[Show abstract][Hide abstract] ABSTRACT: Over-reduction of the photosynthetic electron transport chain can severely damage the photosynthetic apparatus as well as other constituents of the chloroplast and the cell. Here, we exposed Arabidopsis leaves to saturating light either under normal atmospheric conditions or under CO2- and O2-limiting conditions, which greatly increase excitation and electron pressures by draining terminal electron acceptors. The two treatments are found to have very different, often opposing, effects on the structure of the thylakoid membranes, including the width of the granal lumenal compartment. Modulation of the latter is proposed to be related to movements of ions across the thylakoid membrane, which alter the relative osmolarity of the lumen and stroma and affect the partitioning of the proton motive force into its electrical and osmotic components. The resulting changes in thylakoid organization and lumenal width should facilitate the repair of photodamaged photosystem II complexes when the light stress is affected under ambient conditions but are expected to inhibit the repair cycle when the light stress concurs with CO2- and O2-depletion. Under the latter conditions, the changes in thylakoid structure are also predicted to complement other processes that restrict the flow of electrons into the high-potential chain, thus moderating the production of deteriorating reactive oxygen species at photosystem I.This article is protected by copyright. All rights reserved.
No preview · Article · Jan 2015 · The Plant Journal
[Show abstract][Hide abstract] ABSTRACT: A crucial component of protein homeostasis in cells is the repair of damaged proteins. The repair of oxygen-evolving photosystem II (PS II) supercomplexes in plant chloroplasts is a prime example of a very efficient repair process that evolved in response to the high vulnerability of PS II to photooxidative damage, exacerbated by high-light (HL) stress. Significant progress in recent years has unraveled individual components and steps that constitute the PS II repair machinery, which is embedded in the thylakoid membrane system inside chloroplasts. However, an open question is how a certain order of these repair steps is established and how unwanted back-reactions that jeopardize the repair efficiency are avoided. Here, we report that spatial separation of key enzymes involved in PS II repair is realized by subcompartmental-ization of the thylakoid membrane, accomplished by the forma-tion of stacked grana membranes. The spatial segregation of kinases, phosphatases, proteases, and ribosomes ensures a certain order of events with minimal mutual interference. The margins of the grana turn out to be the site of protein degradation, well separated from active PS II in grana core and de novo protein synthesis in unstacked stroma lamellae. Furthermore, HL induces a partial conversion of stacked grana core to grana margin, which leads to a controlled access of proteases to PS II. Our study suggests that the origin of grana in evolution ensures high repair efficiency, which is essential for PS II homeostasis. photosynthesis | photoinhibition | PS II repair cycle | thylakoid membrane |
Full-text · Article · Oct 2014 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: The process of oxygenic photosynthesis enabled and still sustains aerobic life on Earth. The most elaborate form of the apparatus that carries out the primary steps of this vital process is the one present in higher plants. Here, we review the overall composition and supramolecular organization of this apparatus, as well as the complex architecture of the lamellar system within which it is harbored. Along the way, we refer to the genetic, biochemical, spectroscopic and, in particular, microscopic studies that have been employed to elucidate the structure and working of this remarkable molecular energy conversion device. As an example of the highly dynamic nature of the apparatus, we discuss the molecular and structural events that enable it to maintain high photosynthetic yields under fluctuating light conditions. We conclude the review with a summary of the hypotheses made over the years about the driving forces that underlie the partition of the lamellar system of higher plants and certain green algae into appressed and non-appressed membrane domains and the segregation of the photosynthetic protein complexes within these domains.
[Show abstract][Hide abstract] ABSTRACT: The machinery that conducts the light-driven reactions of oxygenic photosynthesis is hosted within specialized paired membranes called thylakoids. In higher plants, the thylakoids are segregated into two morphological and functional domains called grana and stroma lamellae. A large fraction of the luminal volume of the granal thylakoids is occupied by the oxygen-evolving complex of photosystem II. Electron microscopy data we obtained on dark- and light-adapted Arabidopsis thylakoids indicate that the granal thylakoid lumen significantly expands in the light. Models generated for the organization of the oxygen-evolving complex within the granal lumen predict that the light-induced expansion greatly alleviates restrictions imposed on protein diffusion in this compartment in the dark. Experiments monitoring the redox kinetics of the luminal electron carrier plastocyanin support this prediction. The impact of the increase in protein mobility within the granal luminal compartment in the light on photosynthetic electron transport rates and processes associated with the repair of photodamaged photosystem II complexes is discussed.
Full-text · Article · Nov 2011 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Aerobic life on Earth depends on oxygenic photosynthesis. This fundamentally important process is carried out within an elaborate membranous system, called the thylakoid network. In angiosperms, thylakoid networks are constructed almost from scratch by an intricate, light-dependent process in which lipids, proteins, and small organic molecules are assembled into morphologically and functionally differentiated, three-dimensional lamellar structures. In this review, we summarize the major events that occur during this complex, largely elusive process, concentrating on those that are directly involved in network formation and potentiation and highlighting gaps in our knowledge, which, as hinted by the title, are substantial.
[Show abstract][Hide abstract] ABSTRACT: The extensive and multifaceted traffic between nucleus and cytoplasm is handled by a single type of macromolecular assembly called the nuclear pore complex (NPC). While being readily accessible to ions and metabolites, the NPC imposes stringent selectivity on the passage of proteins and RNA, tightly regulating their traffic between the two major cellular compartments. Here we discuss how shuttling carriers, which mediate the transport of macromolecules through NPCs, cross its permeability barrier. We also discuss the co-existence of receptor-mediated macromolecular transport with the passive diffusion of small molecules in the context of the various models suggested for the permeability barrier of the NPC. Finally, we speculate on how nuclear transport receptors negotiate the dependence of their NPC-permeating abilities on hydrophobic interactions with the necessity of avoiding these promiscuous interactions in the cytoplasm and nucleus.
Preview · Article · Nov 2010 · Nucleus (Austin, Texas)
[Show abstract][Hide abstract] ABSTRACT: The primary events of oxygenic photosynthesis are carried out within intricate membrane lamellar systems called thylakoid
networks. These networks, which are present in cyanobacteria, algae, and higher plants, accommodate all of the molecular complexes
necessary for the light-driven reactions of photosynthesis and provide a medium for energy transduction. Here, we describe
the ultrastructure of thylakoid membranes and their three-dimensional organization in various organisms along the evolutionary
tree. Along the way we discuss issues pertaining to the formation and maintenance of these membranes, the means by which they
enable molecular traffic within and across them, and the manner by which they respond to short- and long-term variations in