Ohkama-Ohtsu Naoko, Radwan Safaa, Peterson Annita, Zhao Ping, Badr Abdel Fattah, Xiang Chengbin, Oliver David J
Department of Genetics, Development and Cell Biology, Iowa State University, Ames, IA 50011, USA.
Plant J. 2007 Mar;49(5):865-77. doi: 10.1111/j.1365-313X.2006.03004.x.
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.
γ-谷氨酰转肽酶（GGT）是已知的唯一能够裂解谷胱甘肽中谷氨酸和半胱氨酸之间γ-肽键的酶，因此是谷胱甘肽降解的关键步骤。拟南芥中有三个功能性GGT基因，这里考虑其中两个。GGT1和GGT2定位于质外体，与质膜和/或细胞壁相关。RNA印迹分析和敲除突变体中的酶活性分析表明，GGT1在叶片中表达最强，但在整个植株中都有发现。一个GGT1::GUS融合构建体仅在维管组织中表达，特别是在叶片、根和花的中脉和小叶脉的韧皮部。通过激光显微切割在叶片中证实了这种定位。GGT2的表达仅限于发育中的角果中的胚、胚乳、外珠被和一小部分珠柄。ggt2突变体没有可检测到的表型，而ggt1敲除植株比野生型更小且开花更早。在ggt1植株中，子叶和老叶早期变黄，谷胱甘肽的氧化形式GSSG在质外体空间中积累。这些观察结果表明，GGT1通过代谢细胞外GSSG在预防氧化应激方面很重要，而GGT2可能在将谷胱甘肽运输到发育中的种子中很重要。
