Contribution to the structural characterization of eucaryotic PSI reaction centre - II. Characterization of a highly purified photoactive SDS-CP1 complex

Screening of monoclonal antibodies using antigens labeled with acetylcholinesterase: application to the peripheral proteins of photosystem 1

An original immunoenzymatic screening method, based on the use of antigens labeled with the stable enzyme acetylcholinesterase (AChE, EC 3.1.1.7), is described. The high turnover of this enzyme results in a very sensitive detection of antibodies. In this method, monoclonal antibodies from the supernatants of hybridoma cultures are immobilized on a solid phase coated with anti-mouse immunoglobulins and react simultaneously with the appropriate antigen labeled with biotin molecules. In a second step, biotinylated acetylcholinesterase is in turn associated to the system via avidin interactions and subsequently detected by a colorimetric assay. The method appears more sensitive and easier to use than either the corresponding radioimmunological test using a 125I-iodinated antigen or the same type of enzymatic immunoassay performed with biotinylated horseradish peroxidase instead of biotinylated AChE. The combined use of microtiter plates, solid-phase separation, and colorimetric detection allows a high level of automation of the method which makes it very efficient to process a large number of samples. This technique has been successfully applied to the screening of monoclonal antibodies directed against peripheral proteins of the photosystem 1 (PS1) membrane complex in photosynthesis. A complete set of antibodies recognizing these PS1 components was selected. The same technique was also tested in competition immunoassays and appears to be a very precise and useful tool for quantifying PS1 polypeptides in different biological extracts, including sodium dodecyl sulfate-denatured membranes. This can be of special interest for studying the biogenesis of membrane complexes.

The two genes for the P700 chlorophyll a apoproteins on the Euglena gracilis chloroplast genome contain multiple introns

We have determined the complete nucleotide sequence of the two genes encoding the P700 chlorophyll a apoproteins of the photosystem I reaction center of the Euglena gracilis chloroplast genome. The two genes are separated by 77 bp, are of the same polarity, and span a region which is greater than 9.0 kbp. The psaA gene (751 codons) is interrupted by three introns and the psaB gene (734 codons) by six introns. The introns range in size from 361 to 590 bp, whereas the exons range in size from 42 to 1,194 bp. The introns are extremely AT rich with a pronounced base bias of T greater than A greater than G greater than C in the RNA-like strand. Like other interrupted protein genes in the Euglena chloroplast genome, the psaA and psaB introns are similar to mitochondrial group II introns in having the splice junction consensus sequence, 5' GTGCGNTTCG ..... INTRON ..... TTAATTTTAT 3' and conserved secondary structural features. Except for the placement of the first intron, the intron-exon organization of these two highly homologous genes is not conserved. The other introns fall at or near putative surface domains of the predicted gene products. The psaA and psaB gene products are 74% homologous to one another and 93% and 95% homologous, respectively, to the psaA and psaB gene products of higher plant chloroplasts. The predicted secondary structure derived from the primary amino acid sequence has 11 potential membrane-spanning domains.

The chlorophyll-protein complexes of higher plant photosynthetic membranes or Just what green band is that?

Higher plant thylakoid membranes can be fractionated into a bewildering array of macrocomplexes, chlorophyll-protein complexes and chlorophyll-proteins with various deteregents and separations techniques. The chemical nature of each of these entities depends on the particular methods used to obtain them. This review summarizes the current status of the biochemical identification and characterization of individual chlorophyll-proteins and chlorophyll-protein complexes, and attempts to clarify the relationships among them.

Sequence of two genes in pea chloroplast DNA coding for 84 and 82 kD polypeptides of the photosystem I complex

The genes encoding the two P700 chlorophyll a-apoproteins of the photosystem I complex were localized on the pea (Pisum sativum) chloroplast genome. The nucleotide sequence of the genes and the flanking regions has been determined. The genes are separated by 25 bp and are probably cotranscribed. The 5' terminal gene (psaA1) codes for a 761-residue protein (MW 84.1 kD) and the 3' terminal gene (psaA2) for a 734-residue protein (MW 82.4 kD). Both proteins are highly hydrophobic and contain eleven putative membrane-spanning domains. The homology to the corresponding polypeptides from maize are 89% and 95% for psaA1 and psaA2, respectively. A putative promoter has been identified for the psaA1 gene, and potential ribosome binding sites are present before both genes.

Freeze-fracture analysis of membrane appression and protein segregation in model membranes containing the chlorophyll-protein complexes from chloroplasts

Cation-induced membrane appression and lateral segregation of chlorophyll-protein complexes have been investigated by freeze-fracture analysis of model membranes containing photosystem 1 and the light-harvesting complex of photosystem 2. In light-harvesting complex proteoliposomes, cations caused extensive membrane adhesion and a segregation of protein into appressed regions. A marked flattening of the appressed membranes, sometimes together with a co-alignment of the particles on the opposing membrane faces, strongly suggests a direct transmembrane attraction between the protein particles. Photosystem 1 membranes were not appressed by cations but some clustering of the particles occurred, together with their exclusion from some regions of the lipid. By incorporating dipalmitoylglycerophosphocholine into the membranes, it is shown that a similar exclusion of the particles can occur due to liquid-crystalline to gel state transitions of the lipids. Proteoliposomes containing both the light-harvesting complex and photosystem 1 displayed cation-induced membrane appression, but only between regions containing the light-harvesting complex. Photosystem 1 was largely confined to unappressed membranes. Destacking occurred at low salt concentrations irrespective of whether photosystem 1 was present, showing that in proteoliposomes, and probably in thylakoids, this process does not require the presence of highly charged complexes from unappressed regions.

The action of lipase on chloroplast membranes: II. Polypeptide patterns of bean galactolipase- and phospholipase A2-treated thylakoid membranes

Thylakoid membranes obtained from bean chloroplasts treated with bean galactolipase or phospholipase A2 (from Crotalus terr. terr.) showed marked changes in their polypeptide patterns when separated on SDS-PAGE. The obtained results have been discussed with regard to the relationship between chloroplast lipids and polypeptides originating from chlorophyll-protein complexes of bean thylakoids. A coexistence between galactolipids and the peripheral antennae in PS I complex and LHCP(3) as well as a conspicuous role of phospholipids in PSI and PSII centre chlorophyll-protein complexes has to be underlined.

The action of lipase on chloroplast membranes: II. Polypeptide patterns of bean galactolipase- and phospholipase A2-treated thylakoid membranes

Thylakoid membranes obtained from bean chloroplasts treated with bean galactolipase or phospholipase A2 (from Crotalus terr. terr.) showed marked changes in their polypeptide patterns when separated on SDS-PAGE. The obtained results have been discussed with regard to the relationship between chloroplast lipids and polypeptides originating from chlorophyll-protein complexes of bean thylakoids. A coexistence between galactolipids and the peripheral antennae in PS I complex and LHCP(3) as well as a conspicuous role of phospholipids in PSI and PSII centre chlorophyll-protein complexes has to be underlined.

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.

A single step separation of PS 1, PS 2 and chlorophyll-antenna particles from spinach chloroplasts

After solubilization of photosynthetic membranes by digitonin, three main protein pigment complexes were isolated by electrophoresis with deoxycholate as detergent.The band with the slowest mobility, fraction 1, had PS 1 activity and was devoid of PS 2 activity. This fraction was four times enriched in P700 when compared with chloroplasts. Fraction 1 had little chl b, a long wavelength absorption maximum in the red, a maximum of low temperature emission fluorescence at 730nm, and a circular dichroism spectrum characteristic of PS 1 enriched fraction.Fraction 2 exhibited a PS 2 activity and no PS 1 activity. It was enriched five times in PS 2 reaction centre and had little chl b and carotenoids. The absorption maximum was at 674 nm and the low temperature fluorescence emission maximum was at 700 nm. Fraction 2 might be useful PS 2 enriched particle because of the great stability of this fraction with regard to photochemical activity and also rapidity and simplicity of its preparation.Fraction 3, which had the fastest migration, was devoid of photochemical activities; It was rich in chl b and had the fluorescence and the circular dichroism spectrum characteristic of an antenna complex.

Protein identification of purified particles isolated from spinach thylakoids by deoxycholate electrophoresis

Three thylakoid complexes were isolated by deoxycholate preparative electrophoresis. The protein composition of each fraction was analyzed by SDS analytical electrophoresis. No protein of the PS 1 enriched fraction (fraction 1) was found in the PS 2 enriched fraction (fraction 2) and inversely. The antenna complex (fraction 3) did not have any contamination by proteins of fraction 1 or fraction 2. Fraction 1 was mainly composed of the CP1, the reaction center complex of the PS1, and by low molecular weight proteins, previously found in other PS 1 preparations. Tentative assignments of these proteins are presented; among them are iron sulfur proteins. After analytical SDS electrophoresis of fraction 2, the reaction center complex was dissociated. Nevertheless three proteins of 50 kD, 42 kD and 35 kD were assigned to this complex. Fraction 2 contained also the three cytochromes of the thylakoid membranes: cyt f, cyt b6, cyt b559. Fraction 3 was exclusively composed of one protein pigment complex, CP2.

Effect of various denaturing treatments on the structure and activity of a purified CP1 complex

Spinach CP1 complex, purified as previously described [16], was submitted to various dissociating treatments. Chaotropic agents, like urea and thiocyanate salts, remained without effect on the structure and photooxidation of the complex, just SDS at very high concentrations was able to dissociate the chlorophyll from the polypeptides and to abolish the photoreaction. Proteolytic enzymes have no more action on the apparent structure and activity of CP1, but some of them do cleave the large polypeptides (65 kD) into smaller ones, as observed after pigments dissociation. This last result might be an important step in the search for a smaller active P700 protein complex.