molecular mechanism of quenching (quenching) and recovery from quenching. Finally, a recent working model of NPQ, consistent with recent findings, is been described.

Engineering a carotenoid-binding site in Dokdonia sp. PRO95 Na+-translocating rhodopsin by a single amino acid substitution

Light-driven H⁺, Cl^- and Na⁺ rhodopsin pumps all use a covalently bound retinal molecule to capture light energy. Some H⁺-pumping rhodopsins (xanthorhodopsins; XRs) additionally contain a carotenoid antenna for light absorption. Comparison of the available primary and tertiary structures of rhodopsins pinpointed a single Thr residue (Thr216) that presumably prevents carotenoid binding to Na⁺-pumping rhodopsins (NaRs). We replaced this residue in Dokdonia sp. PRO95 NaR with Gly, which is found in the corresponding position in XRs, and produced a variant rhodopsin in a ketocarotenoid-synthesising Escherichia coli strain. Unlike wild-type NaR, the isolated variant protein contained the tightly bound carotenoids canthaxanthin and echinenone. These carotenoids were visible in the absorption, circular dichroism and fluorescence excitation spectra of the Thr216Gly-substituted NaR, which indicates their function as a light-harvesting antenna. The amino acid substitution and the bound carotenoids did not affect the NaR photocycle. Our findings suggest that the antenna function was recently lost during NaR evolution but can be easily restored by site-directed mutagenesis.

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 significant change in its structure and spectral characteristics. Such conformational changes open the possibility for protein-protein interactions between OCP and the PBs core (i.e., activation of photoprotection mechanisms) or the fluorescence 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-protein interactions between the main participants of photoprotection reactions and on certain properties of representatives of newly discovered families of OCP homologs.

The photocycle of orange carotenoid protein conceals distinct intermediates and asynchronous changes in the carotenoid and protein components

The 35-kDa Orange Carotenoid Protein (OCP) is responsible for photoprotection in cyanobacteria. It acts as a light intensity sensor and efficient quencher of phycobilisome excitation. Photoactivation triggers large-scale conformational rearrangements to convert OCP from the orange OCP^O state to the red active signaling state, OCP^R, as demonstrated by various structural methods. Such rearrangements imply a complete, yet reversible separation of structural domains and translocation of the carotenoid. Recently, dynamic crystallography of OCP^O suggested the existence of photocycle intermediates with small-scale rearrangements that may trigger further transitions. In this study, we took advantage of single 7 ns laser pulses to study carotenoid absorption transients in OCP on the time-scale from 100 ns to 10 s, which allowed us to detect a red intermediate state preceding the red signaling state, OCP^R. In addition, time-resolved fluorescence spectroscopy and the assignment of carotenoid-induced quenching of different tryptophan residues derived thereof revealed a novel orange intermediate state, which appears during the relaxation of photoactivated OCP^R to OCP^O. Our results show asynchronous changes between the carotenoid- and protein-associated kinetic components in a refined mechanistic model of the OCP photocycle, but also introduce new kinetic signatures for future studies of OCP photoactivity and photoprotection.

The Unique Protein-to-Protein Carotenoid Transfer Mechanism

Orange Carotenoid Protein (OCP) is known as an effector and regulator of cyanobacterial photoprotection. This 35 kDa water-soluble protein provides specific environment for blue-green light absorbing keto-carotenoids, which excitation causes dramatic but fully reversible rearrangements of the OCP structure, including carotenoid translocation and separation of C- and N-terminal domains upon transition from the basic orange to photoactivated red OCP form. Although recent studies greatly improved our understanding of the OCP photocycle and interaction with phycobilisomes and the fluorescence recovery protein, the mechanism of OCP assembly remains unclear. Apparently, this process requires targeted delivery and incorporation of a highly hydrophobic carotenoid molecule into the water-soluble apoprotein of OCP. Recently, we introduced, to our knowledge, a novel carotenoid carrier protein, COCP, which consists of dimerized C-domain(s) of OCP and can combine with the isolated N-domain to form transient OCP-like species. Here, we demonstrate that in vitro COCP efficiently transfers otherwise tightly bound carotenoid to the full-length OCP apoprotein, resulting in formation of photoactive OCP from completely photoinactive species. We accurately analyze the peculiarities of this process that, to the best of our knowledge, appears unique, a previously uncharacterized protein-to-protein carotenoid transfer mechanism. We hypothesize that a similar OCP assembly can occur in vivo, substantiating specific roles of the COCP carotenoid carrier in cyanobacterial photoprotection.

Deletion of the short N-terminal extension in OCP reveals the main site for FRP binding

The orange carotenoid protein (OCP) plays a key role in cyanobacterial photoprotection. Photoconversion entails structural rearrangements in OCP that are required for its binding to phycobilisome, thereby inducing excitation energy dissipation. Detachment of OCP from phycobilisome requires the fluorescence recovery protein (FRP). It is considered that OCP interacts with FRP only in the photoactivated state; however, the binding site for FRP is currently unknown. As an important stabilizing element in orange OCP, the short αA-helix within the N-terminal extension (NTE) binds to the C-terminal domain (CTD), but unfolds upon photoactivation and interferes with phycobilisome binding. Here, we demonstrate that the NTE shares specific structural and functional similarities with FRP and discover the main site of OCP-FRP interactions in the OCP-CTD.

A Molecular Mechanism for Nonphotochemical Quenching in Cyanobacteria

The cyanobacterial orange carotenoid protein (OCP) protects photosynthetic cyanobacteria from photodamage by dissipating excess excitation energy collected by phycobilisomes (PBS) as heat. Dissociation of the PBS-OCP complex in vivo is facilitated by another protein known as the fluorescence recovery protein (FRP), which primarily exists as a dimeric complex. We used various mass spectrometry (MS)-based techniques to investigate the molecular mechanism of this FRP-mediated process. FRP in the dimeric state (dFRP) retains its high affinity for the C-terminal domain (CTD) of OCP in the red state (OCP^r). Site-directed mutagenesis and native MS suggest the head region on FRP is a candidate to bind OCP. After attachment to the CTD, the conformational changes of dFRP allow it to bridge the two domains, facilitating the reversion of OCP^r into the orange state (OCP^o) accompanied by a structural rearrangement of dFRP. Interestingly, we found a mutual response between FRP and OCP; that is, FRP and OCP^r destabilize each other, whereas FRP and OCP^o stabilize each other. A detailed mechanism of FRP function is proposed on the basis of the experimental results.