Biogenesis of thylakoid networks in angiosperms: knowns and unknowns.
ABSTRACT Aerobic life on Earth depends on oxygenic photosynthesis. This fundamentally important process is carried out within an elaborate membranous system, called the thylakoid network. In angiosperms, thylakoid networks are constructed almost from scratch by an intricate, light-dependent process in which lipids, proteins, and small organic molecules are assembled into morphologically and functionally differentiated, three-dimensional lamellar structures. In this review, we summarize the major events that occur during this complex, largely elusive process, concentrating on those that are directly involved in network formation and potentiation and highlighting gaps in our knowledge, which, as hinted by the title, are substantial.
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ABSTRACT: Chloroplasts in plants and some green algae contain a continuous thylakoid membrane system that is structurally differentiated into stacked granal membranes interconnected by unstacked thylakoids, the stromal lamellae. Experiments were conducted to test the hypothesis that the thermodynamic tendency to increase entropy in chloroplasts contributes to thylakoid stacking to form grana. We show that the addition of bovine serum albumin or dextran, two very different water-soluble macromolecules, to a suspension of envelope-free chloroplasts with initially unstacked thylakoids induced thylakoid stacking. This novel restacking of thylakoids occurred spontaneously, accompanied by lateral segregation of PSII from PSI, thereby mimicking the natural situation. We suggest that such granal formation, induced by the macromolecules, is partly explained as a means of generating more volume for the diffusion of macromolecules in a crowded stromal environment, i.e., greater entropy overall. This mechanism may be relevant in vivo where the stroma has a very high concentration of enzymes of carbon metabolism, and where high metabolic fluxes are required.Biochimica et Biophysica Acta 07/2005; 1708(2):187-95. · 4.66 Impact Factor
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ABSTRACT: The lipid monogalactosyl diacylglycerol (MGD) is a major structural component of photosynthetic membranes in chloroplasts. Its formation is catalyzed by the enzyme MGD synthase. In many plants, MGD derives from two different biosynthetic pathways: the prokaryotic pathway, which operates entirely within the plastid, and the eukaryotic pathway, which involves steps in the endoplasmic reticulum. Here, we describe the identification and characterization of an Arabidopsis mutant with a defective MGD synthase gene (MGD1). The mutant was identified in a screen of T-DNA lines for individuals with defects in chloroplast biogenesis. It has a yellow-green phenotype that correlates with a approximately 50% deficiency in total chlorophyll per plant. A single T-DNA insertion is located adjacent to the transcription initiation site of the MGD1 gene, and the abundance of MGD1 mRNA is reduced by 75% compared with wild type. Correlation between steady-state MGD1 transcript levels and MGD synthase activity (also reduced by 75% in mgd1) suggests that MGD1 is the most important MGD synthase in green tissues. The amount of MGD in mutant leaves is reduced by 42% compared with wild type. MGD from the mutant contains 23% less 16:3 fatty acid and 10% more 18:3 fatty acid. Because 16:3 is a characteristic feature of MGD from the prokaryotic pathway, it is possible that MGD1 operates with some preference in the prokaryotic pathway. Finally, the MGD-deficiency of mgd1 is correlated with striking defects in chloroplast ultrastructure, strongly suggesting a unique role for MGD in the structural organization of plastidic membranes.Proceedings of the National Academy of Sciences 08/2000; 97(14):8175-9. · 9.74 Impact Factor
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ABSTRACT: The NADPH:protochlorophyllide oxidoreductase of barley has been solubilized from etioplast membranes and purified to apparent homogeneity. The highest specific activity measured for the purified enzyme was 1.6 nmol chlorophyllide formed (mg protein-1) per flash. Electrophoretic analysis of the purified enzyme on sodium dodecylsulfate/polyacrylamide gels revealed only one polypeptide of Mr 36000. Durig glycerol gradient centrifugation the enzyme migrates as a low-molecular-weight component. It is proposed that each enzyme molecule contains only one polypeptide chain. Assuming a molecular weight of 36000 for the enzyme, it was calculated that two or three protochlorophyllide molecules are bound to each enzyme molecule.European Journal of Biochemistry 11/1980; 111(1):251-8. · 3.58 Impact Factor