Chlorophyll-Protein Complexes of the Cyanophyte, Nostoc sp

Phycobilisome structure and function

Phycobilisomes are aggregates of light-harvesting proteins attached to the stroma side of the thylakoid membranes of the cyanobacteria (blue-green algae) and red algae. The water-soluble phycobiliproteins, of which there are three major groups, tetrapyrrole chromophores covalently bound to apoprotein. Several additional protiens are found within the phycobilisome and serve to link the phycobiliproteins to each other in an ordered fashion and also to attach the phycobilisome to the thylakoid membrane. Excitation energy absorbed by phycoerythrin is transferred through phycocyanin to allophycocyanin with an efficiency approximating 100%. This pathway of excitation energy transfer, directly confirmed by time-resolved spectroscopic measurements, has been incorporated into models describing the ultrastructure of the phycobilisome. The model for the most typical type of phycobilisome describes an allophycocyanin-containing core composed of three cylinders arranged so that their longitudinal axes are parallel and their ends form a triangle. Attached to this core are six rod structures which contain phycocyanin proximal to the core and phycoerythrin distal to the core. The axes of these rods are perpendicular to the longitudinal axis of the core. This arrangement ensures a very efficient transfer of energy. The association of phycoerythrin and phycocyanin within the rods and the attachment of the rods to the core and the core to the thylakoid require the presence of several 'linker' polypeptides. It is recently possible to assemble functionally and structurally intact phycobilisomes in vitro from separated components as well as to reassociate phycobilisomes with stripped thylakoids. Understanding of the biosynthesis and in vivo assembly of phycobilisomes will be greatly aided by the current advances in molecular genetics, as exemplified by recent identification of several genes encoding phycobilisome components.Combined ultrastructural, biochemical and biophysical approaches to the study of cyanobacterial and red algal cells and isolated phycobilisome-thylakoid fractions are leading to a clearer understanding of the phycobilisome-thylakoid structural interactions, energy transfer to the reaction centers and regulation of excitation energy distribution. However, compared to our current knowledge concerning the structural and functional organization of the isolated phycobilisome, this research area is relatively unexplored.

Isolation of chlorophyll-protein complexes and quantification of electron transport components in Synura petersenii and Tribonema aequale

The chlorophyll-protein complexes of the yellow alga Synura petersenii (Chrysophyceae) and the yellow-green alga Tribonema aequale (Xanthophyceae) were studied. The sodiumdodecylsulfate/sodiumdesoxycholate solubilized photosynthetic membranes of these species yielded three distinct pigment-protein complexes and a non-proteinuous zone of free pigments, when subjected to SDS polyacrylamid gel electrophoresis. The slowest migrating protein was identical to complex I (CP I), the P-700 chlorophyll a-protein, which possessed 60 chlorophyll a molecules per reaction center in Tribonema and 108 in Synura. The zone of intermediate mobility contained chlorophyll a and carotenoids. The absorption spectrum of this complex was very similar to the chlorophyll a-protein of photosystem II (CP a), which is known from green plants. The fastest migrating pigment protein zone was identified as a light-harvesting chlorophyll-protein complex. In Synura this protein was characterized by the content of chlorophyll c and of fucoxanthin. Therefore this complex will be named as LH Chl a/c-fucocanthin protein. In addition to the separation of the chlorophyll-protein complexes the cellular contents of P-700, cytochrome f (bound cytochrome) and cytochrome c-553 (soluble cytochrome) were measured. The stoichiometry of cytochrome f: cytochrome c-553:P-700 was found to be 1:4:2.4 in Tribonema and 1:6:3.4 in Synurá.

Two chlorophyll-binding subunits of the photosystem 2 reaction center complex isolated from the thermophilic cyanobacterium Synechococcus sp

The reaction center of photosystem 2 has been highly purified from digitonin-solubilized thylakoid membranes of the thermophilic cyanobacterium Synechococcus sp. by means of sucrose density gradient centrifugation and electrophoresis on polyacrylamide gels containing digitonin. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of isolated reaction center complex yielded four chlorophyll a proteins named CP2-a, CP2-b, CP2-c, and CP2-d. When reelectrophoresed, CP2-a was transformed to CP2-d, and CP2-b was converted to CP2-a and CP2-d. The reaction center complex consisted of two major polypeptides of 47,000 and 40,000 Da and several minor polypeptides. CP2-b contained a 47,000-Da polypeptide together with 66,000- and 31,000-Da polypeptides, while CP2-a and CP2-d had only a 47,000-Da polypeptide. The apoprotein of CP2-c was a 40,000-Da polypeptide. Absorption spectra of CP2-a, -b, and -d were similar to each other but distinctly different from those of CP2-c at liquid nitrogen temperature. The reaction center complex showed two fluorescence emission bands at 686 and 694 nm at 77 degrees K. CP2-a, -b, and -d emitted the band at 694 nm, whereas the fluorescence peak at 686 nm was associated with CP2-c. It is concluded that the photosystem 2 reaction center complex contains two chlorophyll-binding subunits, CP2-d (or CP2-a) which may be the site of the primary photochemistry of photosystem 2 and CP2-c which may function as the antenna of the reaction center of photosystem 2.

Photosynthetic Membranes of Porphyridium cruentum: An Analysis of Chlorophyll-Protein Complexes and Heme-Binding Proteins

Three chlorophyll-protein complexes (CP I, CP III, CP IV) were electrophoretically separated from thylakoids of the eukaryotic red alga Porphyridium cruentum. CP I contained the primary photochemical reaction center of photosystem I as judged by its light-induced reversible absorbance change at 700 nanometers, by its fluorescence emission maximum at 720 nanometers (-196 degrees C), and by the molecular weight of its apoprotein (68,000 daltons). CP III and CP IV appeared to belong with photosystem II as suggested by the absence of light-reversible absorbance at 700 nanometers, by their fluorescence maximum at 690 nanometers (-196 degrees C), and by the presence of a chlorophyll-binding polypeptide with a molecular weight of about 52,000 daltons. CP IV when completely denatured had two additional polypeptides of about 40,000 and 48,000 daltons. All three chlorophyll-protein complexes contained carotenoids: the chlorophyll/carotenoid molar ratio of 15:1 for CP I, and 20:1 for CP III and CP IV. The thylakoid membranes of P. cruentum contained four cytochromes, detected by heme-dependent peroxidase activity, but there was no observed association with the electrophoretically separated chlorophyll-protein complexes.

Isolation of chlorophyll-protein complexes and quantification of electron transport components in Synura petersenil and Tribonema aequale

The chlorophyll-protein complexes of the yellow alga Synura petersenii (Chrysophyceae) and the yellow-green alga Tribonema aequale (Xanthophyceae) were studied. The sodiumdodecylsulfate/sodiumdesoxycholate solubilized photosynthetic membranes of these species yielded three distinct pigment-protein complexes and a non-proteinous zone of free pigments, when subjected to SDS polyacrylamid gel electrophoresis. The slowest migrating protein was identical to complex I (CP I), the P-700 chlorophyll a-protein, which possessed 60 chlorophyll a molecules per reaction center in Tribonema and 108 in Synura. The zone of intermediate mobility contained chlorophyll a and carotenoids. The absorption spectrum of this complex was very similar to the chlorophyll a-protein of photosystem II (CP a), which is known from green plants. The fastest migrating pigment protein zone was identified as a light-harvesting chlorophyll-protein complex. In Synura this protein was characterized by the content of chlorophyll c and of fucoxanthin. Therefore this complex will be named as LH Chl a/c-fucocanthin protein. In addition to the separation of the chlorophyll-protein complexes the cellular contents of P-700, cytochrome f (bound cytochrome) and cytochrome c-553 (soluble cytochrome) were measured. The stoichiometry of cytochrome f: cytochrome c-553:P-700 was found to be 1:4:2.4 in Tribonema and 1:6:3.4 in Synurá.

Chlorophyll-protein organization of membranes from the cyanobacterium Anacystis nidulans

Six chlorophyll-containing bands were observed upon electrophoretic analysis of Anacystis nidulans thylakoid membranes. These ranged in apparent molecular weights from approximately 360 to 45 kdalton. Measurements of the light absorption and chlorophyll fluorescence properties of these bands revealed numerous differences among the aggregates. The larger chlorophyll-protein complexes had a chlorophyll absorption maximum at 676 nm while the smallest band, band VI, at approximately 45 kdalton, absorbed at 668 nm. The chlorophyll-protein organization of four submembrane particles was also examined. Digitonin and N-tetradecyl-N,N-dimethyl-3-ammonio-1-pro-panesulfonate were used to fractionate thylakoids and each treatment yielded two green fractions after sucrose density gradient centrifugation. The upper green fractions of both procedures were enriched in band VI. In addition, these fractions showed low temperature fluorescence emission at 686 nm. Conversely, the lower green fractions were enriched in the larger bands (bands I and II), and yielded fluorescence emission at 696 and 716 nm. The gel electrophoresis analysis of these chlorophyll-protein bands revealed 11 peptides ranging in size from less than 10 to 64 kdaltons. The larger CP bands contained as many as five to six polypeptides, whereas band VI contained only two species (at 45 and 48 kdalton). These data suggest that the only proteins in band V (approximately 75 kdalton) and band VI are the chlorophyll binding proteins for photosystems I and II, respectively. We present a model which correlates chlorophyll-protein organization and specific fluorescence emission peaks. Central to this model is the interaction of the larger chlorophyll-protein complexes with bands V and VI to yield fluorescence at 696 and 716 nm, respectively. In addition, the polypeptide composition of each complex allows us to construct a topological model of these complexes within the Anacystis thylakoid.

Allophycocyanin I and the 95 Kilodalton Polypeptide : The Bridge between Phycobilisomes and Membranes

Allophycocyanin was isolated from dissociated phycobilisomes from Nostoc sp. and was separated into allophycocyanin I, II, III, and B as described elsewhere. If the separation of the proteins following phycobilisome isolation is done in the presence of the protease inhibitor, phenylmethylsulfonylfluoride, associated with allophycocyanin I are two colored polypeptides of 95 kilodalton (kD) and 80 kD, belonging to the class of Group I polypeptides as defined by Tandeau de Marsac and Cohen-Bazire (Proc Natl Acad Sci USA 1977 74: 1635-1639). Allophycocyanin I has a fluorescence maximum of 680 nanometers as do intact phycobilisomes and has thus been suggested to be the final emitter of excitation energy in phycobilisomes. Thylakoid membranes washed in low ionic strength buffer containing phenylmethylsulfonylfluoride lose all biliproteins, but retain the 95 kD and 80 kD polypeptides. As suggested by Tandeau de Marsac and Cohen-Bazire, these are likely to be the polypeptides involved in binding the phycobilisome to the membrane. As these polypeptides are isolated with allophycocyanin I, structural evidence is provided for placing allophycocyanin I as the bridge between the phycobilisome and the membrane. These Group I polypeptides and the 29 kD polypeptide (involved in rod attachment to the APC core) are particularly susceptible to proteolytic breakdown. It is thought that in vivo the active protease may be selectively attacking these polypeptides to detach the phycobilisome from the membrane and release the phycoerythrin and phycocyanin containing rods from the allophycocyanin core for greater susceptibility of the biliproteins to protease attack.

Role of the colorless polypeptides in phycobilisome reconstitution from separated phycobiliproteins

A phycoerythrin (PE) and phycocyanin (PC) mixture was separated from allophycocyanin on calcium phosphate chromatography from completely dissociated phycobilisomes of the blue-green alga, Nostoc sp. After dialysis of the PE-PC mixture in 0.75 m potassium phosphate, pH 7, which allows reassociation of the dissociated pigment-proteins, complexes of PE and PC in a 2:1 m ratio (PE/PC complex) as well as complexes predominantly of PC (PC/PE complex) were then separated by sedimentation on linear sucrose gradients. These complexes resemble the rods of intact phycobilisomes and transfer energy efficiently from PE to PC. They contain the Group II colorless polypeptides described by Tandeau de Marsac and Cohen-Bazire (1977 Proc Natl Acad Sci USA 74: 1635 61639). Phycobilisomes can be reconstituted by combining the allophycocyanin pool with (a) the PE-PC mixture, (b) the PE/PC complex, or (c) the PC/PE complex. Successful reconstitution is measured by absorption, fluorescence, circular dichroism, and electron microscopy. The major requirement for reconstitution is the 29-kilodalton colorless polypeptide. In its absence, no phycobilisomes are formed. It is the only colorless polypeptide common to both the PE/PC complex and the PC/PE complex, and appears to be the polypeptide responsible for rod attachment to the allophycocyanin. In addition, high phosphate concentrations and 20 degrees C temperatures are needed for reconstitution.

Transbilayer organization of the chlorophyll-proteins of spinach thylakoids

To investigate the transverse bilayer organization of the chlorophyll-proteins of the three intrinsic chlorophyll-protein complexes, the effects of proteolytic enzymes, and an antibody against the light-harvesting complex were compared using right-side-out and inside-out thylakoid vesicles. The vesicles were isolated by aqueous polymer phase partitioning following the fragmentation of spinach thylakoids by passage through a Yeda press. Both vesicle types were agglutinated by an antiserum specific for the light-harvesting complex, although proteolytic degradation of the complex occurred only in right-side-out vesicles. In addition, there are different antigenic sites for the light-harvesting complex on the inner and outer thylakoid surfaces. Polypeptides of the chlorophyll-alpha-protein complex of photosystem II were degraded by proteases at both membrane surfaces. We concluded that both these chlorophyll-protein complexes are membrane spanning and transversely asymmetric, but that the light-harvesting complex polypeptides accessible at the inner thylakoid surface are more resistant to proteolytic attack. In contrast, the main chlorophyll-containing polypeptide (Mr = 64 500) of photosystem I complex was resistant to proteolytic attack at both the outer and inner thylakoid surfaces.

Light Harvesting in Anacystis nidulans Studied in Pigment Mutants

Spontaneous pigment mutants of Anacystis nidulans were self-selected for improved growth in far red light (> 650 nanometers). Questions were asked about those features of the light-harvesting mechanism which altered to give the mutants improved photosynthetic performance in far red. Answers were sought by comparing pigment and reaction center concentrations for the parent and six mutants grown in gold fluorescent and in far red light. Three significant results emerged. The ratio of reaction centers for photoreactions I and II (RC1/RC2) varied by a value of about 2.1 for all clones grown in gold and a value of about 1.1 for all clones grown in far red. Alteration of the ratio was not evident in any of the mutants.Phycobilisome alterations were evident as decreased phycocyanin content in all mutants. In three mutants, allophycocyanin became the major remaining phycobilisome component. Action spectra for photoreactions I and II allowed estimates of chlorophylls serving each of the two reaction centers. Ratios of chlorophylls to reaction centers within each photosystem were chlorophyll I/RC1 = 118 +/- 11 and chlorophyll II/RC2 = 52 +/- 9 for all seven clones grown in both gold and far red light. Remarkable constancy of these ratios, in spite of wide variation in cell material, supports an hypothesis that in A. nidulans there are two chlorophyll proteins, each bearing a reaction center and chlorophylls in fixed ratio.

Spectral analysis of allophycocyanin I, II, III and B from Nostoc sp. phycobilisomes

Low temperature (-196C) and room temperature (25C) absorption spectra of a family of allophycocyanin spectral forms isolated from Nostoc sp. phycobilisomes as well as of the phycobilisomes themselves have been analyzed by Gaussian curve-fitting. Allophycocyanin I and B share long wavelength components at 668 and 679 nm, bands that are absent from allophycocyanin II and III. These long wavelength absorption components are apparently responsible for the 20 nm difference between the 680 nm fluorescence emission maximum of allophycocyanin I and B and the 660 nm maximum of II and III. This indicates that allophycocyanin I and B are the final acceptors of excitation energy in the phycobilisome and the excitation energy transfer bridge linking the phycobilisome with the chlorophyll-containing thylakoid membranes. These Gaussian components are also found in resolved spectra of phycobilisomes, are arguing against this family of allophycocyanin molecules being artifactual products of protein purification procedures.