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The curious case of the H2-protein in Arabidopsis thaliana
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International audience. In all eukaryotic organisms, the glycine cleavage system (GCS) is a four subunits key mitochondrial enzyme that participates in the formation of one-carbon (C1) units, essential for nucleotide synthesis, epigenetic maintenance and reductive metabolism. In photosynthetic tissues, GCS is also involved in the photorespiratory cycle. Both for C1-metabolism and photorespiration, GCS acts in conjunction with the serine hydroxymethyltransferase (SHMT), to catalyses the tetrahydrofolate (THF)-dependent oxidation of glycine to produce serine, the C1 donor compound 5,10-methylene-THF, as well as the side products CO2, NH3 and NADH. The four subunits are defined as a P-protein (a glycine decarboxylase), a T-protein (an aminomethyl-transferase), an L-protein (a dihydrolipoamide dehydrogenase) and a carrier lipoic acid-dependent protein named H-protein. The mechanistic core of this complex relies on the lipoyl arm of the H-protein, which serves as a mobile co-substrate for the three other enzymes. So far, in plants the GCS has been mainly described in leaves. However, the Arabidopsis thaliana genome contains three H genes named H1, H2 and H3. Analysis of transcriptomic data showed that H1 and H3 are mainly expressed in photosynthetic organs, whereas H2 is primarily expressed in the seed and the roots of germinating seedlings. In addition, while H1 and H3 proteins share approximatively 90% protein sequence identity, H2 has only 60% identity with the other two isoforms; however, the lysine that serves to anchor the lipoic-acid is conserved. Interestingly, a phylogenetic analysis revealed that the H2 gene is highly conserved in plant species, ranging from algae to higher plants. We isolated two independent insertion lines leading to a knockout phenotype for the H2 protein. Resulting plants have a severe growth impairment which cannot be rescued by non-photorespiratory conditions. We are currently investigating the reasons for such drastic dwarf phenotype, as well as why the phenotype cannot be rescued by metabolic intermediates produced in leaves since H1 and H3 should be functional.
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