Sub-etioplast localization of the enzyme NADPH: protochlorophyllide oxidoreductase

Organization of chlorophyll biosynthesis and insertion of chlorophyll into the chlorophyll-binding proteins in chloroplasts

Oxygenic photosynthesis requires chlorophyll (Chl) for the absorption of light energy, and charge separation in the reaction center of photosystem I and II, to feed electrons into the photosynthetic electron transfer chain. Chl is bound to different Chl-binding proteins assembled in the core complexes of the two photosystems and their peripheral light-harvesting antenna complexes. The structure of the photosynthetic protein complexes has been elucidated, but mechanisms of their biogenesis are in most instances unknown. These processes involve not only the assembly of interacting proteins, but also the functional integration of pigments and other cofactors. As a precondition for the association of Chl with the Chl-binding proteins in both photosystems, the synthesis of the apoproteins is synchronized with Chl biosynthesis. This review aims to summarize the present knowledge on the posttranslational organization of Chl biosynthesis and current attempts to envision the proceedings of the successive synthesis and integration of Chl into Chl-binding proteins in the thylakoid membrane. Potential auxiliary factors, contributing to the control and organization of Chl biosynthesis and the association of Chl with the Chl-binding proteins during their integration into photosynthetic complexes, are discussed in this review.

Proteomic analysis of highly purified prolamellar bodies reveals their significance in chloroplast development

The prolamellar body (PLB) proteome of dark-grown wheat leaves was characterized. PLBs are formed not only in etioplasts but also in chloroplasts in young developing leaves during the night, yet their function is not fully understood. Highly purified PLBs were prepared from 7-day-old dark-grown leaves and identified by their spectral properties as revealed by low-temperature fluorescence spectroscopy. The PLB preparation had no contamination of extra-plastidal proteins, and only two envelope proteins were found. The PLB proteome was analysed by a combination of 1-D SDS-PAGE and nano-LC FTICR MS. The identification of chlorophyll synthase in the PLB fraction is the first time this enzyme protein was found in extracts of dark-grown plants. This finding is in agreement with its previous localization to PLBs using activity studies. NADPH:protochlorophyllide oxidoreductase A (PORA), which catalyses the reduction of protochlorophyllide to chlorophyllide, dominates the proteome of PLBs. Besides the identification of the PORA protein, the PORB protein was identified for the first time in dark-grown wheat. Altogether 64 unique proteins, representing pigment biosynthesis, photosynthetic light reaction, Calvin cycle proteins, chaperones and protein synthesis, were identified. The in number of proteins' largest group was the one involved in photosynthetic light reactions. This fact strengthens the assumption that the PLB membranes are precursors to the thylakoids and used for the formation of the photosynthetic membranes during greening. The present work is important to enhance our understanding of the significance of PLBs in chloroplast development.

Light-induced changes in the distribution of the 36000-Mr polypeptide of NADPH-protochlorophyllide oxidoreductase within different cellular compartments of barley (Hordeum vulgare L.) : I. Localization by immunoblotting in isolated plastids and total leaf extracts

Changes in the relative content of NADPH-protochlorophyllide oxidoreductase during the light-induced greening of barley plants were measured both in the total leaf extract as well as in intact and broken plastids. The enzyme protein was identified by its apparent molecular weight and its immunological crossreactivity with an antiserum directed against the NADPH-protochlorophyllide oxidoreductase. The monospecificity of the antiserum was tested by two different criteria: i. The antiserum was purified by affinity chromatography. ii. It was demonstrated that the antiserum crossreacts with only those polypeptides which appear to be enzymatically active. In the fraction of broken plastids isolated from leaves of briefly illuminated barley plants the concentration of the enzyme protein was reduced drastically. Our results indicate that this decrease in enzyme protein content is the consequence of an artificial proteolytic breakdown of the membrane-bound enzyme protein. In intact plastids and in the total leaf extract the concentration of the enzyme protein did not change dramatically during the first 4 to 6 h of illumination. However, when the exposure to continuous white light was extended further the concentration of the enzyme protein in intact plastids began to decline rapidly while in total leaf extracts the concentration remained almost constant for the next 10 h of light. These results indicate that part of the enzyme protein may be localized outside of the plastid compartment.

The NADPH-protochlorophyllide oxidoreductase is the major protein constituent of prolamellar bodies in wheat (Triticum aestivum L.)

A fraction of highly purified prolamellar bodies was isolated from etioplasts of wheat (Triticum aestivum L. cv. Starke II, Weibull), as previously described by Ryberg and Sundqvist (1982, Physiol. Plant., 56, 125-132). Studies on the protein composition revealed that only one major polypeptide of an apparent molecular weight of 36000 is present in the fraction of prolamellar bodies. This polypeptide was identified as the NADPH-protochlorophyllide oxidoreductase. The highest specific activity of the enzyme in etiolated leaf tissue was confirmed to be in the fraction of prolamellar bodies.

Reaggregation of etioplast lipids and the formation of prolamellar bodies and thylakoids: An ultrastructural study

Etioplasts of dark-grownAvena sativa plants were used to prepare either saponin-free or saponin-containing prolamellar bodies. Lipid extracts from both fractions were studied in reaggregation experiments: extracts containing saponins showed liposomes as well as tubules, while saponin-free samples formed only liposomes. Purified PLB lipids in reaggregation experiments were either studied in the presence or in the absence of saponins. Best tubule formation was found with samples containing MGDG+saponin. However, the reconstruction of PLB-like structures was not possible. The long tubules, protruding from isolated PLBs, are seen as a result of the reaction of saponins (originally located in vacuoles) with MGDG.

Prolamellar body and saponins: Avenacosides are not constituents of Avena etioplasts

Etiolated Avena leaf cells were homogenized, then fractionated into four fractions in the presence of salts by differential centrifugation, and intact etioplasts were prepared by Percoll or sucrose density gradient centrifugation. Using thin layer chromatography steroidal saponins, avenacoside A and B were found in the leaf cells and heavy cell fractions which were rich in other cell structures besides intact etioplasts, but not detected in the purified etioplasts. We concluded that saponins are not constituents of the prolamellar bodies in etioplasts.

Regulation of photosynthesis by reversible phosphorylation of the light-harvesting chlorophyll a/b protein

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