Functions of serine hydroxymethyltransferases in plant metabolism and gene regulation
Zusammenfassung der Projektergebnisse
Serine hydroxymethyltransferase (SHMT) is an important enzyme in most if not all organisms. In mammals, cytoplasmic (SHMT1) and mitochondrial (SHMT2) isozymes belong to metabolic networks required for the biosynthesis of purines, thymidylate and methionine. In plants, for example Arabidopsis, isoforms of SHMT are present in the mitochondrion (SHM1 and SHM2), the cytosol (SHM4 and SHM5), the plastids (SHM3), and possibly nuclei (SHM6 and SHM7). At the beginning of this project, only one plant isoSHMT, the mitochondrial SHM1, had been more exactly examined. This enzyme is responsible for the photorespiratory glycine-into-serine conversion and cannot be replaced by SHM2. Apart from their likely involvement in cellular onecarbon metabolism, little is known about the exact biological functions of the other five isozymes, in particular SHM2, SHM4, SHM5, SHM6, and SHM7. In this project, we evaluated the functions of the 'non-photorespiratory' mitochondrial SHM2 and of the likely nuclear located SHM6 and SHM7. We found that SHM2 cannot contribute to photorespiratory metabolism because it is normally not present in leaf mesophyll mitochondria. This is likely due to the operation of an 'import filter' that prevents pre SHM2 to enter these particular mitochondria. However, SHM2 is present in the mitochondria of nonphotorespiratory tissues where it occurs together with SHM1. In leaves, for example, SHM2 is present in the protoxylem and/or adjacent cells of the veins. Unlike the individual shm1 and shm2 null mutants, which require CO2-enriched air to inhibit photorespiration (shm1) or do not show any visible impairment (shm2), double-null mutants cannot survive in CO2-enriched air. It appears that SHM1 and SHM2 operate in a redundant manner in one-carbon metabolism of non-photorespiring cells with a high demand of one-carbon units, for example during lignification of vascular cells. We hypothesize that yet unknown kinetic properties of SHM2 might render this particular enzyme unsuitable for the high-folate conditions of photorespiring mesophyll mitochondria. Concerning SHM6 and SHM7, we could demonstrate nuclear localization but not yet exactly identify the metabolic function of these enzymes. Even the combined deletion of both enzymes had only minor effects on growth and development of a corresponding Arabidopsis double mutant.
Projektbezogene Publikationen (Auswahl)
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(2010). Charakterisierung der GDC- und SHM-Genfamilie in Arabidopsis thaliana als Voraussetzung für die Nachbildung einer primären CO2-Pumpe. Dissertation, Universität Rostock
Engel N
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(2010). Untersuchungen zur Funktion der Proteine SHMT6 und SHMT7 in Arabidopsis thaliana. Dissertation, Universität Rostock
Müller S
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(2011). The presequence of Arabidopsis serine hydroxymethyltransferase SHM2 selectively prevents import into mesophyll mitochondria. Plant Physiology 157: 1711-1720
Engel N, Ewald R, Gupta KJ, Zrenner R, Hagemann M, Bauwe H
