Characterisation of stroma membranes from Zea mays L. chloroplasts

Isolation and characteristics of the PSI-LHCI-LHCII supercomplex under high light

We developed a novel method for the isolation of the PSI-LHCI-LHCII complex from spinach leaves. The supercomplex was resolved into a core complex (CPI), LHCII trimers, LHCI dimers and LHCII monomers using green gel electrophoresis. We then investigate changes in the fluorescence and absorption spectra of PSI-LHCI-LHCII under high light. In addition, we compared light-induced denaturation of the core protein subunits in both PSI-LHCI and PSI-LHCI-LHCII. Differences in denaturation and photochemical activity indicated that binding of LHCII increased the photosensitivity of the PSI core. Increased energy delivered to the PSI core during illumination accelerated damage to the core complex.

Evidences for interaction of PsbS with photosynthetic complexes in maize thylakoids

The PsbS subunit of Photosystem II (PSII) has received much attention in the past few years, given its crucial role in photoprotection of higher plants. The exact location of this small subunit in thylakoids is also debated. In this work possible interaction partners of PsbS have been identified by immunoaffinity and immunoprecipitation, performed with mildly solubilized whole thylakoid membrane. The interacting proteins, as identified by mass spectrometry analysis of the immunoaffinity eluate, include CP29, some LHCII components, but also components of Photosystem I, of the cytochrome b(6)f complex as well as of ATP synthase. These proteins can be co-immunoprecipitated by using highly specific anti-PsbS antibodies and, vice-versa, PsbS is co-immunoprecipitated by antisera against components of the interacting complexes. We also find that PsbS co-migrates with bands containing PSII, ATP synthase and cytochrome b(6)f as well as with LHCII-containing bands on non-denaturing Deriphat PAGE. These results suggest multiple location of PsbS in the thylakoid membrane and point to an unexpected lateral mobility of this PSII subunit. As revealed by immunogold labelling with antibody against PsbS, the protein is associated either with granal membranes or prevalently with stroma lamellae in low or high-intensity light-treated intact leaves, respectively. This finding is consistent with the capability of PsbS to interact with complexes located in stroma lamellae, even though the exact physiological condition(s) under which these interactions may take place remain to be clarified.

Photoinhibitory light-induced changes in the composition of chlorophyll-protein complexes and photochemical activity in photosystem-I submembrane fractions

The effects of irradiation on photosystem (PS)-I submembrane particles using intense white light (2000 micoE x m(-2) x S(-1)) at chilling temperature (4 degrees C) were studied. PSI-dependent oxygen uptake activity was stable during the first 3 h of photoinhibitory illumination in the presence of added superoxide dismutase (SOD). Without added SOD, the oxygen uptake almost doubled during this period, presumably due to the denaturation of native membrane-bound SOD or its release from the PSI membranes. The total chlorophyll (Chl) content and the magnitude of light-induced absorbance changes at 830 nm (deltaA830) were also barely affected during the first 3-3.5 h of photoinhibitory treatment. However, further exposure to strong light markedly accelerated Chl breakdown followed by a decline in oxygen uptake rate and deltaA830. This corresponded with the disappearance of the bands attributed to PsaA/B polypeptides on electrophoretic gels. Despite the invariant maximum magnitude of deltaA830 during the first 3-3.5 h of photoinhibitory treatment, the light-response curves of P700 oxidation gradually altered, demonstrating a several-fold increase in the ability of weak actinic light to oxidize P700. The major Chl a-protein 1 (CP1) band gradually disappeared during the first 4 h of light exposure with a corresponding increase in the Chl content of a band with lower electrophoretic mobility ascribed to the formation of oligomers containing CP1, light-harvesting complex I (LHCI)-680 and LHCI-730. This aggregation of Chl-protein complexes, likely caused by photoinhibitory-induced cross-linking favoring light harvesting, is proposed to explain the enhanced capacity of weak light to oxidize P700 in photoinhibited PSI submembrane fractions compared with untreated ones.

Changes in the structure of chlorophyll-protein complexes and excitation energy transfer during photoinhibitory treatment of isolated photosystem I submembrane particles

The activity of light-induced oxygen consumption, absorption spectra, low temperature (77 K) chlorophyll fluorescence emission and excitation spectra were studied in suspensions of photosystem (PS) I submembrane particles illuminated by 2000 microE m(-2) s(-1) strong white light (WL) at 4 degrees C. A significant stimulation of oxygen uptake was observed during the first 1-4 h of photoinhibitory treatment, which rapidly decreased during further light exposure. Chlorophyll (Chl) content gradually declined during the exposure of isolated PSI particles to strong light. In addition to the Chl photobleaching, pronounced changes were found in Chl absorption and fluorescence spectra. The position of the major peak in the red part of the absorption spectrum shifted from 680 nm towards shorter wavelengths in the course of strong light exposure. A 6-nm blue shift of that peak was observed after 5-h illumination. Even more pronounced changes were found in the characteristics of Chl fluorescence. The magnitude of the dominating long-wavelength emission band at 736 nm located in untreated particles was five times reduced after 2-h exposure, whereas the loss in absolute Chl contents did not exceed 10% of its initial value. The major peak in low-temperature Chl fluorescence emission spectra shifted from 736 to 721 nm after 6-h WL treatment. Individual Chl-protein complexes differed in the response of their absorption spectra to strong WL. Unlike light-harvesting complexes (LHC), LHCI-680 and LHC-730, which did not exhibit changes in the major peak position, its maximum was shifted from 678 to 671 nm in CPIa complex after PSI submembrane particles were irradiated with strong light for 6 h. The results demonstrated that excitation energy transfer represents the stage of photosynthetic utilization of absorbed quanta which is most sensitive to strong light in isolated PSI particles.

Structural organization of the thylakoid membrane: freeze-fracture and immunocytochemical analysis

This article summarizes our ultrastructural studies on the organization of the thylakoid membrane of green algae and higher plants. We have used freeze-fracture and immunogold labeling to investigate the lateral distribution of the components in the membrane, their interactions, and the folding of their polypeptide chains in the membrane.