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  • 1.
    Aronsson, Henrik
    et al.
    Department of Biology, Leicester University, Leicester, United Kingdom.
    Sundqvist, Christer
    Department of Plant Physiology, Göteborg University, Göteborg, Sweden.
    Dahlin, Clas
    Högskolan i Halmstad, Akademin för ekonomi, teknik och naturvetenskap, Bio- och miljösystemforskning (BLESS), Växtcellbiologi: Energiomvandling i växtceller.
    POR – import and membrane association of a key element in chloroplast development2003Ingår i: Physiologia Plantarum: An International Journal for Plant Biology, ISSN 0031-9317, E-ISSN 1399-3054, Vol. 118, nr 1, s. 1-9Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 2.
    Aronsson, Henrik
    et al.
    Department of Plant Physiology, Göteborg University, Göteborg, Sweden.
    Sundqvist, Christer
    Department of Plant Physiology, Göteborg University, Göteborg, Sweden.
    Timko, Michael P.
    Department of Biology, University of Virginia, Charlottesville, United States.
    Dahlin, Clas
    Högskolan i Halmstad, Akademin för ekonomi, teknik och naturvetenskap, Bio- och miljösystemforskning (BLESS).
    Characterisation of the assembly pathway of the pea NADPH:protochlorophyllide (Pchlide) oxidoreductase (POR), with emphasis on the role of its substrate, Pchlide2001Ingår i: Physiologia Plantarum: An International Journal for Plant Biology, ISSN 0031-9317, E-ISSN 1399-3054, Vol. 111, nr 2, s. 239-244Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The homologous import and membrane association of a key enzyme for chlorophyll biosynthesis, the NADPH:protochlorophyllide (Pchlide) oxidoreductase (PAR, EC 1.6.99.1) into pea chloroplasts was investigated in vitro. The co-factor, NADPH, decreased binding of the precursor protein (pPOR) to the envelope membranes in the presence of ATP. The decrease of the binding reaction with NADPH was not observed with the precursor of the small subunit of Rubisco (pSS). To investigate possible substrate-dependency for the import reaction, internal Pchlide concentrations in the plastids were raised by either an addition of ÎŽ-aminolevulinic acid to isolated plastids or etiolation of the seedlings prior to plastid isolation. Increased amounts of plastid-bound Pchlide gave no observable differences in POR import. The capacity of POR and 11 different POR mutants, carrying charged-to-alanine scanning substitutions, to form a catalytically active POR-Pchlide-NADPH complex and to associate with the thylakoid membranes in a protease-resistant way were tested. Wild-type POR, as well as the mutants with charge substitutions in the N-terminal region of the protein, exhibited higher catalytic activity than the POR mutants carrying substitutions in the C-terminal region. Formation of a catalytically active complex did not, however, increase the association efficiency onto the thylakoids. We can, therefore, postulate that the import of pea POR into pea chloroplasts was not substrate-dependent, nor did formation of catalytically active complexes stimulate or inhibit the membrane association reaction of POR.

  • 3.
    Dahlin, C.
    et al.
    Högskolan i Halmstad, Sektionen för ekonomi och teknik (SET).
    Aronsson, H.
    Department of Plant Physiology, Göteborg University, Box 461, SE-405 30 Göteborg, Sweden.
    Almkvist, J.
    Department of Plant Physiology, Göteborg University, Box 461, SE-405 30 Göteborg, Sweden.
    Sundqvist, C.
    Department of Plant Physiology, Göteborg University, Box 461, SE-405 30 Göteborg, Sweden.
    Protochlorophyllide-independent import of two NADPH: Pchlide oxidoreductase proteins (PORA and PORB) from barley into isolated plastids2000Ingår i: Physiologia Plantarum: An International Journal for Plant Biology, ISSN 0031-9317, E-ISSN 1399-3054, Vol. 109, nr 3, s. 298-303Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    The enzyme catalysing the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide), NADPH:Pchlide oxidoreductase (POR; EC 1.6.99.1), is a nuclear-encoded protein that is post-translationally imported to the plastid, In barley and Arabidopsis thaliana, the reduction of Pchlide is controlled by two different PORs, PORA and PORE. To characterise the possible Pchlide dependency for the import reaction, radiolabelled precursor proteins of barley PORA and PORE (pPORA and pPORB, respectively) were used for in vitro assays with isolated plastids of barley and pea with different contents of Pchlide, To obtain plastids with different endogenous levels of Pchlide, several methods were used. Barley plants were grown in darkness or in greenhouse conditions for 6 days. Alternatively, greenhouse-grown pea plants were incubated for 4 days in darkness before plastid isolation, or chloroplasts isolated from greenhouse-grown plants were incubated with delta-aminolevulinic acid (ALA), an early precursor in the Chi biosynthesis resulting in elevated Pchlide contents in the plastids, Both barley pPORA and pPORB were effectively imported into barley and pea chloroplasts isolated from the differentially treated plants, including those isolated from greenhouse-grown plants. The absence or presence of Pchlide did not significantly affect the import capacity of barley pPORA or pPORB, Assays performed on stroma-enriched fractions from chloroplasts and etioplasts of barley indicated that no post-import degradation of the proteins occurred in the stroma, irrespective of whether the incubation was performed in darkness or in light.

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