RESUMO
gamma-Glutamyl transpeptidase (GGT) is the only enzyme known that can cleave the gamma-peptide bond between glutamate and cysteine in glutathione, and is therefore a key step in glutathione degradation. There are three functional GGT genes in Arabidopsis, two of which are considered here. GGT1 and GGT2 are apoplastic, associated with the plasma membrane and/or cell wall. RNA blots and analysis of enzyme activity in knockout mutants suggest that GGT1 is expressed most strongly in leaves but is found throughout the plant. A GGT1::GUS fusion construct showed expression only in vascular tissue, specifically the phloem of the mid-rib and minor veins of leaves, roots and flowers. This localization was confirmed in leaves by laser microdissection. GGT2 expression is limited to embryo, endosperm, outer integument, and a small portion of the funiculus in developing siliques. The ggt2 mutants had no detectable phenotype, while the ggt1 knockouts were smaller and flowered sooner than wild-type. In ggt1 plants, the cotyledons and older leaves yellowed early, and GSSG, the oxidized form of glutathione, accumulated in the apoplastic space. These observations suggest that GGT1 is important in preventing oxidative stress by metabolizing extracellular GSSG, while GGT2 might be important in transporting glutathione into developing seeds.
Assuntos
Proteínas de Arabidopsis/genética , Arabidopsis/enzimologia , Regulação da Expressão Gênica de Plantas , Glutationa/metabolismo , gama-Glutamiltransferase/genética , Arabidopsis/genética , Proteínas de Arabidopsis/metabolismo , Genes Reporter , Dissulfeto de Glutationa/metabolismo , Mutação , Floema/genética , Folhas de Planta/metabolismo , gama-Glutamiltransferase/metabolismoRESUMO
The results in the present study emphasize that in Arabidopsis thaliana genome, gamma-GTase encoded by a gene family comprised of four genes. Three of these genes are functionally expressed while the fourth is a pseudogene. The three functional genes express different active protein isoforms that appeared to have different functions. Two genes (GGT1 and GGT2) are structurally similar and expresses proteins with the same molecular weight and both target to the same cellular compartment (plasma membrane). However, both demonstrated different tissue localization, physiological activities and different patterns of response to environmental stress. The majority of GGTI encoded by the first gene was localized in the rosette leaves, It is also expressed throughout the whole plant.
