Biology subjects

Friedrich, T.

Publications and source records attributed to Friedrich, T..

34 records · Page 2Linked to original sources

bioRxiv2021

Early embryonic loss following intravaginal Zika virus challenge in rhesus macaques

Zika virus (ZIKV) is an arthropod-borne virus (arbovirus) and is primarily transmitted by Aedes species mosquitoes; however, ZIKV can also be sexually transmitted. During the initial epidemic and in places where ZIKV is now considered endemic, it is difficult to disentangle the risks and contributions of sexual versus vector-borne transmission to adverse pregnancy outcomes. To examine the potential impact of sexual transmission of ZIKV on pregnancy outcome, we challenged three rhesus macaques (Macaca mulatta) three times intravaginally with 1 x 107 PFU of a low passage, African lineage ZIKV isolate (ZIKV-DAK) in the first trimester ([~]30 days gestational age). Samples were collected from all animals initially on days 3 through 10 post challenge, followed by twice, and then once weekly sample collection; ultrasound examinations were performed every 3-4 days then weekly as pregnancies progressed. All three dams had ZIKV RNA detectable in plasma on day 3 post-ZIKV challenge. At approximately 45 days gestation (17-18 days post-challenge), two of the three dams were found to have nonviable embryos by ultrasound. Viral RNA was detected in recovered tissues and at the maternal-fetal interface (MFI) in both cases. The remaining viable pregnancy proceeded to near term ([~]155 days gestational age) and ZIKV RNA was detected at the MFI but not in fetal tissues. These results suggest that sexual transmission of ZIKV may represent an underappreciated risk of pregnancy loss during early gestation.

microbiology↗

bioRxiv2020

Transmission of SARS-CoV-2 in domestic cats imposes a narrow bottleneck

The evolutionary mechanisms by which SARS-CoV-2 viruses adapt to mammalian hosts and, potentially, undergo antigenic evolution depend on the ways genetic variation is generated and selected within and between individual hosts. Using domestic cats as a model, we show that SARS-CoV-2 consensus sequences remain largely unchanged over time within hosts, while dynamic sub-consensus diversity reveals processes of genetic drift and weak purifying selection. We further identify a notable variant at amino acid position 655 in Spike (H655Y), which was previously shown to confer escape from human monoclonal antibodies. This variant arises rapidly and persists at intermediate frequencies in index cats. It also becomes fixed following transmission in two of three pairs. These dynamics suggest this site may be under positive selection in this system and illustrate how a variant can quickly arise and become fixed in parallel across multiple transmission pairs. Transmission of SARS-CoV-2 in cats involved a narrow bottleneck, with new infections founded by fewer than ten viruses. In RNA virus evolution, stochastic processes like narrow transmission bottlenecks and genetic drift typically act to constrain the overall pace of adaptive evolution. Our data suggest that here, positive selection in index cats followed by a narrow transmission bottleneck may have instead accelerated the fixation of S H655Y, a potentially beneficial SARS-CoV-2 variant. Overall, our study suggests species- and context-specific adaptations are likely to continue to emerge. This underscores the importance of continued genomic surveillance for new SARS-CoV-2 variants as well as heightened scrutiny for signatures of SARS-CoV-2 positive selection in humans and mammalian model systems. Author summaryThrough ongoing human adaptation, spill-back events from other animal intermediates, or with the distribution of vaccines and therapeutics, the landscape of SARS-CoV-2 genetic variation is certain to change. The evolutionary mechanisms by which SARS-CoV-2 will continue to adapt to mammalian hosts depend on genetic variation generated within and between hosts. Here, using domestic cats as a model, we show that within-host SARS-CoV-2 genetic variation is predominantly influenced by genetic drift and purifying selection. Transmission of SARS-CoV-2 between hosts is defined by a narrow transmission bottleneck, involving 2-5 viruses. We further identify a notable variant at amino acid position 655 in Spike (H655Y), which arises rapidly and is transmitted in cats. Spike H655Y has been previously shown to confer escape from human monoclonal antibodies and is currently found in over 1000 human sequences. Overall, our study suggests species- and context-specific adaptations are likely to continue to emerge, underscoring the importance of continued genomic surveillance in humans and non-human mammalian hosts.

microbiology↗

bioRxiv2020

Mass Photometry of Membrane Proteins

Integral membrane proteins (IMPs) are biologically highly significant but challenging to study because they require maintaining a cellular lipid-like environment. Here, we explore the application of mass photometry (MP) to IMPs and membrane mimetic systems at the single particle level. We apply MP to amphipathic vehicles, such as detergents and amphipols, as well as to lipid and native nanodiscs, characterising the particle size, sample purity and heterogeneity. Using methods established for cryogenic electron microscopy, we eliminate detergent background, enabling high-resolution studies of membrane protein structure and interactions. We find evidence that, when extracted from native membranes using native styrene-maleic acid nanodiscs, the potassium channel KcsA is present as a dimer of tetramers - in contrast to results obtained using detergent purification. Finally, using lipid nanodiscs, we show that MP can help distinguish between functional and non-functional nanodisc assemblies, as well as determine the critical factors for lipid nanodisc formation.

biophysics↗

bioRxiv2019

Quantifying the heterogeneity of macromolecular machines by mass photometry

Sample purity is central to in vitro studies of protein function and regulation, as well as to the efficiency and success of structural studies requiring crystallization or computational alignment of individual molecules. Here, we show that mass photometry (MP) accurately reports on sample heterogeneity using minimal volumes with molecular resolution within minutes. We benchmark our approach by negative stain electron microscopy (nsEM), including workflows involving chemical crosslinking and multi-step purification of a multi-subunit ubiquitin ligase. When applied to proteasome stability, we detect and quantify assemblies invisible to nsEM. Our results illustrate the unique advantages of MP for rapid sample characterization, prioritization and optimization.

biophysics↗

bioRxiv2019

Primary infection with dengue or Zika virus does not affect the severity of heterologous secondary infection in macaques.

Zika virus (ZIKV) and dengue virus (DENV) are genetically and antigenically related flaviviruses that now co-circulate in much of the tropical and subtropical world. The rapid emergence of ZIKV in the Americas in 2015 and 2016, and its recent associations with Guillain-Barre syndrome, birth defects, and fetal loss have led to the hypothesis that DENV infection induces cross-reactive antibodies that influence the severity of secondary ZIKV infections. It has also been proposed that pre-existing ZIKV immunity could affect DENV pathogenesis. We examined outcomes of secondary ZIKV infections in three rhesus and fifteen cynomolgus macaques, as well as secondary DENV-2 infections in three additional rhesus macaques up to a year post-primary ZIKV infection. Although cross-binding antibodies were detected prior to secondary infection for all animals and cross-neutralizing antibodies were detected for some animals, previous DENV or ZIKV infection had no apparent effect on the clinical course of heterotypic secondary infections in these animals. All animals had asymptomatic infections and, when compared to controls, did not have significantly perturbed hematological parameters. Rhesus macaques infected with DENV-2 approximately one year after primary ZIKV infection had higher vRNA loads in plasma when compared with serum vRNA loads from ZIKV-naive animals infected with DENV-2, but a differential effect of sample type could not be ruled out. In cynomolgus macaques, the serotype of primary DENV infection did not affect the outcome of secondary ZIKV infection.\n\nAuthor summaryPre-existing immunity to one of the four DENV serotypes is known to increase the risk of severe disease upon secondary infection with a different serotype. Due to the antigenic similarities between ZIKV and DENV, it has been proposed that these viruses could interact in a similar fashion. Data from in vitro experiments and murine models suggests that pre-existing immunity to one virus could either enhance or protect against infection with the other. These somewhat contradictory findings highlight the need for immune competent animal models for understanding the role of cross-reactive antibodies in flavivirus pathogenesis. We examined secondary ZIKV or DENV infections in rhesus and cynomolgus macaques that had previously been infected with the other virus. We assessed the outcomes of secondary ZIKV or DENV infections by quantifying vRNA loads, clinical and laboratory parameters, body temperature, and weight for each cohort of animals and compared them with control animals. These comparisons demonstrated that within a year of primary infection, secondary infections with either ZIKV or DENV were similar to primary infections and were not associated with enhancement or reduction in severity of disease based on the outcomes that we assessed.

pathology↗

bioRxiv2018

Zika viruses of both African and Asian lineages cause fetal harm in a vertical transmission model

Congenital Zika virus (ZIKV) infection was first linked to birth defects during the American outbreak 1-3. It has been proposed that mutations unique to the Asian/American-genotype explain, at least in part, the ability of Asian/American ZIKV to cause congenital Zika syndrome (CZS) 4,5. Recent studies identified mutations in ZIKV infecting humans that arose coincident with the outbreak in French Polynesia and were stably maintained during subsequent spread to the Americas 5. Here we show that African ZIKV can infect and harm fetuses and that the S139N mutation that has been associated with the American outbreak is not essential for fetal harm. Our findings, in a vertical transmission mouse model, suggest that ZIKV will remain a threat to pregnant women for the foreseeable future, including in Africa, southeast Asia, and the Americas. Additional research is needed to better understand the risks associated with ZIKV infection during pregnancy, both in areas where the virus is newly endemic and where it has been circulating for decades.

microbiology↗

bioRxiv2018

Pre-existing SIV Infection Increases Susceptibility to Tuberculosis in Mauritian Cynomolgus Macaques

Tuberculosis (TB), caused by Mycobacterium tuberculosis (M.tb), is the leading cause of death among HIV positive patients. The precise mechanisms by which HIV impairs host resistance to a subsequent M.tb infection are unknown. We modeled this co-infection in Mauritian cynomolgus macaques (MCM) using SIV as an HIV surrogate. We infected seven MCM with SIVmac239 intrarectally and six months later co-infected them via bronchoscope with ~10 CFU M.tb. Another eight MCM were infected with M.tb alone. TB progression was monitored by clinical parameters, by culturing bacilli in gastric and bronchoalveolar lavages, and by serial 18F-FDG PET/CT imaging. The eight MCM infected with M.tb alone displayed dichotomous susceptibility to TB, with four animals reaching humane endpoint within 13 weeks and four animals surviving >19 weeks post M.tb infection. In stark contrast, all seven SIV+ animals exhibited rapidly progressive TB following co-infection and all reached humane endpoint by 13 weeks. Serial PET/CT imaging confirmed dichotomous outcomes in MCM infected with M.tb alone and marked susceptibility to TB in all SIV+ MCM. Notably, imaging revealed a significant increase in TB granulomas between four and eight weeks post M.tb infection in SIV+, but not in SIV-naive MCM and implies that SIV impairs the ability of animals to contain M.tb dissemination. At necropsy, animals with pre-existing SIV infection had more extrapulmonary TB disease, more overall pathology, and increased bacterial loads than animals infected with M.tb alone. We thus developed a tractable MCM model in which to study SIV-M.tb co-infection and demonstrate that pre-existing SIV dramatically diminishes the ability to control M.tb co-infection.\n\nAuthor summaryMycobacterium tuberculosis (M.tb) is the etiologic agent of tuberculosis (TB) and infects a tremendous number of individuals. TB causes millions of deaths each year and is the leading cause of death in human immunodeficiency virus (HIV)-positive individuals. Currently, the mechanisms by which pre-existing HIV infection increases susceptibility to subsequent M.tb infection and predisposes an individual to TB disease are poorly understood. We developed a simian immunodeficiency virus (SIV) - M.tb co-infection model in Mauritian cynomolgus macaques (MCM) to investigate how SIV impairs the immune response to a subsequent M.tb infection. We show that naive MCM display variable resistance to TB while all SIV-infected MCM failed to control M.tb infection. Using quantitative measures of disease and serial PET/CT imaging, we show that SIV+ co-infected animals uniformly exhibit rapid TB progression, more tuberculosis disease dissemination, and increased mortality. This coinfection model will facilitate studies, provide unique insights into the defects underlying TB susceptibility in HIV+ individuals and will help us develop approaches to overcome these defects.

immunology↗

bioRxiv2018

Radioprotective role of cyanobacterial phycobilisomes

It is now generally accepted that cyanobacteria are responsible for production of oxygen, which led to the so-called \"Great Oxygenation Event\". Appearance of dioxygen in Earths atmosphere resulted in formation of the ozone layer and the ionosphere, which caused significant reduction of ionizing radiation levels at the surface of our planet. This event not only increased biological diversity but also canceled the urgency of previously developed mechanisms of DNA protection, which allowed to survive and develop in harsh environmental conditions including exposure to cosmic rays. In order to test the hypothesis if one of the oldest organisms on Earth retained ancient protection mechanisms, we studied the effect of ionizing radiation (IoR, here: -particles with a kinetic energy of about 30 MeV) and space flight during the mission of the Foton-M4 satellite on cells of Synechocystis sp. PCC6803. By analyzing spectral and functional characteristics of photosynthetic membranes we revealed numerous similarities between cells exposed to IoR and after the space mission. In both cases, we found that excitation energy transfer from phycobilisomes to photosystems was interrupted and the concentration of phycobiliproteins was significantly reduced. Although photosynthetic activity was severely suppressed, the effect was reversible and the cells were able to rapidly recover from stress under normal conditions. Moreover, in vitro experiments demonstrated that the effect of IoR on isolated phycobilisomes was completely different from such in vivo. These observations suggest that the actual existence and the uncoupling of phycobilisomes under irradiation stress could play specific role not only in photo-, but also in radioprotection, which was crucial for early stages of evolution and the development of Life on Earth.

biophysics↗

bioRxiv2018

Doxorubicin induces caspase-mediated proteolysis of KV7.1

The voltage-gated potassium channel Kv7.1 (KCNQ1) co-assembles with KCNE1 to generate the cardiac potassium current IKs. Gain- and loss-of-function mutations in KCNQ1 are associated with atrial fibrillation and long-QT (LQT) syndrome, respectively, highlighting the importance of modulating IKS activity for proper cardiac function. On a post-translational level, IKS can be regulated by phosphorylation, ubiquitination and sumoylation. Here, we report proteolysis of Kv7.1 as a novel, irreversible posttranslational modification. The identification of two C-terminal fragments (CTF1 and CTF2) of Kv7.1 led us to identify an aspartate critical for the generation of CTF2 and caspases as responsible for mediating Kv7.1 proteolysis. Activating caspases by apoptotic stimuli significantly reduced Kv7.1/KCNE1 currents, which was abrogated in cells expressing caspase-resistant Kv7.1 D459A/KCNE1 channels. An increase in cleavage of Kv7.1 could be detected in the case of LQT mutation G460S, which is located adjacent to the cleavage site. Application of apoptotic stimuli or doxorubicin-induced cardiotoxicity provoked caspase-mediated cleavage of endogenous Kv7.1 in human cardiomyocytes. In summary, our findings establish caspases as novel regulatory components for modulating Kv7.1 activity which may have important implications for the molecular mechanism of doxorubicin-induced cardiotoxicity.\n\nNon-standard Abbreviations and Acronyms

biochemistry↗

bioRxiv2018

Functional interaction of low-homology FRPs from different cyanobacteria with Synechocystis OCP

Photosynthesis requires a balance between efficient light harvesting and protection against photodamage. The cyanobacterial photoprotection system uniquely relies on the functioning of the photoactive orange carotenoid protein (OCP) that under intense illumination provides fluorescence quenching of the light-harvesting antenna complexes, phycobilisomes. The recently identified fluorescence recovery protein (FRP) binds to the photoactivated OCP and accelerates its relaxation into the basal form, completing the regulatory circle. The molecular mechanism of FRP functioning is largely controversial. Moreover, since the available knowledge has mainly been gained from studying Synechocystis proteins, the cross-species conservation of the FRP mechanism remains unexplored. Besides phylogenetic analysis, we performed a detailed structural-functional analysis of two selected low-homology FRPs by comparing them with Synechocystis FRP (SynFRP). While adopting similar dimeric conformations in solution and preserving binding preferences of SynFRP toward various OCP variants, the low-homology FRPs demonstrated distinct binding stoichiometries and differentially accentuated features of this functional interaction. By providing clues to understand the FRP mechanism universally, our results also establish foundations for upcoming structural investigations necessary to elucidate the FRP-dependent regulatory mechanism.

biochemistry↗

bioRxiv2017

Infection via mosquito bite alters Zika virus replication kinetics in rhesus macaques

For more than three decades it has been recognized that small amounts of vector saliva can significantly alter the infectivity of vector-borne pathogens and subsequent in vivo dynamics. Mouse and nonhuman primate models now serve as useful platforms to study Zika virus (ZIKV) pathogenesis, candidate therapies, and vaccines, but they rely on needle inoculation of virus: the effects of mosquito-borne infection on disease outcome have not been explored in these models. To model vector-borne transmission of ZIKV in nonhuman primates, we infected Aedes aegypti mosquitoes with ZIKV and allowed them to feed on four ZIKV-naive rhesus macaques. We compared ZIKV replication kinetics and tissue distribution between animals that were subcutaneously inoculated with 104 plaque-forming units of ZIKV and those that were exposed via mosquito bite. Here, we show that infection via mosquito bite delays ZIKV replication to peak viral loads in rhesus macaques. Importantly, in mosquito-infected animals ZIKV tissue distribution was limited to hemolymphatic tissues, female reproductive tract tissues, kidney, and liver, potentially emulating key features of human ZIKV infections, most of which are characterized by mild or asymptomatic disease. This newly developed system will be valuable for studying ZIKV disease because it more closely mimics human infection by mosquito bite than needle-based inoculations.

microbiology↗

bioRxiv2017

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

The 35 kDa water-soluble Orange Carotenoid Protein (OCP) is responsible for photoprotection in cyanobacteria. It acts as a light intensity sensor that simultaneously serves as efficient quencher of phycobilisome excitation energy as well as of reactive oxygen species. Photoactivation triggers large-scale conformational rearrangements to convert OCP from the orange OCPO state to the red active signaling state OCPR, as demonstrated by various structural methods. Eventually, such rearrangements imply complete yet reversible separation of structural domains (C- and N-terminal domain) and significant translocation of the carotenoid cofactor. Very recently, dynamic crystallography of OCPO crystals suggested the existence of photocycle intermediates with small-scale rearrangements that may trigger further transitions in the protein. However, the currently existing gap between the ultra-fast picosecond and 100 millisecond time scale of spectroscopic and structural data precludes knowledge about distinct intermediate states. 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 OCPR. In addition, time-resolved fluorescence spectroscopy and following assignment of carotenoid-induced quenching of different tryptophan residues revealed a novel orange intermediate state, which appears during back-relaxation of photoactivated OCPR to OCPO. Our results show asynchronous changes in the carotenoid and protein components and provide refined mechanistic information about the OCP photocycle as well as introduce new kinetic signatures for future studies of OCP photoactivity and photoprotection.\n\nSignificance statementCyanobacteria utilize the Orange Carotenoid Protein (OCP) to protect their photosynthetic apparatus from the harmful effects of intense sunlight. OCP is a blue light-triggered photoswitch, which undergoes photoconversion from its dark adapted orange to the active red state, the latter being able to interact with the phycobilisome antennae and quench their fluorescence, thus avoiding excessive energy flow to the photosystems. With the help of the fluorescence recovery protein (FRP), OCP detaches from phycobilisomes and can return faster into the orange state. Until now, only the thermodynamically stable orange state and the metastable red state are established in a primitive photocycle. In this work, we apply transient absorption and fluorescence spectroscopy and identify two novel photocycle intermediates of physiological relevance.

biophysics↗

bioRxiv2017

Disruption Of Ankyrin B And Caveolin-1 Interaction Sites Alters Na+,K+-ATPase Lateral Diffusion In HEK293 Cell Plasma Membranes

The Na+,K+-ATPase is a plasma membrane ion transporter of high physiological importance for ion homeostasis and cellular excitability in electrically active tissues. Mutations in the genes coding for Na+,K+-ATPase -subunit isoforms lead to severe human pathologies including Familial Hemiplegic Migraine type 2 (FHM2), Alternating Hemiplegia of Childhood (AHC), Rapid Dystonia Parkinsonism (RDP) or epilepsy. Many of the reported mutations lead to change- or loss-of-function effects, whereas others do not alter the functional properties, but lead to e.g. reduced protein stability, reduced protein expression or defective plasma membrane targeting. Na+,K+-ATPase frequently assembles with other membrane transporters or cellular matrix proteins in specialized plasma membrane microdomains, but the effects of these interactions on targeting or protein mobility are elusive so far. Mutational disruption of established interaction motifs of the Na+,K+-ATPase with ankyrin B and caveolin-1 are expected to result in changes in plasma membrane targeting, changes of the localization pattern, and of the diffusion behavior of the enzyme. We studied the consequences of mutations in these binding sites by monitoring diffusion of eGFP-labeled Na+,K+-ATPase constructs in the plasma membrane of living HEK293T cells by fluorescence correlation spectroscopy (FCS) as well as fluorescence recovery after photobleaching (FRAP) or photoswitching (FRAS) and observed significant differences compared to the wild-type enzyme, with synergistic effects for combinations of interaction site mutations. These measurements expand the possibilities to study the consequences of Na+,K+-ATPase mutations and provide information about the interaction of Na+,K+-ATPase 2-isoform with cellular matrix proteins, the cytoskeleton or other membrane protein complexes.

biophysics↗

bioRxiv2017

The Unique Protein-To-Protein Carotenoid Transfer Mechanism

List of abbreviations\n\nAbstractOrange Carotenoid Protein (OCP) is known to be an effector and regulator of cyanobacterial photoprotection. This 35 kDa water-soluble protein provides specific environment for keto-carotenoids, the excitation of which induced by the absorption of blue-green light 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. While recent studies significantly 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 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 the photoactive OCP from completely photoinactive species. We accurately analyze peculiarities of this carotenoid transfer process which, to the best of our knowledge, seems unique, previously uncharacterized protein-to-protein carotenoid transfer process. We hypothesize that a similar OCP assembly can occur in vivo, substantiating specific roles of the COCP carotenoid carrier in cyanobacterial photoprotection.

biophysics↗

bioRxiv2017

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

The photoactive Orange Carotenoid Protein (OCP) plays a central role in cyanobacterial photoprotection. Photoconversion entails significant structural rearrangements in OCP required for its binding to the phycobilisome to induce excitation energy dissipation, whereas the fluorescence recovery protein (FRP) is required for OCP detachment and restoration of phycobilisome fluorescence. Although key to understanding the whole reversible mechanism of photoprotection, the FRP binding site on OCP has been representing challenge since the discovery of FRP in 2010 and is currently unknown. OCP comprises two structural domains organized into a compact basic orange form due to specific protein-chromophore and inter-domain protein-protein interactions and interacts with FRP tightly only when photoactivated. As an important stabilizing element in the orange OCP, the short A-helix within the N-terminal extension (NTE) binds to OCPs C-terminal domain (CTD), but unfolds upon photoactivation and interferes with phycobilisome binding. By using an alloy of biochemical and biophysical techniques, 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.

biophysics↗

bioRxiv2016

Assembly of photoactive Orange Carotenoid Protein from its domains unravels a carotenoid shuttle mechanism

The Orange Carotenoid Protein (OCP) is indispensable for cyanobacterial photoprotection by quenching phycobilisome fluorescence upon photoconversion from the orange OCPO to the red OCPR form. Cyanobacterial genomes frequently harbor, besides genes for Orange Carotenoid Proteins (OCPs), several genes encoding homologs of OCPs N- or C-terminal domains (NTD, CTD). Unlike the well-studied NTD homologs, called Red Carotenoid Proteins (RCPs), the role of CTD homologs remains elusive. We show how OCP can be reassembled from its functional domains. Expression of Synechocystis OCP-CTD in carotenoid-producing Escherichia coli yielded violet-colored proteins, which, upon mixing with the RCP-apoprotein, produced an orange-like photoswitchable form that further photoconverted into a species spectroscopically indistinguishable from RCP, thus demonstrating a unique carotenoid shuttle mechanism. The CTD itself is a novel, dimeric carotenoid-binding protein, which effectively quenches singlet oxygen and interacts with the Fluorescence Recovery Protein, assigning physiological roles to CTD homologs and explaining the evolutionary process of OCP formation.\n\nOne Sentence SummaryThe C-domain of cyanobacterial OCP dimerizes, binds a carotenoid, and delivers it to the N-domain forming photoactive OCP.

biophysics↗