| 1. |
- Aronsson, Henrik, et al.
(författare)
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POR hits the road : import and assembly of a plastid protein
- 2003
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Ingår i: Plant Molecular Biology. - Berlin : Springer-Verlag. - 0167-4412 .- 1573-5028. ; 51:1, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- The biosynthesis of chlorophyll is a strictly light-dependent multistep process in higher plants. The light-dependent step is catalysed by NADPH:protochlorophyllide oxidoreductase (POR, EC.1.6.99.1), which reduces protochlorophyllide (Pchlide) to chlorophyllide (Chlide). POR is nucleus-encoded and post-translationally imported into plastids. It has been proposed that the import of a POR protein isozyme (PORA) is totally dependent on Pchlide and uses a novel import pathway. This proposal is based on findings that PORA import only occurs in the presence of Pchlide and that the presence of overexpressed precursor of Rubisco small subunit (pSS), a protein which is known to use the general import pathway, does not outcompete PORA import. Another study demonstrated that POR precursor protein (pPOR) can be cross-linked to one of the components in the translocation machinery, Toc75, in the absence of Pchlide, and that its import can be outcompeted by the addition of the pSS. This indicates that pSS and pPOR may use the same translocation mechanism. Thus, POR does not necessarily need Pchlide for import – which is in contrast to earlier observations – and the exact POR import mechanism remains unresolved. Once in the stroma, the POR transit peptide is cleaved off and the mature POR protein is associated to the plastid inner membranes. Formation of the correct membrane–associated, thermolysin-protected assembly is strictly dependent of NADPH. As a final step, the formation of the NADPH-Pchlide-POR complex occurs. When POR accumulates in the membranes of proplastids, an attraction of monogalactosyl diacylglycerol (MGDG) can occur, leading to the formation of prolamellar bodies (PLBs) and the development of etioplasts in darkness.
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| 2. |
- Aronsson, Henrik, et al.
(författare)
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POR – import and membrane association of a key element in chloroplast development
- 2003
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Ingår i: Physiologia Plantarum. - : Blackwell Publishing. - 0031-9317 .- 1399-3054. ; 118:1, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- The development of proplastids or etioplasts to chloroplast is visualized by the accumulation of chlorophyll in leaves of higher plants. The biosynthesis of chlorophyll includes a light-dependent reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide). This light-dependent step is catalysed by the nucleus-encoded NADPH:Pchlide oxidoreductase (POR, EC 1.6.99.1). POR is active within plastids and therefore has to be translocated over the plastid envelope membranes. The import of chloroplast proteins seems to follow a general import pathway using translocons at the outer and inner envelope membrane. POR cross-linking to Toc75, one of the major translocon components at the outer envelope membrane, indicates its use of the general import pathway. However, since variations exist within the so-called general import pathway one has to consider previous data suggesting a novel totally Pchlide-dependent import pathway of one POR isoform, PORA. The suggested Pchlide dependency of POR import is discussed since recent observations contradict this idea. In the stroma the POR transit peptide is cleaved off and the mature POR protein is targeted to the plastid inner membranes. The correct and stable association of POR to the membrane requires the cofactor NADPH. Functional activity of POR calls for formation of an NADPH–Pchlide–POR complex, a formation that probably takes place after the membrane association and is dependent on a phosphorylation reaction.
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| 3. |
- Dahlin, Clas
(författare)
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Surface charge densities and membrane fluidities in thylakoids with different degrees of thylakoid appression after Norflurazon treatment
- 2003
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Ingår i: Photosynthetica (Praha). - : Springer-Verlag. - 0300-3604 .- 1573-9058. ; 41:4, s. 635-639
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Tidskriftsartikel (refereegranskat)abstract
- Wheat seedlings (Triticum aestivum L.) develop plastids (etioplasts and chloroplasts) which exhibit alterations in inner membrane organisation after treatment with Norflurazon (NF), an inhibitor of carotenoid biosynthesis. In dark-grown plants, results in a decreased amount of partitions (contact zones) between prothylakoids. In weak red light, the results in plants containing chloroplasts devoid of grana. Assays have been performed to investigate the membrane surface charge density in these membranes, and relate possible differences to the absence of (pro-)thylakoid overlap after NF teatment. Using the fluorescent probe 9- amino acridine (9-AA), the average surface charge density of isolated PTs was -21.8±3.2 mC m-2 and - 27.4±2.6 mC m-2 in the control and after, respectively. Thylakoid membranes isolated from plants grown in weak red light exhibited slightly more negative values, -23.5±2.9 mC m-2 and -29.0±2.1 mC m-2, in control and after, respectively. The surface charge density of destacked thylakoids from greenhouse-grown untreated plants, containing extensive grana stacking, was -34.3±2.5 mC m-2. Assays using the fluorescent probe of DPH (1,6- diphenyl- 1,3,5- hexatriene) showed that this probe exhibits a higher polarisation value when incorporated into thylakoids from NF- treated plants compared to untreated plants grown in weak red light. The highest polarisation value was found in untreated plants grown in the greenhouse. This indicates a lower rotation transition of the probe in the lipid environment of thylakoids after NF treatment, which can be interpreted as more rigid membranes. These results suggest that the surface charge density and the mobility of membrane components may play a major role for the formation of partitions in dark-grown plants and in the formation of grana in plants grown in weak red light. 23 Additional key words: chloroplasts; etioplasts; (pro-)thylakoids; Triticum; stacking; wheat; 9- amino acridine.
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