Phycobiliproteins: Molecular Aspects of Photosynthetic Antenna System. In: Fong FK, editor. Light Reaction Path of Photosynthesis. Berlin: Springer Berlin Heidelberg; 1982. pp. 7-45. [DOI: 10.1007/978-3-642-81795-3_2]

A rising tide of blue-absorbing biliprotein photoreceptors: characterization of seven such bilin-binding GAF domains in Nostoc sp. PCC7120

Cyanobacteriochromes are photochromic sensory photoreceptors in cyanobacteria that are related to phytochromes but cover a much broader spectral range. Using a homology search, a group of putative blue-absorbing photoreceptors was identified in Nostoc sp. PCC 7120 that, in addition to the canonical chromophore-binding cysteine of cyanobacteriochromes, have a conserved extra cysteine in a DXCF motif. To assess their photochemical activities, putative chromophore-binding GAF domains were expressed in Escherichia coli together with the genes for phycocyanobilin biosynthesis. All except one covalently bound a chromophore and showed photoreversible photochromic responses, with absorption at approximately 420 nm for the 15Z states formed in the dark, and a variety of red-shifted absorption peaks in the 490-600 nm range for the 15E states formed after light activation. Under denaturing conditions, the covalently bound chromophores were identified as phycocyanobilin, phycoviolobilin or mixtures of both. The canonical cysteines and those of the DXCF motifs were mutated, singly or together. The canonical cysteine is responsible for stable covalent attachment of the bilin to the apo-protein at C3(1) . The second linkage from the cysteine in the DXCF motif, probably to C10 of the chromophore, yields blue-absorbing rubin-type 15Z chromophores, but is lost in most cases upon photoconversion to the 15E isomers of the chromophores, and also when denatured with acidic urea.

Toward a mechanism for biliprotein lyases: revisiting nucleophilic addition to phycocyanobilin

Biliprotein lyases attach linear-tetrapyrrolic bilins covalently to apoproteins, which is a prerequisite for the assembly of phycobiliproteins into phycobilisomes, the light-harvesting complexes of cyanobacteria. On the basis of the addition of thiol and imidazole to phycocyanobilin, we propose a generalized lyase reaction mechanism. The adducts contain isomerized phycocyanobilin that can be transferred by the lyase to apoproteins by either back-isomerization, generating phycocyanobilin-containing proteins, or direct transfer, generating phycoviolobilin-containing proteins.

Biliprotein maturation: the chromophore attachment

Biliproteins are a widespread group of brilliantly coloured photoreceptors characterized by linear tetrapyrrolic chromophores, bilins, which are covalently bound to the apoproteins via relatively stable thioether bonds. Covalent binding stabilizes the chromoproteins and is mandatory for phycobilisome assembly; and, it is also important in biliprotein applications such as fluorescence labelling. Covalent binding has, on the other hand, also considerably hindered biliprotein research because autocatalytic chromophore additions are rare, and information on enzymatic addition by lyases was limited to a single example, an EF-type lyase attaching phycocyanobilin to cysteine-alpha84 of C-phycocyanin. The discovery of new activities for the latter lyases, and of new types of lyases, have reinvigorated research activities in the subject. So far, work has mainly concentrated on cyanobacterial phycobiliproteins. Methodological advances in the process, however, as well as the finding of often large numbers of homologues, opens new possibilities for research on the subsequent assembly/disassembly of the phycobilisome in cyanobacteria and red algae, on the assembly and organization of the cryptophyte light-harvesting system, on applications in basic research such as protein folding, and on the use of phycobiliproteins for labelling.

Phycobilin:cystein-84 biliprotein lyase, a near-universal lyase for cysteine-84-binding sites in cyanobacterial phycobiliproteins

Phycobilisomes, the light-harvesting complexes of cyanobacteria and red algae, contain two to four types of chromophores that are attached covalently to seven or more members of a family of homologous proteins, each carrying one to four binding sites. Chromophore binding to apoproteins is catalyzed by lyases, of which only few have been characterized in detail. The situation is complicated by nonenzymatic background binding to some apoproteins. Using a modular multiplasmidic expression-reconstitution assay in Escherichia coli with low background binding, phycobilin:cystein-84 biliprotein lyase (CpeS1) from Anabaena PCC7120, has been characterized as a nearly universal lyase for the cysteine-84-binding site that is conserved in all biliproteins. It catalyzes covalent attachment of phycocyanobilin to all allophycocyanin subunits and to cysteine-84 in the beta-subunits of C-phycocyanin and phycoerythrocyanin. Together with the known lyases, it can thereby account for chromophore binding to all binding sites of the phycobiliproteins of Anabaena PCC7120. Moreover, it catalyzes the attachment of phycoerythrobilin to cysteine-84 of both subunits of C-phycoerythrin. The only exceptions not served by CpeS1 among the cysteine-84 sites are the alpha-subunits from phycocyanin and phycoerythrocyanin, which, by sequence analyses, have been defined as members of a subclass that is served by the more specialized E/F type lyases.

Biliprotein chromophore attachment: chaperone-like function of the PecE subunit of alpha-phycoerythrocyanin lyase

Biliproteins are post-translationally modified by chromophore addition. In phycoerythrocyanin, the heterodimeric lyase PecE/F covalently attaches phycocyanobilin (PCB) to cysteine-alpha84 of the apoprotein PecA, with concomitant isomerization to phycoviolobilin. We found that: (a) PecA adds autocatalytically PCB, yielding a low absorbance, low fluorescence PCB.PecA adduct, termed P645 according to its absorption maximum; (b) In the presence of PecE, a high absorbance, high fluorescence PCB.PecA adduct is formed, termed P641; (c) PecE is capable of transforming P645 to P641; (d) When in stop-flow experiments, PecA and PecE were preincubated before chromophore addition, a red-shifted intermediate (P650, tau=32 ms) was observed followed by a second, which was blue-shifted (P605, tau=0.5 s), and finally a third (P638, tau=14 s) that yielded the adduct (P641, tau=20 min); (e) The reaction was slower, and P605 was missing, if PecA and PecE were not preincubated; (f) Gel filtration gave no evidence of a stable complex between PecA and PecE; however, complex formation is induced by adding PCB; and (g) A red-shifted intermediate was also formed, but more slowly, with phycoerythrobilin, and denaturation showed that this is not yet covalently bound. We conclude, therefore, that PecA and PecE form a weak complex that is stabilized by PCB, that the first reaction step involves a conformational change and/or protonation of PCB, and that PecE has a chaperone-like function on the chromoprotein.

Chromophore attachment in phycocyanin. Functional amino acids of phycocyanobilin--alpha-phycocyanin lyase and evidence for chromophore binding

Covalent attachment of phycocyanobilin (PCB) to the alpha-subunit of C-phycocyanin, CpcA, is catalysed by the heterodimeric PCB : CpcA lyase, CpcE/F [Fairchild CD, Zhao J, Zhou J, Colson SE, Bryant DA & Glazer AN (1992) Proc Natl Acad Sci USA89, 7017-7021]. CpcE and CpcF of the cyanobacterium, Mastigocladus laminosus PCC 7603, form a 1 : 1 complex. Lyase-mutants were constructed to probe functional domains. When in CpcE (276 residues) the N terminus was truncated beyond the R33YYAAWWL motif, or the C terminus beyond amino acid 237, the enzyme became inactive. Activity decreases to 20% when C-terminal truncations went beyond L275, which is a key residue: the K(m) of CpcE(L275D) and (L276D) increased by 61% and 700%, k(cat)/K(m) decreased 3- and 83-fold, respectively. The enzyme also lost activity when in CpcF (213 residues) the 20 N-terminal amino acids were truncated; truncation of 53 C-terminal amino acids inhibited complex formation with CpcE, possibly due to misfolding. According to chemical modifications, one accessible arginine and one accessible tryptophan are essential for CpcE activity, and one carboxylate for CpcF. Both subunits bind PCB, as assayed by Ni2+ affinity chromatography, SDS/PAGE and Zn2+-induced fluorescence. The bound PCB could be transferred to CpcA to yield alpha-CPC. The PCB transfer capacity correlates with the activity of the lyase, indicating that PCB bound to CpcE/F is an intermediate of the enzymatic reaction. A catalytic mechanism is proposed, in which a CpcE/F complex binds PCB and adjusts via a salt bridge the conformation of PCB, which is then transferred to CpcA.

Photochromic chromopeptides derived from phycoerythrocyanin: biophysical and biochemical characterization

Truncated chromopeptides have been prepared from the small photo- and redox-switchable biliprotein alpha-phycoerythrocyanin (alpha-PEC). The native chromoprotein consists of a C-terminal globin domain containing the chromophore and the regulatory cysteins 98 and 99, and a two-helix (X,Y) N-terminal domain responsible for aggregation. Digestion with chymotrypsin-free trypsin leads to three chromopeptides, (N-30, N-33 and N-35), basically lacking the two N-terminal helices X and Y. The photo- and redox chemistry of the major product (N-33) is identical, qualitatively and quantitatively, to that of native alpha-PEC. A series of N- and C-terminally truncated polypeptides were expressed in E. coli and subjected to autocatalytic and enzymatic reconstitution with phycocyanobilin. Enzymatic reconstitution was possible with N-terminally truncated polypeptides up to 45 aa, while neither a more extensively shortened (N-63) peptide, nor two C-terminally shortened polypeptides could be reconstituted. All chromopeptides recovered from enzymatic reconstitution contained the native phycoviolobilin chromophore and showed the photochemical and redox reactivity of alpha-PEC, albeit quantitatively reduced in the N-45 chromopeptide.

Light and adaptive responses in red macroalgae: an overview

The most recent research shows an increased interest in algal photobiology following the modifications of light spectral composition induced by the 'ozone depletion' in the atmosphere. The consequences of this change on the macrophytic red algae, which possess accessory light-harvesting complexes, are currently uncertain. Moreover the underwater light field of coastal waters, where most macroalgae are living, has been well characterized only in the last few years. The presence and the variability of light components, such as red, far-red, green, blue and ultraviolet radiation, in these environments, at different depths, have raised the question of whether red macroalgae are 'light-intensity or/and light-quality adapters'. In this paper various adaptive responses of red algae, both in the field and under culture, are discussed in order to compare the physiological and ecological results. All these studies seem to suggest that red algae are both 'light-intensity and light-quality adapters'. In particular, the light quality and, overall, the modulation of the ratios between spectral components seem to play the role of photomorphogenic 'signals' regulating algal metabolism and growth. Short- and long-term responses, and potential strategies and mechanisms for adaptation to light at individual, cellular and molecular levels, are discussed with special reference to the photosynthetic equipment.

E. Antonini Plenary lecture. A structural basis of light energy and electron transfer in biology

Aspects of intramolecular light energy and electron transfer will be discussed for three protein cofactor complexes, whose three-dimensional structures have been elucidated by X-ray crystallography: components of light-harvesting cyanobacterial phycobilisomes, the purple bacterial reaction centre and the blue multi-copper oxidases. A wealth of functional data is available for these systems which allow specific correlations between structure and function, and general conclusions about light energy and electron transfer in biological materials to be made.

Nobel lecture. A structural basis of light energy and electron transfer in biology

Aspects of intramolecular light energy and electron transfer will be discussed for three protein cofactor complexes, whose three-dimensional structures have been elucidated by x-ray crystallography: Components of light harvesting cyanobacterial phycobilisomes, the purple bacterial reaction centre, and the blue multi-copper oxidases. A wealth of functional data is available for these systems which allow specific correlations between structure and function and general conclusions about light energy and electron transfer in biological materials to be made.