TAAC is readily expressed in dark-grown Arabidopsis seedlings, and its level remains stable throughout the greening process. Its expression is highest in developing green tissues and in leaves undergoing senescence or abiotic stress. We propose that the TAAC protein supplies ATP for energy-dependent reactions during thylakoid biogenesis and turnover in plants.Chloroplasts perform oxygenic photosynthesis in algae and plants and have evolved by endosymbiosis from cyanobacteria. Chloroplasts have two distinct membrane systems, the double envelope surrounding the organelle and an internal membrane system named thylakoids. The envelope membrane represents the interface between the cytoplasm and chloroplast stroma, whereas the thylakoid membrane separates the stroma and the lumenal space. Altogether ϳ800 membrane proteins have been identified by proteomics in the envelope and thylakoid membranes of Arabidopsis thaliana (for reviews, see Refs. 1 and 2). As expected, the main function for the identified envelope proteins was transport of ions and metabolites, whereas photosynthesis was attributed to most of the identified thylakoid proteins. The major protein complexes in thylakoids are photosystems (PS) 4 I and II, the cytochrome b 6 f complex, and the proton-translocating ATP synthase. These photosynthetic complexes contain not only proteins but also pigments and other cofactors. Their assembly, activity, and removal require a large number of auxiliary, regulatory, and transport proteins (3, 4). Many biochemical reports pointed to the existence of transport activities in the thylakoid membrane, such as calcium transport (5), copper transport (6), anion channels (7), cation channels (8, 9), and nucleotide transport (10). Only the thylakoid copper transporter was identified at the genetic level in Arabidopsis (11). No hydrophobic proteins related to the above-mentioned transport activities were identified in the previous proteomic works on Arabidopsis thylakoid membranes (for a review, see Ref.2). Therefore, genetic strategies are required for identification and elucidation of their role in optimal function of the thylakoid.ATP is produced during the light-dependent photosynthetic reactions on the stromal side of the thylakoid membrane. Besides its utilization during CO 2 fixation in the stroma, ATP drives many energy-dependent processes in thylakoids, including protein phosphorylation, folding, import, and degradation.

Besides an essential role in optimizing water oxidation in photosystem II (PSII), it has been reported that the spinach PsbO protein binds GTP [C. Spetea, T. Hundal, B. Lundin, M. Heddad, I. Adamska, B. Andersson, Proc. Natl. Acad. Sci. U.S.A. 101 (2004) 1409-1414]. Here we predict four GTP-binding domains in the structure of spinach PsbO, all localized in the beta-barrel domain of the protein, as judged from comparison with the 3D-structure of the cyanobacterial counterpart. These domains are not conserved in the sequences of the cyanobacterial or green algae PsbO proteins. MgGTP induces specific changes in the structure of the PsbO protein in solution, as detected by circular dichroism and intrinsic fluorescence spectroscopy. Spinach PsbO has a low intrinsic GTPase activity, which is enhanced fifteen-fold when the protein is associated with the PSII complex in its dimeric form. GTP stimulates the dissociation of PsbO from PSII under light conditions known to also release Mn(2+) and Ca(2+) ions from the oxygen-evolving complex and to induce degradation of the PSII reaction centre D1 protein. We propose the occurrence in higher plants of a PsbO-mediated GTPase activity associated with PSII, which has consequences for the function of the oxygen-evolving complex and D1 protein turnover.

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A diminished IFN-gamma secretion and the association of fasting C-peptide levels with cytokine response in children with type 1 diabetes suggest that factors related to beta-cell function in type 1 diabetes may modify T-cell function. Thus, the T-cell responses detected at or after diagnosis may not reflect the pathogenic process leading to type 1 diabetes.

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