Spectroscopic study of the light-harvesting protein C-phycocyanin associated with colorless linker peptides

Direct Energy Transfer from Allophycocyanin-Free Rod-Type CpcL-Phycobilisome to Photosystem I

Phycobilisomes (PBSs) are photosynthetic antenna megacomplexes comprising pigment-binding proteins (cores and rods) joined with linker proteins. A rod-type PBS that does not have a core is connected to photosystem I (PSI) by a CpcL linker protein, which stabilizes a red-form of the phycocyanobilin (red-PCB) in the rod. However, quantitative information on the energy transfer from red-type PBS to PSI has not been determined. Herein, the isolated supercomplex of the rod-type PBS and the PSI tetramer from Anabaena sp. PCC 7120 were probed by time-resolved spectroscopy at 77 K and by decay-associated spectral analysis to show that red-PCB mediates the fast and efficient (time constant = 90 ps, efficiency = 95%) transfer of excitation energy from PCB to chlorophyll a (Chl a). According to the Förster energy transfer mechanism, this high efficiency corresponds to a 4 nm distance between red-PCB and Chl a, suggesting that β-84 PCB in the rod acts as red-PCB.

Predicting cyanobacterial biovolumes from phycocyanin fluorescence using a handheld fluorometer in the field

Toxic cyanobacterial blooms are becoming more prevalent in freshwater systems, increasing the need for monitoring to protect human health. Phycocyanin fluorescence sensors have been developed as tools for providing fast and cost-effective proxy measurements for cyanobacterial biomass. However, poor precision and low sensitivity in many of the probe sensors assessed to-date has restricted their potential for practical application in cyanobacterial monitoring programmes. In the present study, the sensitivity and accuracy of a handheld fluorometer, the CyanoFluor, was assessed using 12 cyanobacterial strains and samples from four different lakes collected weekly for 12 weeks. After the initial measurements, the samples were lysed by sonication, which we hypothesised would reduce inter and intra-specific differences. The CyanoFluor displayed high sensitivity (limit of quantification = 3.5 µg L^-1 of phycocyanin) and was able to detect cyanobacterial biovolumes to levels much lower than the threshold levels in current recreational guidelines worldwide. There were strong and significant phycocyanin to biovolume relationships (r² ≥ 0.88, P < 0.05) for all 12 cyanobacterial cultures. Collectively, strong relationships between phycocyanin fluorescence and cyanobacterial biovolumes were also identified in environmental samples (r² ≥ 0.78, P < 0.001), although weaker relationships were identified when lakes were analysed separately (r² = 0.06 - 0.90). There were differences in phycocyanin per biovolume between both cultured strains and lakes, highlighting innate interspecific differences that exist between cyanobacterial species. Lysis of samples consistently reduced variability between technical replicates, in cyanobacteria cultures (up to 87% reduction in sample variability) and environmental samples (71 - 93% reduction), indicating that it would be a useful methodological step to improve the repeatability of results. When guideline thresholds (aligned with currently enforced risk assessment categories) were modelled based on the most successful linear regression model, 74% of samples were assigned to the correct risk category. The sensitivity of the CyanoFluor and accuracy of the phycocyanin threshold models, indicates high potential for this method to be integrated into cyanobacterial monitoring programmes.

Excitation Energy Transfer in Intact CpcL-Phycobilisomes from Synechocystis sp. PCC 6803

This work highlights spectroscopic studies performed on a CpcL-phycobilisome (CpcL-PBS) light-harvesting complex from cyanobacterium Synechocystis sp. PCC 6803 ΔAB strain. The CpcL-PBS antenna has the form of a single rod made up exclusively of phycocyanins (PCs), a structure that is much simpler compared to the better known and broadly studied CpcG-PBS that consists of a cylindrical core with a set of protruding PC rods. Steady-state and time-resolved fluorescence studies demonstrated that the CpcL-PBS antenna comprises two spectral forms of phycocyanobilin (PCB), one emitting at 650 nm and a second emitting at 670 nm. The latter one presumably serves as the so-called terminal energy emitter without allophycocyanin. Studies of excitation energy migration between those two PCB forms demonstrated that even small buffer alterations, commonly applied by spectroscopists to tweak buffers to be more friendly for a certain type of spectroscopy, may lead to very different experimental outcomes and, in consequence, to differences in models of excitation migration pathway in this antenna complex.

Application of Laser-Induced Fluorescence in Functional Studies of Photosynthetic Biofilms

Biofilms are a ubiquitous form of life for microorganisms. Photosynthetic biofilms such as microphytobenthos (MPB) and biological soil crusts (BSC) play a relevant ecological role in aquatic and terrestrial ecosystems, respectively. On the other hand, photosynthetic epilithic biofilms (PEB) are major players in the microbial-induced decay of stone structures of cultural heritage. The use of fluorescence techniques, namely, pulse-amplitude-modulated fluorometry, was crucial to understanding the photophysiology of these microbial communities, since they made it possible to measure biofilms’ photosynthetic activity without disturbing their delicate spatial organization within sediments or soils. The use of laser-induced fluorescence (LIF) added further technical advantages, enabling measurements to be made at a considerable distance from the samples, and under daylight. In this Perspective, we present state-of-the-art LIF techniques, show examples of the application of LIF to MPB and present exploratory results of LIF application to BSC, as well as to PEB colonizing stone structures of cultural heritage. Thereafter, we discuss the perspectives of LIF utilization in environmental research and monitoring, in cultural heritage conservation and assessment, and in biotechnological applications of photosynthetic biofilms.

The γ33 subunit of R-phycoerythrin from Gracilaria chilensis has a typical double linked phycourobilin similar to β subunit

Phycobilisomes (PBS) are accessory light harvesting protein complexes formed mainly by phycobiliproteins (PBPs). The PBPs absorb light that is efficiently transferred to Photosystems due to chromophores covalently bound to specific cysteine residues. Besides phycobiliproteins (PE), the PBS contains linker proteins responsible for assembly and stabilization of the whole complex and the tuning of energy transfer steps between chromophores. The linker (γ³³) from Gracilaria chilensis, is a chromophorylated rod linker associated to (αβ)₆ hexamers of R-phycoerythrin (R-PE). Its role in the energy transfer process is not clear yet. Structural studies as well as the composition and location of the chromophores are essential to understand their involvement in the energy transfer process in PBS. To achieve this, the coding gene of γ³³ was cloned and sequenced. The sequence was analyzed by informatics tools, to obtain preliminary information which leaded the next experiments. The protein was purified from R-phycoerythrin, and the sequence confirmed by mass spectrometry. The coding sequence analysis revealed a protein of 318 aminoacid residues containing a chloroplastidial transit peptide (cTP) of 39 aminoacids at the N-terminus. The conservation of cysteines revealed possible chromophorylation sites. Using α and β R-PE subunits as spectroscopic probes in denaturation assays, we deduced a double bonded phycourobilin (PUB) on γ³³ subunit that were confirmed between Cys62 and Cys73 (DL-PUB^62/73) by mass spectrometry. The cysteines involved in the double link are located in a helical region, in a conformation that reminds the position of the DL-PUB^50/61 in the β subunit of R-PE. The position of single linked PUB at Cys⁹⁵ and a single linked PEB at Cys¹⁷² were also confirmed. Spectroscopic studies show the presence of both types of chromophores and that there are not energy transfer by FRET among them.

Attachment of phycobilisomes in an antenna-photosystem I supercomplex of cyanobacteria

Oxygenic photosynthesis is driven by photosystems I and II (PSI and PSII, respectively). Both have specific antenna complexes and the phycobilisome (PBS) is the major antenna protein complex in cyanobacteria, typically consisting of a core from which several rod-like subcomplexes protrude. PBS preferentially transfers light energy to PSII, whereas a PSI-specific antenna has not been identified. The cyanobacterium Anabaena sp. PCC 7120 has rod-core linker genes (cpcG1-cpcG2-cpcG3-cpcG4). Their products, except CpcG3, have been detected in the conventional PBS. Here we report the isolation of a supercomplex that comprises a PSI tetramer and a second, unique type of a PBS, specific to PSI. This rod-shaped PBS includes phycocyanin (PC) and CpcG3 (hereafter renamed "CpcL"), but no allophycocyanin or CpcGs. Fluorescence excitation showed efficient energy transfer from PBS to PSI. The supercomplex was analyzed by electron microscopy and single-particle averaging. In the supercomplex, one to three rod-shaped CpcL-PBSs associate to a tetrameric PSI complex. They are mostly composed of two hexameric PC units and bind at the periphery of PSI, at the interfaces of two monomers. Structural modeling indicates, based on 2D projection maps, how the PsaI, PsaL, and PsaM subunits link PSI monomers into dimers and into a rhombically shaped tetramer or "pseudotetramer." The 3D model further shows where PBSs associate with the large subunits PsaA and PsaB of PSI. It is proposed that the alternative form of CpcL-PBS is functional in harvesting energy in a wide number of cyanobacteria, partially to facilitate the involvement of PSI in nitrogen fixation.

Phycobilisome: architecture of a light-harvesting supercomplex

The phycobilisome (PBS) is an extra-membrane supramolecular complex composed of many chromophore (bilin)-binding proteins (phycobiliproteins) and linker proteins, which generally are colorless. PBS collects light energy of a wide range of wavelengths, funnels it to the central core, and then transfers it to photosystems. Although phycobiliproteins are evolutionarily related to each other, the binding of different bilin pigments ensures the ability to collect energy over a wide range of wavelengths. Spatial arrangement and functional tuning of the different phycobiliproteins, which are mediated primarily by linker proteins, yield PBS that is efficient and versatile light-harvesting systems. In this review, we discuss the functional and spatial tuning of phycobiliproteins with a focus on linker proteins.

Biosynthesis of a stable allophycocyanin beta subunit in metabolically engineered Escherichia coli

Allophycocyanin (APC) is widely used as a fluorescent tag for fluorescence detection techniques. In this study, the apcB gene from a thermophilic cyanobacterium strain was cloned and ligated into an expression vector to construct a pathway for the biosynthesis of an allophycocyanin beta subunit (holo-apcBT) in Escherichia coli. Isopropyl β-d-1-thiogalactopyranoside induction successfully reconstituted holo-apcBT in E. coli. The recombinant holo-apcB showed spectroscopic properties similar to native APC. The stability and spectroscopic properties of this protein were then compared with another recombinant allophycocyanin beta subunit (holo-apcBM) whose apcB gene was cloned from mesophilic cyanobacterium. At high temperatures and during the course of storage, holo-apcBT was significantly more stable than holo-apcBM. In addition, holo-apcBT had an unexpectedly higher extinction coefficient and fluorescence quantum yield than holo-apcBM, suggesting that holo-apcBT would be a promising fluorescent tag and serve as a substitute for native APC.

Isolation, characterization and antioxidative activity of C-phycocyanin from Limnothrix sp. strain 37-2-1

C-phycocyanin (C-PC) is a blue colored accessory photosynthetic pigment found in cyanobacteria. Some of the medicinal properties of Spirulina have been attributed to this pigment, which includes anticancer, antioxidant, and anti-inflammatory activity. We have screened cyanobacteria isolated from freshwater habitats in Florida for their high content of C-PC. Of 125 strains tested, one filamentous strain identified as Limnothrix sp. was selected for further research. This strain produced 18% C-PC of total dry biomass. Here we describe a simple method for obtaining C-PC of high purity without the use of ion exchange chromatography. The procedure is based on pigment precipitation from the cell lysate with an appropriate concentration of ammonium sulfate, then purification with activated carbon and chitosan, followed by a sample concentration using tangential flow filtration. We have shown that when the lower concentration of ammonium sulfate was used, C-PC with higher purity index was recovered. Characterization of C-PC from Limnothrix showed that it had an absorbance maximum at 620nm and fluorescence at 639nm. The molecular mass of intact C-PC was estimated to be ~50kDa with α and β subunits forming dimmers. When C-PC content per unit biomass was compared to that of marketed Spirulina powder, we found that Limnothrix was superior. C-phycocyanin from Limnothrix had an antioxidative activity on DPPH free radicals similar to that found in a natural antioxidant - rutin.

Allophycocyanin trimer stability and functionality are primarily due to polar enhanced hydrophobicity of the phycocyanobilin binding pocket

Allophycocyanin (APC) is the primary pigment-protein component of the cores of the phycobilisome antenna complex. In addition to an extremely high degree of amino acid sequence conservation, the overall structures of APC from both mesophilic and thermophilic species are almost identical at all levels of assembly, yet APC from thermophilic organisms should have structural attributes that prevent thermally induced denaturation. We determined the structure of APC from the thermophilic cyanobacterium Thermosynechococcus vulcanus to 2.9 A, reaffirming the conservation of structural similarity with APC from mesophiles. We provide spectroscopic evidence that T. vulcanus APC is indeed more stable at elevated temperatures in vitro, when compared with the APC from mesophilic species. APC thermal and chemical stability levels are further enhanced when monitored in the presence of high concentrations of buffered phosphate, which increases the strength of hydrophobic interactions, and may mimic the effect of cytosolic crowding. Absorption spectroscopy, size-exclusion HPLC, and native gel electrophoresis also show that the thermally or chemically induced changes in the APC absorption spectra that result in the loss of the prominent 652-nm band in trimeric APC are not a result of physical monomerization. We propose that the bathochromic shift that occurs in APC upon trimerization is due to the coupling of the hydrophobicity of the alpha84 phycocyanobilin cofactor environment created by a deep cleft formed by the beta subunit with highly charged flanking regions. This arrangement also provides the additional stability required by thermophiles at elevated temperatures. The chemical environment that induces the bathochromic shift in APC trimers is different from the source of shifts in the absorption of monomers of the terminal energy acceptors APC(B) and L(CM), as visualized by the building of molecular models.

The structure at 2 Å resolution of Phycocyanin from Gracilaria chilensis and the energy transfer network in a PC–PC complex

Phycocyanin is a phycobiliprotein involved in light harvesting and conduction of light to the reaction centers in cyanobacteria and red algae. The structure of C-phycocyanin from Gracilaria chilensis was solved by X-ray crystallography at 2.0 A resolution in space group P2(1). An interaction model between two PC heterohexamers was built, followed by molecular dynamic refinement. The best model showed an inter-hexamer rotation of 23 degrees . The coordinates of a PC heterohexamer (alphabeta)(6) and of the PC-PC complex were used to perform energy transfer calculations between chromophores pairs using the fluorescence resonance energy transfer approach (FRET). Two main intra PC ((I)beta(3)(82)-->(I)alpha(1)(84)-->(I)alpha(5)(84)-->(I)beta(6)(82) and (I)beta(3)(153)-->(I)beta(5)(153)) and two main inter PC ((I)beta(6)(82)-->(II)beta(3)(82) and (I)beta(5)(153)-->(II)beta(3)(153)) pathways were proposed based on the values of the energy transfer constants calculated for all the chromophore pairs in the hexamer and in the complex.

A semiempirical approach to the intra-phycocyanin and inter-phycocyanin fluorescence resonance energy-transfer pathways in phycobilisomes

A semiempirical methodology to model the intra-phycocyanin and inter-phycocyanin fluorescence resonance energy-transfer (FRET) pathways in the rods of the phycobilisomes (PBSs) from Fremyella diplosiphon is presented. Using the Förster formulation of FRET and combining experimental data and PM3 calculation of the dipole moments of the aromatic portions of the chromophores, transfer constants between pairs of chromophores in the phycocyanin (PC) structure were obtained. Protein docking of two PC hexamers was used to predict the optimal distance and axial rotation angle for the staked PCs in the PBSs' rods. Using the distance obtained by the docking process, transfer constants between pairs of chromophores belonging to different PC hexamers were calculated as a function of the angle of rotation. We show that six preferential FRET pathways within the PC hexameric ring and 15 pathways between hexamers exist, with transfer constants consistent with experimental results. Protein docking predicted the quaternary structure for PCs in rods with inter-phycocyanin distance of 55.6 A and rotation angle of 20.5 degrees . The inter-phycocyanin FRET constant between chromophores at positions beta(155) is maximized at the rotation angle predicted by docking revealing the crucial role of this specific inter-phycocyanin channel in defining the complete set of FRET pathways in the system.

Characterization, structure and function of linker polypeptides in phycobilisomes of cyanobacteria and red algae: An overview

Cyanobacteria and red algae have intricate light-harvesting systems comprised of phycobilisomes that are attached to the outer side of the thylakoid membrane. The phycobilisomes absorb light in the wavelength range of 500-650 nm and transfer energy to the chlorophyll for photosynthesis. Phycobilisomes, which biochemically consist of phycobiliproteins and linker polypeptides, are particularly wonderful subjects for the detailed analysis of structure and function due to their spectral properties and their various components affected by growth conditions. The linker polypeptides are believed to mediate both the assembly of phycobiliproteins into the highly ordered arrays in the phycobilisomes and the interactions between the phycobilisomes and the thylakoid membrane. Functionally, they have been reported to improve energy migration by regulating the spectral characteristics of colored phycobiliproteins. In this review, the progress regarding linker polypeptides research, including separation approaches, structures and interactions with phycobiliproteins, as well as their functions in the phycobilisomes, is presented. In addition, some problems with previous work on linkers are also discussed.

Elucidation of the molecular structures of components of the phycobilisome: reconstructing a giant

The molecular architectures of photosynthetic complexes are rapidly becoming available through the power of X-ray crystallography. These complexes are comprised of antenna complexes, which absorb and transfer energy into photochemical reaction centers. Most reaction centers, found in both oxygenic and non-oxygenic species, are connected to transmembrane chlorophyll containing antennas, and the crystal structures of these antennas contain information on the structure of the entire complex as well as clear indications on their modes of functional association. In cyanobacteria and red alga, most of the Photosystem II associated light harvesting is performed by an enormous (3-7 MDa) membrane attached complex called the phycobilisome (PBS). While the crystal structures of many isolated components of different PBSs have been determined, the structure of the entire complex as well as its manner of association with Photosystem II can only be suggested. In this review, the structural information obtained on the isolated components will be described. The structural information obtained from the components provides the basis for the modeled reconstruction of this giant complex.

Allophycocyanin complexes from the phycobilisome of a thermophilic blue-green alga Myxosarcina concinna Printz

The core polypeptide components of the intact phycobilisomes (PBSs) prepared by the sucrose gradients in 0.9 M phosphate buffer from a thermophilic cyanobacterium Myxosarcina concinna Printz were investigated. Three allophycocyanins, designated AP1, AP2, and AP3, of the PBS cores were successfully prepared by using the gradient polyacrylamide gel electrophoresis (PAGE) performed in neutral, instead of alkaline, buffer system. The spectral properties of AP2 and AP3 demonstrated that they both had fluorescence emission maxima at 684/685 nm at 77 K, which was identical to those of the intact PBSs, and showed the absorption of allophycocyanin B (AP-B) subunit. Sodium dodecyl sulfate-PAGE revealed that the three biliprotein complexes were all composed of heterogeneous subunits and two more linker polypeptides (Ls), AP1 alpha(22.3)alpha(19.5)beta(17.4)beta(15.7)L(13.8)L(11.3)L(9.5), AP2 alpha(22.3)alpha(19.5)beta(17.4)beta(15.7)beta(15.1)L(11.3)L(9.5), and AP3 alpha(22.3)alpha(19.5)beta(17.4)beta(15.7)beta(15.1)L(11.3)L(9.5)L(8.3). Compared with the characteristics of AP1, beta(15.1), which belonged to the beta subunit group, was the AP-B subunit of AP2 and AP3. Because AP2 was only obtained together with the PBS by the aid of 2% (v/v) Triton X-100, but not AP3, it was closely related to anchoring the PBS core on thylakoid membranes though the polypeptide analysis showed that AP2 had no core-membrane linker (LCM). Aggregates of the three AP biliproteins were proposed based on the present results, and their functions in the PBS core construction and the energy transfer to PS II and PS I were discussed.