Glutamine synthetase and glutamate dehydrogenase isoforms in maize leaves: localization, relative proportion and their role in ammonium assimilation or nitrogen transport |
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Authors: | Thomas W Becker Elisa Carrayol Bertrand Hirel |
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Institution: | Lehrstuhl für Genetik, Fakult?t für Biologie, Universit?t Bielefeld, Postfach 10 01 31, 33501 Bielefeld, Germany, DE Unité de Nutrition Azotée des Plantes, Institut National de la Recherche Agronomique, Route de Saint-Cyr, 78026 Versailles, France, FR
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Abstract: | Mesophyll cells (MCs) and bundle-sheath cells (BSCs) of leaves of the C4 plant maize (Zea mays L.) were separated by cellulase digestion to determine the relative proportion of the glutamine synthetase (GS; EC 6.3.1.2)
or the NADH-glutamate dehydrogenase (GDH; EC 1.4.1.2) isoforms in each cell type. The degree of cross-contamination between
our MC and BSC preparations was checked by the analysis of marker proteins in each fraction. Nitrate reductase (EC 1.6.6.1)
proteins (110 kDa) were found only in the MC fraction. In contrast, ferredoxin-dependent glutamate synthase (Fd-GOGAT; EC
1.4.7.1) proteins (160 kDa) were almost exclusively present in the BSC fraction. These results are consistent with the known
intercellular distribution of nitrate reductase and Fd-GOGAT proteins in maize leaves and show that the cross-contamination
between our MC and BSC fractions was very low. Proteins corresponding to cytosolic GS (GS-1) or plastidic GS (GS-2) were found
in both the MC and BSC fractions. While equal levels of GS-1 (40 kDa) and GS-2 (44 kDa) polypeptides were present in the BSC
fraction, the GS-1 protein level in the MC fraction was 1.8-fold higher than the GS-2 protein pool. Following separation of
the GS isoforms by anion-exchange chromatography of MC or BSC soluble protein extracts, the relative GS-1 activity in the
MC fraction was found to be higher than the relative GS-2 activity. In the BSC fraction, the relative GS-1 activity was very
similar to the relative GS-2 activity. Two isoforms of GDH with apparent molecular weights of 41 kDa and 42 kDa, respectively,
were detected in the BSC fraction of maize leaves. Both GDH isoenzymes appear to be absent from the MC fraction. In the BSCs,
the level of the 42-kDa GDH isoform was 1.7-fold higher than the level of the 41-kDa GDH isoform. A possible role for GS-1
and GDH co-acting in the synthesis of glutamine for the transport of nitrogen is discussed.
Received: 25 January 2000 / Accepted: 30 March 2000 |
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Keywords: | : C4-plant – Cell type specificity – Glutamate dehydrogenase – Glutamine synthetase – Nitrogen metabolism – Zea (ammonium assimilation) |
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