Stroma-exposed Membranes Research Articles

Light-Harvesting Complex Stress-Related Proteins Catalyze Excess Energy Dissipation in Both Photosystems of Physcomitrella patens.

Two LHC-like proteins, Photosystem II Subunit S (PSBS) and Light-Harvesting Complex Stress-Related (LHCSR), are essential for triggering excess energy dissipation in chloroplasts of vascular plants and green algae, respectively. The mechanism of quenching was studied in Physcomitrella patens, an early divergent streptophyta (including green algae and land plants) in which both proteins are active. PSBS was localized in grana together with photosystem II (PSII), but LHCSR was located mainly in stroma-exposed membranes together with photosystem I (PSI), and its distribution did not change upon high-light treatment. The quenched conformation can be preserved by rapidly freezing the high-light-treated tissues in liquid nitrogen. When using green fluorescent protein as an internal standard, 77K fluorescence emission spectra on isolated chloroplasts allowed for independent assessment of PSI and PSII fluorescence yield. Results showed that both photosystems underwent quenching upon high-light treatment in the wild type in contrast to mutants depleted of LHCSR, which lacked PSI quenching. Due to the contribution of LHCII, P. patens had a PSI antenna size twice as large with respect to higher plants. Thus, LHCII, which is highly abundant in stroma membranes, appears to be the target of quenching by LHCSR.

The Mobile Thylakoid Phosphoprotein TSP9 Interacts with the Light-harvesting Complex II and the Peripheries of Both Photosystems

The localization of the plant-specific thylakoid-soluble phosphoprotein of 9 kDa, TSP9, within the chloroplast thylakoid membrane of spinach has been established by the combined use of fractionation, immunoblotting, cross-linking, and mass spectrometry. TSP9 was found to be exclusively confined to the thylakoid membranes, where it is enriched in the stacked grana membrane domains. After mild solubilization of the membranes, TSP9 migrated together with the major light-harvesting antenna (LHCII) of photosystem II (PSII) and with PSII-LHCII supercomplexes upon separation of the protein complexes by either native gel electrophoresis or sucrose gradient centrifugation. Studies with a cleavable cross-linking agent revealed the interaction of TSP9 with both major and minor LHCII proteins as identified by mass spectrometric sequencing. Cross-linked complexes that in addition to TSP9 contain the peripheral PSII subunits CP29, CP26, and PsbS, which form the interface between LHCII and the PSII core, were found. Our observations also clearly suggest an interaction of TSP9 with photosystem I (PSI) as shown by both immunodetection and mass spectrometry. Sequencing identified the peripheral PSI subunits PsaL, PsaF, and PsaE, originating from cross-linked protein complexes of around 30 kDa that also contained TSP9. The distribution of TSP9 among the cross-linked forms was found to be sensitive to conditions such as light exposure. An association of TSP9 with LHCII as well as the peripheries of the photosystems suggests its involvement in regulation of photosynthetic light harvesting.

Ultraviolet B Exposure of Whole Leaves of Barley Affects Structure and Functional Organization of Photosystem II

This study examines the effects of ecologically important levels of ultraviolet B radiation on protein D1 turnover and stability and lateral redistribution of photosystem II. It is shown that ultraviolet B light supported only limited synthesis of protein D1, one of the most important components of photosystem II, whereas it promoted significant degradation of proteins D1 and D2. Furthermore, dephosphorylation of photosystem II subunits was specifically elicited upon exposure to ultraviolet B light. Structural modifications of photosystem II and changes in its lateral distribution between granum membranes and stroma-exposed lamellae were found to be different from those observed after photoinhibition by strong visible light. In particular, more complete dismantling of photosystem II cores was observed. Altogether, the data reported here suggest that ultraviolet B radiation alone fails to activate the photosystem II repair cycle, as hypothesized for visible light. This failure may contribute to the toxic effect of ultraviolet B radiation, which is increasing as a consequence of depletion of stratospheric ozone.

Identification and Characterization of D1 and D2 Protein Breakdown Fragments in Cotyledon Thylakoids from Ceratonia siliqua L.

Summary Breakdown products of Photosystem II reaction centre D1 and D2 proteins have been detected and characterized in cotyledon thylakoids from the epigeal species Ceratonia siliqua . Breakdown fragments are mainly localized in the stroma-exposed membranes. Cleavage of D1 protein occurs in the hydrophylic loop connecting the 4th and 5th putative transmembrane segments, giving rise to fragments very similar, if not identical, to those observed during light-induced turnover or photoinhibition. Fragments from D2 protein originate from cleavage somewhere between the 3rd and 4th transmembrane segments.

Dynamics of Photosystem II heterogeneity during photoinhibition: depletion of PS II β from non-appressed thylakoids during strong-irradiance exposure of Chlamydomonas reinhardtii

Thylakoid membranes of Chlamydomonas reinhardtii were fractionated into appressed membranes and non-appressed vesicles. The latter had a Chl a/b ratio of 3.2 and contained 1.12 mmol PS II (mol Chl a+b)−1. The PS II centers in the non-appressed regions displayed the kinetic characteristics of PS IIβ centers. They were able to generate a stable charge-separation to QA, but they were unable to transfer electrons from Q−A to QB (QB-nonreducing centers). After a strong-irradiance treatment (SIT) of cells by exposure to 2000 μmol · m−2 · s−1 for 15 min, a portion of the PS IIα centers in the appressed membranes was damaged due to photoinhibition. PS IIβ centers were resistant to photoinhibitory damage; however, SIT caused a lowering of the PS II content in the non-appressed fraction to 0.60 mmol PS II (mol Chl a+b)−1. Concomitantly, there was a conversion of QB-nonreducing centers to a form able to reduce the plastoquinone pool (QB-reducing centers). Low-temperature during SIT inhibited both the depletion of PS IIβ from the non-appressed membrane region and the activation of QB-nonreducing centers. These results are consistent with a ‘PS II repair cycle’ which translocates PS II units from the stroma-exposed membranes into the appressed thylakoids during activation of PS IIβ from a QB-nonreducing to a QB-reducing form. Since the overall distribution of Chl-protein complexes between the stroma-exposed and appressed membrane regions was unaffected by SIT, it is suggested that damaged PS II units from the appressed membrane region are exchanged with PS IIβ from the stroma-exposed membranes.

A study on the lateral distribution of the plastoquinone pool with respect to Photosystem II in stacked and unstacked spinach chloroplasts

The quenching of Photosystem II (PS II) chlorophyll fluorescence by oxidised plastoquinone has been used in an attempt to determine their relative distribution in the partition zone and stroma-exposed thylakoid membranes. Thus, the PS II-plastoquinone interaction was determined in stacked (2.5 mM MgCl 2) and largely unstacked (0.25 mM MgCl 2) membranes. A method to correct for spillover or other quenching changes at the different MgCl 2 concentrations, which would compete with the plastoquinone-induced quenching, was devised utilising the quinone dibromothymoquinone. This compound is demonstrated to behave as an ideal (theoretically) PS II quencher at both high and low MgCl 2 concentrations, which indicates that it distributes itself homogeneously between partition zone and stroma-exposed membrane regions. In passing from the stacked to the unstacked configuration, the PS II-plastoquinone interaction decreases less than the PS II-dibromothymoquinone interaction. This is interpreted to mean that plastoquinone is present in both the partition zone and stroma-exposed membranes, with somewhat higher concentrations in the stroma-exposed membranes. Thus, plastoquinone is well placed to transport reducing equivalents from the partition zones to the stroma-exposed membranes.