Phycoerythrocyanin Research Articles

Structure, function and applications of phycoerythrin: a unique light harvesting protein in algae

<p indent="0mm">Photosynthesis is an ancient and important chemical reaction process. Under light conditions, plants, algae, and certain bacteria convert solar energy into chemical energy. And then, chemical energy is directly used by living cells. A special light-harvesting pigment protein, phycobiliprotein (PBP), exists in red algae, cyanobacteria (blue-green algae), and some cryptophytes to capture light energy in the water environment. Since the first discovered of PBP, scientists have conducted in-depth research on the structure, function and application of PBP. PBP is a water-soluble protein. It is formed by the covalent connection of apoprotein and phycobilin. According to the spectral characteristics, PBP can be divided into four types: Phycoerythrin (PE), phycocyanin (PC), allophycocyanin (APC) and phycoerythrocyanin (PEC). Through the coordination of four PBPs, more than 95% of the energy can be efficiently transmitted. PE is the beginning of the energy transfer of the PBP system. It is composed of three subunits α, β and γ, and the molecular weight is about 240 kD. Among them, the γ subunit has the function of connecting and stabilizing, so that PE can exist in a stable (αβ) 6γ form. PE can covalently connect chromophores such as phycoerythrobilin (PEB) and phycourobilin (PUB), and has a strong absorption efficiency for short-wavelength blue and green light. It enables red algae and cyanobacteria to efficiently capture and transfer light energy in the deep blue and green light environment. PE is a bright red fluorescent protein. When excited by a specific wavelength, PE will emit strong fluorescence. Food-grade PE can be used as a natural colorant in food, cosmetics and other fields. High-purified PE is combined with proteins such as biotin and monoclonal antibodies to make fluorescent probes for immunodetection and other technologies; PE can also be used as a new photosensitizer for photodynamic therapy of deep tissues. In addition, PE has also been proven to have anti-oxidant and anti-inflammatory biological activities, and has a certain therapeutic effect on Alzheimer’s disease, cancer, diabetes, liver and kidney toxicity and other diseases. But there is no doubt that PE has great potential in the field of biotechnology. The application of PE is closely related to its purity. China’s abundant red algae resources provide a large amount of raw materials for large-scale preparation of PE. However, there are many obstacles in the preparation of high-purified PE. The most commonly used column chromatography has the problem of expansion, while other purification processes are not mature. In view of the above problems, this review highlights an overview on the structure, function and purification process of PE. Then the applications of PE in optical and biological activities are reviewed and prospected. This article provides a reference for the high value processing and application of marine algae.

Diverse Chromatic Acclimation Processes Regulating Phycoerythrocyanin and Rod-Shaped Phycobilisome in Cyanobacteria

Cyanobacteria have evolved various photoacclimation processes to perform oxygenic photosynthesis under different light environments. Chromatic acclimation (CA) is a widely recognized and ecologically important type of photoacclimation, whereby cyanobacteria alter the absorbing light colors of a supermolecular antenna complex called the phycobilisome. To date, several CA variants that regulate the green-absorbing phycoerythrin (PE) and/or the red-absorbing phycocyanin (PC) within the hemi-discoidal form of phycobilisome have been characterized. In this study, we identified a unique CA regulatory gene cluster encoding yellow–green-absorbing phycoerythrocyanin (PEC) and a rod-membrane linker protein (CpcL) for the rod-shaped form of phycobilisome. Using the cyanobacterium Leptolyngbya sp. PCC 6406, we revealed novel CA variants regulating PEC (CA7) and the rod-shaped phycobilisome (CA0), which maximize yellow–green light-harvesting capacity and balance the excitation of photosystems, respectively. Analysis of the distribution of CA gene clusters in 445 cyanobacteria genomes revealed eight CA variants responding to green and red light, which are classified based on the presence of PEC, PE, cpcL, and CA photosensor genes. Phylogenetic analysis further suggested that the emergence of CA7 was a single event and preceded that of heterocystous strains, whereas the acquisition of CA0 occurred multiple times. Taken together, these results offer novel insights into the diversity and evolution of the complex cyanobacterial photoacclimation mechanisms.

Content of phycoerythrin, phycocyanin, alophycocyanin and phycoerythrocyanin in some cyanobacterial strains: Applications.

Cyanobacteria are unique prokaryotes, which are capable to perform oxygenic photosynthesis. Within these organisms, phycobilisomes (PBS) act as an antenna of the photosynthetic pigment apparatus. Phycobilisomes contain several phycobiliproteins (PBP): phycoerythrin (PE), phycocyanin (PC), allophycocyanin (APC) and phycoerythrocyanin (PEC). The application of phycobiliproteins in the biotechnology, food industry and medicine during the last years is rapidly increasing. The aim of our study was to assess the qualitative and quantitative content of phycoerythrin, phycocyanin, allophycocyanin and phycoerythrocyanin in 14 cyanobacterial strains kept in Plovdiv Algal Culture Collection (PACC) and 4 strains purchased from the Culture Collection of Autotrophic Organisms (CCALA). Our data demonstrated that three strains of Microcoleus autumnalis (PACC 5505, PACC 5522 and PACC 5527) have high potential to produce phycoerythrins (0.132, 0.201 and 0.136 mg/mL, respectively). Similarly, one Microcoleus autumnalis strain (PACC 5522) and one strain of Leptolyngbya boryana (CCALA 084) are suitable for biotechnological production of phycocyanins (0.051 and 0.264 mg/mL, respectively) as well as allophycocyanins (0.102 and 0.171 mg/mL, respectively). In addition, the data about the pigment content could be used as a biochemical marker for taxonomic purposes within the group.

Characterization and responses to environmental cues of a photosynthetic antenna-deficient mutant of the filamentous cyanobacterium Anabaena sp. PCC 7120

The cyanobacterial phycobilisome (PBS) is a giant pigment-protein complex which harvests light energy for photosynthesis and comprises two structures: a core and peripheral rods. Most studies on PBS structure and function are based on mutants of unicellular strains. In this report, we describe the phenotypic and genetic characterization of a transposon mutant of the filamentous Anabaena sp. strain PCC 7120, denoted LC1, which cannot synthesize the phycobiliprotein phycocyanin (PC), the main component of the rods; in this mutant, the transposon had inserted into the cpcB gene (orf alr0528) which putatively encodes PC-β chain. Mutant LC1 was able to synthesize phycoerythrocyanin (PEC), a phycobiliprotein (PBP) located at the terminal region of the rods; but in the absence of PC, PEC did not attach to the PBSs that only retained the allophycocyanin (APC) core; ferredoxin: NADP+-oxidoreductase (FNR) that is associated with the PBS in the wild type, was not found in isolated PBSs from LC1. The performance of the mutant exposed to different environmental conditions was evaluated. The mutant phenotype was successfully complemented by cloning and transfer of the wild type complete cpc operon to mutant LC1. Interestingly, LC1 compensated its mutation by significantly increasing the number of its core-PBS and the effective quantum yield of photosystem II (PSII) photochemistry; this feature suggests a more efficient energy conversion in the mutant which may be useful for biotechnological applications.

Comparison of the photosynthetic characteristics of two developmental stages inNostoc sphaeroides Kützing (cyanophyta)

The photosynthetic activities between two main developmental stages, colony and hormogonium, of the edible cyanobacteriumNostoc sphaeroides Kutzing, were compared. Hormogonia have a higher content of chlorophyll than that of colonies. It showed that the ratios of phycocyain (PC), allophycocyain (APC) and phycoerythrocyanin (PEC) in hormogonia and colonies were different. The room temperature chlorophyll fluorescence, 77 K chlorophyll fluorescence, measurements of PS I and PS II activities all showed that colony has higher photosynthetic competence than hormogonia. Hormogonia had a higher respiration rate were very close. The responses of hormogonia and colonies to high light illuminations also were different. Both of their oxygen evolution rates decreased quickly with the prolonged high light illumination, but hormogonia can keep relatively higher PS II activity (Fv/Fm) than that of colonies. The results suggested that colony was photosynthetically more competent than hormogonia, while the ability of hormogonia to tolerate high light illumination was higher than that of colony.

Energy transfer in monomeric phycoerythrocyanin

Phycoerythrocyanin (PEC) is part of the phycobilisome of cyanobacteria. Its monomer carries one phycoviolobilin and two phycocyanobilins (PCB) as chromophores. For an understanding of the complicated energy transfer in phycobilisomes, a detailed knowledge of the processes in the constituting building proteins is indispensable. We report the experimental data necessary for the description of Förster energy transfer in monomeric PEC, including fluorescence lifetimes and quantum yields of the two subunits. The bulk experiments are complemented by studies of single PEC molecules. Förster energy calculations and Monte Carlo simulations based on the bulk data are presented. They reveal that earlier experimental findings of energy transfer heterogeneities in single PEC molecules originate in spectral shifts between the contributing chromophores.

Spectroscopy of Single Phycoerythrocyanin Monomers: Dark State Identification and Observation of Energy Transfer Heterogeneities

Phycoerythrocyanin (PEC) is part of the light harvesting system of cyanobacteria. The PEC monomer contains one phycoviolobilin chromophore, which transfers excitation energy onto two phycocyanobilin chromophores. Many spectroscopical methods have been used in the past to study the bulk properties of PEC. These methods average over many molecules. Therefore, differences in the behavior of individual molecules remain hidden. The energy transfer within photosynthetic complexes is however sensitive to changes in the spectroscopic properties of the participating subunits. Knowledge about heterogeneities is therefore important for the description of the energy transfer in photosynthetic systems. Here, the recording of the fluorescence emission of single PEC molecules is used as a tool to obtain such information. Spectrally resolved detection as well as double resonance excitation of single PEC molecules is used to investigate their bleaching behavior. The trans isomer of the phycoviolobilin chromophore is identified as a short-lived dark state of monomeric PEC. Polarization sensitive single molecule detection is used for the direct observation of the energy transfer in individual PEC molecules. The experiments reveal that more than one-half of the PEC molecules exhibit an energy transfer behavior significantly different from the bulk. These heterogeneities persist on a time scale of several seconds. Model calculations lead to the conclusion that they are caused by minor shifts in the spectra of the chromophores.

Structure and function of PBS model complex

A complex of phycobiliproteins, containing phycoerythrocyanin (PEC), C-phycovyanin (C-PC) and allo-phycocyanin (APC) as well as some linker polypeptides, was reconstructed. The absorption and fluoreacence spectra of the complex were compared with those of native phycobilisomes (PBS) and the phycobiliproteins. Based on the measured data, it can be concluded that the complex can be taken as a model of PBS and is an entirely functional group for excitation energy transfer step by step from peripheral PEC to APC. The single terminal emitter feature of the complex makes it favorable for clarifying energy transfer pathways and the kinetics in comparison with native PBS. Further research is carried on in the lab.

Z,E isomerization of the α-84 phycoviolobilin chromophore of phycoerythrocyanin from Mastigocladus laminosus investigated by Fourier-transform infrared difference spectroscopy

The photoreaction of the phycoviolobilin (PVB) chromophore-containing part of phycoerythrocyanin (PEC) from Mastigocladus laminosus was investigated by Fourier-transform infrared spectroscopy (FT-IR). Difference spectra between the parent states P566 and P507 were obtained in 1H 2O and 2H 2O for the first time. The spectra are generally characterised by large changes in the range between 1710 and 1590 cm −1 and by a strong difference band around 1270 cm −1. In order to study the influence on the PVB chromophore upon aggregation, spectra of the α-subunit and the (αβ) 3 trimer are compared, showing distinct differences which may be of relevance for the chromophore-protein and protein-protein interactions. The difference spectra demonstrate many similarities to the spectra recently obtained for the P r→meta-R c transition of phytochrome [Foerstendorf et al. (1996) Biochemistry 35, 10793–10799]. In particular, a band around 1710 cm −1, which was tentatively assigned to the C=O stretch of ring D is also observed in the spectra of PEC. It strongly supports this identification and the deduced molecular interpretation on the protonation state of the chromophore.

Chromophore assignment in phycoerythrocyanin from Mastigocladus laminosus

The component spectra (maxima of absorption, circular and linear dichroism) of individual chromophores have been assigned for phycoerythrocyanin (PEC) trimer, monomer(s), and its subunits (α-PEC and β-PEC) by titration with p-chloromercury-benzene-sulfonate (PCMS), linear dichroism and photochemical transformations, as well as by deconvolution using a ‘bilin’ line-shape spectrum based on the α-84 phycoviolobilin-chromophore in the α-subunit. The level ordering PVB-α-84 → PCB-β-155 → PCB-β-84 is the same irrespective of aggregation. Two different monomers (αβ) were observed. In 4 M urea, the spectra are appropriately weighted sums of the subunit spectra, whereas in the monomer obtained in 1 M KSCN, both β-chromophores are red-shifted by 4–5 nm. Formation of trimer (αβ)_3gives considerable spectral changes: (1) the absorption is narrowed, which has been rationalized by excitonic coupling between neighbouring monomers, (2) the short wavelength part in the CD spectrum is missing and (3) a fourth band (+) at 528 in the LD spectrum appears. A deconvolution of the trimeric aggregation state using only the ‘bilin’ line-shape model is not possible.

Candidate Genes for the Phycoerythrocyanin α Subunit Lyase.: BIOCHEMICAL ANALYSIS OF pecE AND pecF INTERPOSON MUTANTS

The rod substructures of the Anabaena sp. PCC 7120 phycobilisome contain the light harvesting proteins C-phycocyanin and phycoerythrocyanin (PEC). Even at low light intensities, PEC represents no more than 5% of the phycobilisome protein. The beta subunits of both proteins carry thioether-linked phycocyanobilin (PCB) at beta-Cys-82 and beta-Cys-155; however, C-phycocyanin has PCB at alpha-Cys-84 whereas PEC alpha subunit carries phycobiliviolin at this position. The Anabaena sp. PCC 7120 pec operon is made up of five genes. PecB and pecA encode the beta and alpha subunits of PEC, pecC encodes a linker polypeptide associated with PEC in the rod substructure, and pecE and pecF are genes of unknown function that show a high degree of homology to cpcE and cpcF, that encode a C-phycocyanin alpha subunit PCB lyase (Fairchild, C. D., Zhao, J., Zhou, J., Colson, S. E., Bryant, D. A., and Glazer, A. N. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 7017-7021). Insertional mutants in pecE and pecF, and an interposon mutant in which a portion of both pecE and pecF was deleted, were constructed. All three types of mutants grew 1.3 times slower than wild-type under limiting light conditions and showed a 20% reduction in the PCB content of whole cells relative to chlorophyll alpha. Holo-PEC was missing from the phycobilisomes of all three types of mutants and the level of the PEC linker polypeptide was reduced relative to the wild-type. However, approximately 30% of the wild-type level of the PEC beta subunit was present in all of these phycobilisomes. In contrast, the PEC alpha subunit was barely detectable in the pecE and pecF mutants, but was present in the pecEF deletion mutant as a PCB-adduct in a 1:1 ratio with the PEC beta subunit. The identity of this "unnatural" adduct was confirmed by isolation of the subunit and amino-terminal sequencing. These biochemical results support the inference that pecE and pecF encode a PEC alpha subunit phycobiliviolin lyase, and, in conjunction with earlier findings, demonstrate that phycobiliprotein bilin lyases show high selectivity (rather than absolute specificity) for both the bilin and the polypeptide substrate.

Photophysics of phycoerythrocyanins from the cyanobacterium Westiellopsis prolifica studied by time-resolved fluorescence and coherent anti-Stokes Raman scattering spectroscopy

Three building blocks of the antenna complexes of the cyanobacterium Westiellopsis prolifica were studied: PEC(X), which is similar to the α-subunit of phycoerythrocyanin (PEC), trimers of PEC and monomers derived from these by deaggregation with KSCN. The fit of the fluorescence decay curve of PEC(X) requires at least four exponentials, although it supposedly contains only one chromophore. The coherent anti-Stokes Raman scattering (CARS) spectra indicate that the heterogeneity observed is due to geometrical isomers, which are in part generated by photoinduced processes. A similar heterogeneity in chromophore structure and properties is also found in the monomers, where four exponentials are needed to fit the fluorescence decay curve. As in trimers, there is a long-lived, low-amplitude component, which can be assigned to impurities and/or oxidation products. The energy transfer time between the two phyocyanobilin chromophores in the β-subunit is about 500 ps; the lifetime of the fluorescing β-chromophore is 1.5 ns. The phycoviolobilin chromophore in the α-subunit adopts different geometries characterized by fluorescence lifetimes of about 240 and 800 ps. No evidence was found for energy transfer between the α-chromophore and the β-chromophores. This energy transfer occurs in trimers on a time scale of less than 20 ps; the energy transfer time between the two different types of β-chromophore is about 250 ps and the lifetime of the terminal emitter is about 1.5 ns. The excited state kinetics are therefore similar to those of PEC trimers from Mastigocladus laminosus, as are the CARS spectra, indicating a similar chromophore—protein arrangement. In comparison with phycocyanin, the ordering of the excited states of chromophores β84 and β155 may be changed. Although PEC trimers of Westiellopsis prolifica show almost as good a photostability as trimers of Mastigocladus laminosus, monomers are so photolabile that no CARS spectra could be recorded.

Spectral properties of Mastigocladus laminosus phycobilisomes embedded in poly(vinyl alcohol) films

Polarized absorption, fluorescence and delayed time-resolved luminescence spectra were measured of phycobilisomes isolated from the cyanobacterium Mastigocladus laminosus and embedded in isotropic and anisotropic (stretched) poly(vinyl alcohol) (PVA) films. Stretching induced orientation of phycobilisomes and allowed differently oriented groups of chromophores to be photoselected by polarized light. From the set of polarized emission components, the anisotropy of fluorescence coefficients for the pools of differently oriented molecules was calculated. These coefficients are different in various regions of biliprotein emission. Contributions to emission anisotropy from pools of differently oriented chromophores are especially well resolved for a group of molecules oriented at large angles with respect to the PVA stretching axis. Polarized fluorescence spectra suggest strongly that more than one chain of pigment engaged in the energy transfer process is present in the investigated phycobilisomes. Delayed luminescence was observed in a region of prompt fluorescence and/or in the longer-wavelength region. Its intensity is about 10% of that of the prompt fluorescence. Decay times and yields of delayed luminescence excited in the phycoerythrocyanin (PEC) and C-phycocyanin (C-PC) absorption regions are different (τ PEC = 12 μs, τ C−PC = 38 μs). Delayed emission is probably, as for phycobilisomes without phycoerythrin, due to ionization of chromophores and delayed recombination.

Molecular assembly of the phycobilisomes from the cyanobacterium Mastigocladus laminosus

Five phycocyanin- and two phycoerythrocyanin-associated linker polypeptides were resolved by two-dimensional gel electrophoresis from isolated phycobilisomes of the thermophilic cyanobacterium Mastigocladus laminosus. These linker polypeptides were located in the substructure of the phycobilisome by analysis of isolated high molecular weight allophycocyanin—phycocyanin (AP—PC) complexes and by examination of different phycobilisome “types” induced by alteration of the culture conditions. The core fractions reveal two different rod—core linker polypeptides L RC 29.5PC and L RC 32PC and one rod linker L R 31PC associating the first and the following “trimeric” PC complexes at the profile sides of the core. The “minimal” phycobilisome type with an apparent molecular weight M r of 4 500 000 is characterized by the occurrence of only one additional rod linker polypeptide L R 34.5PC. Further sequential elongation of the rods occurs by PC and phycoerythrocyanin (PEC) “trimers” in cooperation with two low molecular weight linker polypeptides L R 11PC and L R 11PEC. The structure of the “maximal” phycobilisome with M r 8 500 000 is completed by the association of PEC “hexamers” via L R 34.5PEC. This sequential assembly could be demonstrated by the polypeptide composition of different “intermediate” and “maximal” phycobilisomes. The large number of linker polypeptides and the stoichiometric calculations of core fractions and of different phycobilisome “types” indicate a partial “trimeric” organization of the rod complexes. With respect to polypeptide stoichiometry, the “trimeric” arrangement of PC·L RC 29.5/32, PC·L R 31, PC·L R 11 and PEC·L R 11 in the structure of the rods is proposed. The structural and functional consequences of the study are represented in a model of the “maximal” phycobilisome.

FLUORESCENCE AND CIRCULAR DICHROISM STUDIES ON THE PHYCOERYTHROCYANINS FROM THE CYANOBACTERIUM

AbstractTwo phycoerythrocyanin (PEC) fractions have been obtained from the phycobilisomes of the cyanobac‐terium Westiellopsis prolifica ARM 365. They have been characterized by absorption, fluorescence and circular dichroism spectroscopy. One of them is spectroscopically similar to a PEC trimer known from other organisms. Whereas efficient energy transfer from its violin (α‐84) to the cyanin (β‐84, 155) chromophores is efficient in the trimer (αβ it is impeded after dissociation to the monomer (α,β). A second fraction of PEC which we earlier termed PEC(X) (Maruthi Sai et al., Photochem. Photobiol.55,119–124, 1992), exhibited the spectral properties similar to that of the α‐subunit of PEC from Mastigocladus laminosus. With this highly photoactive fraction, the circular dichroism spectra of the violobilin chromophore in both photoreversible states were obtained.

Genes encoding both subunits of phycoerythrocyanin, a light-harvesting biliprotein from the cyanobacterium Mastigocladus laminosus

Phycocyanin (PC) and phycoerythrocyanin (PEC) are light-harvesting components of the phycobilisome (PbS) from the cyanobacterium Mastigocladus laminosus. These two biliproteins are closely related, and show a particularly high degree of sequence homology in the C-terminal part of their β-subunits. A 198-bp gene fragment encoding this region of PC from M. laminosus was therefore used as a heterologous hybridization probe to identify the genes coding for PEC from the same organism. A 1.7-kb HindIII fragment was cloned and its sequence determined. Three open reading frames (ORFs) were found on this fragment. The gene coding for the β-subunit of PEC ( pecB) was followed downstream by the α-subunit encoding gene ( pecA). This gene arrangement had also been found in the PC-encoding ( cpc) gene pair from M. laminosus, and is conserved in cpc genes from other organisms. This finding is compatible with a model of evolution of the cpc and pec gene pairs as the product of gene duplication of an ancestral β- and α-subunit-encoding pair. A third ORF starts downstream from pecA. It codes for the 34.5-kDa linker protein, which forms complexes with PEC with a 1:6 stoichiometry in the PbS. Biliprotein- and linker protein-encoding genes are frequently clustered, and this provides mechanisms for the production of the different stoichiometric amounts of these gene products required in the PbS and for coregulation by environmental factors.

Crosslinking of phycobiliproteins from the cyanobacterium Mastigocladus laminosus with bis-imidates: localization of an intrasubunit and an intersubunit crosslink in C-phycocyanin.

The light-harvesting pigment-protein complexes allophycocyanin (AP), C-phycocyanin (PC) and phycoerythrocyanin (PEC) of the cyanobacterium Mastigocladus laminosus consist of alpha- and beta-subunits containing about 170 amino-acid residues each. These two subunits form an alpha,beta-monomer, three of which build up a disc-shaped trimer. In this study these phycobiliproteins were crosslinked with bis-imidates. Various spacer lengths of the reagent and various aggregation states of the phycobiliprotein were tested. An intersubunit crosslink could be verified in all three phycobiliproteins. PC-trimers were crosslinked with the homobifunctional reagent dimethyl pimelimidate having a maximal crosslinking distance of 10 A. Two crosslinks could be identified: an intramonomer intersubunit crosslink with a yield of 48% and an intrasubunit crosslink within alpha PC (57%). These products were chemically and enzymatically fragmented and the small crosslinked peptides were isolated and then identified by amino-acid analysis. The following amino acids were crosslinked: alpha-Val 1 with beta-Ala 1 and alpha-Lys 62 with alpha-Lys 134. Both crosslinks could be localized within the known three-dimensional structure of PC.

Linker polypeptides of the phycobilisome from the cyanobacterium Mastigocladus laminosus. II. Amino-acid sequences and functions.

The complete primary structure of an 80-residue linker polypeptide, LR(C)8.9, from the phycobilisome of the cyanobacterium Mastigocladus laminosus was determined as well as the 44 N-terminal residues of the two linker polypeptides LR34.5,PEC and LR34.5,PC and the 114 C-terminal residues of LR34.5,PEC. A brief description of the structure determination and an extensive discussion of the relationships of these polypeptides have been published recently (Füglistaller, P., Suter, F. & Zuber, H. (1985) Biol. Chem. Hoppe-Seyler 366, 993-1001). In this paper we report in detail about the elucidation of the primary structures. Limited digestion of the hexameric phycobiliprotein-linker polypeptide complex (alpha PEC beta PEC)6LR34.5,PEC with various proteases resulted in a linker polypeptide diminished by a 1-5 kDa segment, while the phycobiliproteins remained intact. By N-terminal sequence analysis of the residual part of the linker polypeptide in the complex, LR34.5-5,PEC, it was concluded that the C-terminus of the polypeptide had been attacked by the proteases. This C-terminal part of the protein influences the hexamer formation of phycoerythrocyanin (PEC) and is responsible for the linkage between two phycobiliprotein hexamers. From the function of the C-terminal segment of LR34.5,PEC and its homology to the LR(C)8.9 polypeptide, it was concluded that LR(C)8.9 is located at the end of the peripheral phycobilisomal rods distal to the allophycocyanin core.