We investigated the function of chlorophyll a/b binding antenna proteins Chlorophyll Protein 26 (CP26) and CP24 in light harvesting and regulation of photosynthesis by isolating Arabidopsis thaliana knockout lines that completely lacked one or both of these proteins. All three mutant lines had a decreased efficiency of energy transfer from trimeric light-harvesting complex II (LHCII) to the reaction center of photosystem II (PSII) due to the physical disconnection of LHCII from PSII and formation of PSII reaction center depleted domains in grana partitions. Photosynthesis was affected in plants lacking CP24 but not in plants lacking CP26: the former mutant had decreased electron transport rates, a lower DpH gradient across the grana membranes, reduced capacity for nonphotochemical quenching, and limited growth. Furthermore, the PSII particles of these plants were organized in unusual two-dimensional arrays in the grana membranes. Surprisingly, overall electron transport, nonphotochemical quenching, and growth of the double mutant were restored to wild type. Fluorescence induction kinetics and electron transport measurements at selected steps of the photosynthetic chain suggested that limitation in electron transport was due to restricted electron transport between Q A and Q B , which retards plastoquinone diffusion. We conclude that CP24 absence alters PSII organization and consequently limits plastoquinone diffusion.

SummaryThe neutrophil-activating protein (HP-NAP) of Helicobacter pylori is a major 17 kDa antigen of the immune response of infected individuals. Amino acid sequence comparison indicated a high similarity between HP-NAP and both bacterial DNA-protecting proteins (Dps) and ferritins. The structure prediction and spectroscopic analysis presented here indicate a close similarity between HP-NAP and Dps. Electron microscopy revealed that HP-NAP forms hexagonal rings of 9± 10 nm diameter with a hollow central core as seen in Dps proteins, clearly different from the 12 nm icositetrameric (24 subunits) ferritins. However, HP-NAP is resistant to thermal and chemical denaturation similar to the ferritin family of proteins. In addition, HP-NAP binds up to 40 atoms of iron per monomer and does not bind DNA. We therefore conclude that HP-NAP is an unusual, small, ferritin that folds into a four-helix bundle that oligomerizes into dodecamers with a central hole capable of binding up to 500 iron atoms per oligomer.

We investigated the structural effects induced by Al3+ on different beta-amyloid (Abeta) fragments at pH 7.4 and T=25 degrees C, with particular attention given to the sequences 1-40 and 1-42. Al3+ caused peptide enrichment in beta sheet structure and formation of solvent-exposed hydrophobic clusters. These intermediates evolved to polymeric aggregates which organized in fibrillar forms in the case of the Al3+-Abeta(1-42) complex. Comparative studies showed that Zn2+ and Cu2+ were much less efficient than Al3+ in stimulating the spontaneous aggregation/fibrillogenesis of Abetas. Studies with liposomes as membrane models showed dramatic changes in the structural properties of the lipid bilayer in the presence of Al3+-Abeta complexes, suggesting a major role of Al3+ in Abeta-induced cell dysfunction. Al3+ effects were abolished by desferrioxamine mesylate (DFO) only in solution. We concluded that, in vivo, DFO may act as a protective agent by preventing or reverting Abeta aggregation in the extracellular spaces.

The three-dimensional structure of photosystem I1 (PSII) has been determined by conventional transmission electron microscopy and computerized three-dimensional reconstruction. Both the complete system and that lacking the oxygen-evolving complex have been analyzed. The PSI1 complex has a four-lobed structure with twofold symmetry. An estimate of the molecular mass and the results of DeriphatPAGE analysis suggest that a reaction centre is present in each half of the structure resolved by electron microscopy. Stepwise removal of components of the complex showed that the removal of CP47 (a 47-kDa chlorophyll-protein complex) induces monomerization of PSII, which indicates the importance of this subunit for the dimeric structure.The light reactions of photosynthesis are driven by the excitation energy absorbed by the antenna pigments and transferred to the reaction centres. In higher plants as many as 210 chlorophyll (chl) molecules are associated with photosystem (PS) I (Melis and Anderson, 1983) and 230 with PSII (McCauley and Melis, 1986). Each of the two supramolecular complexes consists of a chl-a-binding core complex where charge separation occurs, surrounded by antenna complexes which bind both chl a and chl b and serve as antennae . Although photosynthetic organisms have antenna proteins quite different from each other, all oxygenevolving organisms contain an homologous PSII core complex which is able to oxidize water and reduce plastoquinone (Erickson and Rochaix, 1992). Four chlorophyll proteins called D1, D2, CP47 and CP43 are included in this core complex, together with cytochrome 0-559 and three oxygenevolving complex (OEC) polypeptides. Little is known about how the chlorophyll proteins of reaction centres are organized and how antenna complexes interact with the reaction centres to form photosynthetic units. Models for PSII organization have been proposed which provide a basis for further experimentation (Dainese and Bassi, 1991 ;Peter and Thornber, 1991 ;Bassi and Dainese, 1992), although direct structural information is limited. Electron microscopic analysis of cyanobacterial PSII core complex has shown a dimeric organization (Morschel and Schatz, 1987). Contrasting results, however, have been reported in the case of chl-b-conCorrespondence to R. Bassi, Istituto di Biotecnologie Vegetali, Fax: +39 45 8203710.