Evidence for a structural role for chlorophyll in chlorophyll-protein complexes

A comparison between plant photosystem I and photosystem II architecture and functioning

Oxygenic photosynthesis is indispensable both for the development and maintenance of life on earth by converting light energy into chemical energy and by producing molecular oxygen and consuming carbon dioxide. This latter process has been responsible for reducing the CO2 from its very high levels in the primitive atmosphere to the present low levels and thus reducing global temperatures to levels conducive to the development of life. Photosystem I and photosystem II are the two multi-protein complexes that contain the pigments necessary to harvest photons and use light energy to catalyse the primary photosynthetic endergonic reactions producing high energy compounds. Both photosystems are highly organised membrane supercomplexes composed of a core complex, containing the reaction centre where electron transport is initiated, and of a peripheral antenna system, which is important for light harvesting and photosynthetic activity regulation. If on the one hand both the chemical reactions catalysed by the two photosystems and their detailed structure are different, on the other hand they share many similarities. In this review we discuss and compare various aspects of the organisation, functioning and regulation of plant photosystems by comparing them for similarities and differences as obtained by structural, biochemical and spectroscopic investigations.

Hierarchical organization and structural flexibility of thylakoid membranes

Chloroplast thylakoid membranes accommodate densely packed protein complexes in ordered, often semi-crystalline arrays and are assembled into highly organized multilamellar systems, an organization warranting a substantial degree of stability. At the same time, they exhibit remarkable structural flexibility, which appears to play important - yet not fully understood - roles in different short-term adaptation mechanisms in response to rapidly changing environmental conditions. In this review I will focus on dynamic features of the hierarchically organized photosynthetic machineries at different levels of structural complexity: (i) isolated light harvesting complexes, (ii) molecular macroassemblies and supercomplexes, (iii) thylakoid membranes and (iv) their multilamellar membrane systems. Special attention will be paid to the most abundant systems, the major light harvesting antenna complex, LHCII, and to grana. Two physical mechanisms, which are less frequently treated in the literature, will receive special attention: (i) thermo-optic mechanism -elementary structural changes elicited by ultrafast local heat transients due to the dissipation of photon energy, which operates both in isolated antenna assemblies and the native thylakoid membranes, regulates important enzymatic functions and appears to play role in light adaptation and photoprotection mechanisms; and (ii) the mechanism by which non-bilayer lipids and lipid phases play key role in the functioning of xanthophyll cycle de-epoxidases and are proposed to regulate the protein-to-lipid ratio in thylakoid membranes and contribute to membrane dynamics. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.

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.

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.