Phosphorylation of thylakoids and isolated subthylakoid vesicles derived from different structural domains of the thylakoid membrane from spinach chloroplast

Sublocalization of Cytochrome b6f Complexes in Photosynthetic Membranes

It is well established that the majority of energy-converting photosynthetic protein complexes in plant thylakoid membrane are nonhomogenously distributed between stacked and unstacked membrane regions. Yet, the sublocalization of the central cytochrome b₆f complex remains controversial. We present a structural model that explains the variation in cytochrome b₆f sublocalization data. Small changes in the distance between adjacent membranes in stacked grana regions either allow or restrict access of cytochrome b₆f complexes to grana. If the width of the gap falls below a certain threshold, then the steric hindrance prevents cytochrome b₆f access to grana. Evidence is presented that the width of stromal gap is variable, demonstrating that the postulated mechanism can regulate the lateral distribution of the cytochrome b₆f complexes.

Defining the far-red limit of photosystem I: the primary charge separation is functional to 840 nm

The far-red limit of photosystem I (PS I) photochemistry was studied by EPR spectroscopy using laser flashes between 730 and 850 nm. In manganese-depleted spinach thylakoid membranes, the primary donor in PS I, P700, was oxidized simultaneously with tyrosine Z, the secondary donor in PS II. It was found that at 295 K PS I photochemistry, observed as P700 (+) formation, was functional up to 840 nm. This is 30 nm further to the red region than was reported for PS II photochemistry (Thapper, A., Mamedov, F., Mokvist, F., Hammarström, L., and Styring, S. (2009) Plant Cell 21, 2391-2401). The same far-red limit for the P700 (+) formation was observed in a PS I reaction center core preparation from Nostoc punctiforme. The reduction of the acceptor side of PS I, observed as reduction of the iron-sulfur centers FA and FB by low temperature EPR measurements, was also functional at 15 K with light up to >830 nm. Taken together, these results, obtained from both plants and cyanobacteria, most likely rule out involvement of the red-absorbing antenna chlorophylls in this reaction. Instead we propose the existence of weak charge transfer bands absorbing in the far-red region in the ensemble of excitonically coupled chlorophyll a molecules around P700 similar to what has been found in the reaction center of PS II. These charge transfer bands could be responsible for the far-red light absorption leading to PS I photochemistry at wavelengths up to 840 nm.

Separation and identification of the light harvesting proteins contained in grana and stroma thylakoid membrane fractions

This paper presents the results of a study performed to develop a rapid and straightforward method to resolve and simultaneously identify the light-harvesting proteins of photosystem I (LHCI) and photosystem II (LHCII) present in the grana and stroma of the thylakoid membranes of higher plants. These hydrophobic proteins are embedded in the phospholipid membrane, and their extraction usually requires detergent and time consuming manipulations that may introduce artifacts. The method presented here makes use of digitonin, a detergent which causes rapid (within less than 3 min) cleavage of the thylakoid membrane into two subfractions: appressed (grana) and non-appressed (stroma) membranes, the former enriched in photosystem II and the latter containing mainly photosystem I. From these two fractions identification of the protein components was performed by separating them by reversed-phase high-performance liquid chromatography (RP-HPLC) and determining the intact molecular mass by electrospray ionization mass spectrometry (ESI-MS). By this strategy the ion suppression during ESI-MS that normally occurs in the presence of membrane phospholipids was avoided, since RP-HPLC removed most phospholipids from the analytes. Consequently, high quality mass spectra were extracted from the reconstructed ion chromatograms. The specific cleavage of thylakoid membranes by digitonin, as well as the rapid identification and quantification of the antenna composition of the two complexes facilitate future studies of the lateral migration of the chlorophyll-protein complexes along thylakoid membranes, which is well known to be induced by high intensity light or other environmental stresses. Such investigations could not be performed by sodium dodecylsulfate-polyacrylamide gel electrophoresis because of insufficient resolution of the proteins having molecular masses between 22,000 and 25,000.

A quantitative model of the domain structure of the photosynthetic membrane

A model is presented that gives a quantitative picture of the distribution of the photosynthetic components in the photosynthetic membrane of higher plants. A salient feature of the model is that most of the pigments are located in the grana where photosystem I and II carry out linear electron transport, whereas the stroma lamellae, which harbour <20% of the pigments, carry out photosystem-I-mediated cyclic electron transport. This arrangement derives from the observation that more pigments are associated with photosystem I, which therefore captures more quanta than photosystem II. The excess pigments associated with photosystem I are thought to be located in the stroma lamellae.

Photosystem II in different parts of the thylakoid membrane: a functional comparison between different domains

The electron transport properties of photosystem II (PSII) from five different domains of the thylakoid membrane were analyzed by flash-induced fluorescence kinetics. These domains are the entire grana, the grana core, the margins from the grana, the stroma lamellae, and the Y100 fraction (which represent more purified stroma lamellae). The two first fractions originate from appressed grana membranes and have PSII with a high proportion of O(2)-evolving centers (80-90%) and efficient electron transport on the acceptor side. About 30% of the granal PSII centers were found in the margin fraction. Two-thirds of those PSII centers evolve O(2), but the electron transfer on the acceptor side is slowed. PSII from the stroma lamellae was less active. The fraction containing the entire stroma has only 43% O(2)-evolving PSII centers and slow electron transfer on the acceptor side. In contrast, PSII centers of the Y100 fraction show no O(2) evolution and were unable to reduce Q(B). Flash-induced fluorescence decay measurements in the presence of DCMU give information about the integrity of the donor side of PSII. We were able to distinguish between PSII centers with a functional Mn cluster and without any Mn cluster, and PSII centers which undergo photoactivation and have a partially assembled Mn cluster. From this analysis, we propose the existence of a PSII activity gradient in the thylakoid membrane. The gradient is directed from the stroma lamellae, where the Mn cluster is absent or inactive, via the margins where photoactivation accelerates, to the grana core domain where PSII is fully photoactivated. The photoactivation process correlates to the PSII diffusion along the membrane and is initiated in the stroma lamellae while the final steps take place in the appressed regions of the grana core. The margin domain is seemingly very important in this process.

Fractionation of the thylakoid membranes from tobacco. A tentative isolation of 'end membrane' and purified 'stroma lamellae' membranes

Thylakoids isolated from tobacco were fragmented by sonication and the vesicles so obtained were separated by partitioning in aqueous polymer two-phase systems. By this procedure, grana vesicles were separated from stroma exposed membrane vesicles. The latter vesicles could be further fractionated by countercurrent distribution, with dextran-polyethylene glycol phase systems, and divided into two main populations, tentatively named 'stroma lamellae' and 'end membrane'. Both these vesicle preparations have high chlorophyll a/b ratio, high photosystem (PS) I and low PS II content, suggesting their origin from stroma exposed regions of the thylakoid. The two vesicle populations have been compared with respect to biochemical composition and photosynthetic activity. The 'end membrane' has a higher chlorophyll a/b ratio (5.7 vs. 4.7), higher P700 content (4.7 vs. 3.3 mmol/mol of chlorophyll). The 'end membrane' has the lowest PS II content, the ratio PS I/PS II being more than 10, as shown by EPR measurements. The PS II in both fractions is of the beta-type. The decay of fluorescence is different for the two populations, the 'stroma lamellae' showing a very slow decay even in the presence of K3Fe(CN)6 as an acceptor. The two vesicle populations have very different surface properties: the end membranes prefer the upper phase much more than the stroma lamellae, a fact which was utilized for their separation. Arguments are presented which support the suggestion that the two vesicle populations originate from the grana end membranes and the stroma lamellae, respectively.

Chromatographic approaches to liposomes, proteoliposomes and biomembrane vesicles

Size-exclusion chromatography has been used for fractionation of liposomes, proteoliposomes and biomembrane vesicles of up to approximately 500 nm in size and for separation of these entities from smaller components. Liposome sizes, encapsulation stability, and solute affinities for membrane proteins have been determined. Counter-current distribution in aqueous two-phase systems has widened the range of applications to larger structures. Immobilized biomembrane vesicles and (proteo)liposomes provide stationary phases for chromatographic analysis of specific or nonspecific membrane-solute interactions.