A protein tyrosine kinase of chloroplast thylakoid membranes phosphorylates light harvesting complex II proteins

On the Extent of Tyrosine Phosphorylation in Chloroplasts

Reanalysis of published mass spectrometry data on Tyr-phosphorylated chloroplast proteins indicates that the majority of peptide spectrum matches reporting Tyr phosphorylation are ambiguous.

Oxidation-reduction signalling components in regulatory pathways of state transitions and photosystem stoichiometry adjustment in chloroplasts

State transitions and photosystem stoichiometry adjustment are two oxidation-reduction (redox)-regulated acclimatory responses in photosynthesis. State transitions are short-term adaptations that, in chloroplasts, involve reversible post-translational modification by phosphorylation of light-harvesting complex II (LHC II). Photosystem stoichiometry adjustments are long-term responses involving transcriptional regulation of reaction centre genes. Both responses are initiated by changes in light quality and are regulated by the redox state of plastoquinone (PQ). The LHC II kinase involved in the state 2 transition is a serine/threonine kinase known as STT7 in Chlamydomonas, and as STN7 in Arabidopsis. The phospho-LHC II phosphatase that produces the state 1 transition is a PP2C-type protein phosphatase currently termed both TAP38 and PPH1. In plants and algae, photosystem stoichiometry adjustment is governed by a modified two-component sensor kinase of cyanobacterial origin - chloroplast sensor kinase (CSK). CSK is a sensor of the PQ redox state. Chloroplast sigma factor 1 (SIG1) and plastid transcription kinase (PTK) are the functional partners of CSK in chloroplast gene regulation. We suggest a signalling pathway for photosystem stoichiometry adjustment. The signalling pathways of state transitions and photosystem stoichiometry adjustments are proposed to be distinct, with the two pathways sensing PQ redox state independently of each other.

The role of mass spectrometry in plant systems biology

Large-scale analyses of proteins and metabolites are intimately bound to advancements in MS technologies. The aim of these non-targeted "omic" technologies is to extend our understanding beyond the analysis of only parts of the system. Here, metabolomics and proteomics emerged in parallel with the development of novel mass analyzers and hyphenated techniques such as gas chromatography coupled to time-of-flight mass spectrometry (GC-TOF-MS) and multidimensional liquid chromatography coupled to mass spectrometry (LC-MS). The analysis of (i) proteins (ii) phosphoproteins, and (iii) metabolites is discussed in the context of plant physiology and environment and with a focus on novel method developments. Recently published studies measuring dynamic (quantitative) behavior at these levels are summarized; for these works, the completely sequenced plants Arabidopsis thaliana and Oryza sativa (rice) have been the primary models of choice. Particular emphasis is given to key physiological processes such as metabolism, development, stress, and defense. Moreover, attempts to combine spatial, tissue-specific resolution with systematic profiling are described. Finally, we summarize the initial steps to characterize the molecular plant phenotype as a corollary of environment and genotype.

Redox regulation of chloroplast transcription

Chloroplasts are the important plant cell organelles where photosynthesis takes place. Throughout this process, reaction center proteins are degraded and subsequently replenished by redox-responsive gene expression. In addition to well defined posttranscriptional mechanisms at the RNA and protein level, the transcription of chloroplast DNA into RNA precursors has been a focal point of studies in this area. Evidence has become available for a central role of a redox-responsive protein kinase named plastid transcription kinase (PTK) because of its association with the chloroplast transcription complex. The recent cloning of the PTK gene has resulted in a full-length cDNA for a protein related to the catalytic alpha subunit of nucleocytoplasmic casein kinase (CK2), yet with an additional chloroplast transit peptide. The corresponding protein, termed cpCK2alpha, was shown to be associated with the major organellar RNA polymerase, PEP-A. Both authentic PTK and recombinant cpCK2alpha have comparable general properties in vitro, and both are subject to regulation by the redox-reactive reagent glutathione. Based on the physical and functional equivalence, it is anticipated that the cloned protein can help clarify the functional role of the transcription kinase in vivo, including the identification of interaction partners at the interface between photosynthetic redox signaling and gene expression.

Redox regulation of thylakoid protein phosphorylation

The photosystem II of chloroplast thylakoid membranes contains several proteins phosphorylated by redox-activated protein kinases. The mechanism of the reversible activation of the light-harvesting antenna complex II (LHCII) kinase(s) is one of the best understood and related to the regulation of energy transfer to photosystem II or I, thereby optimizing their relative excitation (state transition). The deactivated LHCII protein kinase(s) is associated with cytochrome b(6)f and dissociates from the complex upon activation. Activation of the LHCII protein kinase occurs via dynamic conformational changes in the cytochrome b(6)f complex taking place during plastoquinol oxidation. Deactivation of the kinase involves its reassociation with an oxidized cytochrome complex. A fine-tuning redox-dependent regulatory loop inhibits the activation of the kinase via reduction of protein disulfide groups, possibly involving the thioredoxin complex. Phosphorylation of LHCII is further modulated by light-induced conformational changes of the LHCII substrate. The reversible phosphorylation of LHCII and other thylakoid phosphoproteins, catalyzed by respective kinases and phosphatases, is under strict regulation in response to environmental changes.

Will the real LHC II kinase please step forward?

Many laboratories have searched for the protein kinase responsible for phosphorylation of the chloroplast light-harvesting complex of photosynthesis, LHC II. The LHC II kinase provides a vital link in a redox signaling pathway of ecological, developmental, and evolutionary significance. Various candidates for the LHC II kinase, some stronger than others, have come and gone. Recently, a family of three thylakoid-associated kinases (TAKs) has been identified and purified; they too catalyze in vitro phosphorylation of LHC II. The LHC II kinase is part of an integrated network of signal transduction to which input is provided by a number of environmental factors. The implications of understanding these processes stretch beyond the important, central question of how plants adapt their photosynthetic machinery to changing wavelengths of light.

Molecular recognition in thylakoid structure and function

In photosynthesis, light-harvesting chlorophyll molecules are shunted between photosystems by phosphorylation of the protein to which they are bound. An anchor for the phosphorylated chlorophyll-protein complex has now been identified in the reaction centre of chloroplast photosystem I. This finding supports the idea that molecular recognition, not membrane surface charge, governs the architecture of the chloroplast thylakoid membrane. We describe a model for the chloroplast thylakoid membrane that is consistent with recent structural data that specify the relative dimensions of intrinsic protein complexes and their dispositions within the membrane. Control of molecular recognition accommodates membrane stacking, lateral heterogeneity and regulation of light-harvesting function by means of protein phosphorylation during state transitions--adaptations that compensate for selective excitation of photosystem I or photosystem II. High-resolution structural description of membrane protein-protein interactions is now required to understand thylakoid structure and regulation of photosynthesis.

Balancing the two photosystems: photosynthetic electron transfer governs transcription of reaction centre genes in chloroplasts

Chloroplasts are cytoplasmic organelles whose primary function is photosynthesis, but which also contain small, specialized and quasi-autonomous genetic systems. In photosynthesis, two energy converting photosystems are connected, electrochemically, in series. The connecting electron carriers are oxidized by photosystem I (PS I) and reduced by photosystem II (PS II). It has recently been shown that the oxidation reduction state of one connecting electron carrier, plastoquinone, controls transcription of chloroplast genes for reaction centre proteins of the two photosystems. The control counteracts the imbalance in electron transport that causes it: oxidized plastoquinone induces PS II and represses PS I; reduced plastoquinone induces PS I and represses PS II. This complementarity is observed both in vivo, using light favouring one or other photosystem, and in vitro, when site-specific electron transport inhibitors are added to transcriptionally and photosynthetically active chloroplasts. There is thus a transcriptional level of control that has a regulatory function similar to that of purely post-translational 'state transitions' in which the redistribution of absorbed excitation energy between photosystems is mediated by thylakoid membrane protein phosphorylation. The changes in rates of transcription that are induced by spectral changes in vivo can be detected even before the corresponding state transitions are complete, suggesting the operation of a branched pathway of redox signal transduction. These findings suggest a mechanism for adjustment of photosystem stoichiometry in which initial events involve a sensor of the redox state of plastoquinone, and may thus be the same as the initial events of state transitions. Redox control of chloroplast transcription is also consistent with the proposal that a direct regulatory coupling between electron transport and gene expression determines the function and composition of the chloroplast's extra-nuclear genetic system.