Functional characterization of the arabidopsis' γ-glutamyltransferase gene family.

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γ-Glutamyltransferase (GGT) has been proposed to catalyze the first committed step in glutathione (GSH) catabolism, degradation of glutathione S-conjugates (GSX) and cysteine (Cys) recycling. However, there are still many aspects about the GGT function, especially concerning the in vivo role of the enzyme in plants, that are not clear. In the Arabidopsis genome there are three genes (AtGGT1, AtGGT2, AtGGT3) coding for GGT. RT-PCR gene expression analysis suggested that AtGGT1 and AtGGT3 are expressed constitutively. In contrast, AtGGT2 is mainly expressed in siliques. Analysis of the distribution of GGT activity by centrifugation indicated that Arabidopsis has soluble and bound GGT activities. Bound GGT was not localized to the microsomal fraction. Besides, the fact that bound GGT was solubilized by high ionic strength buffer containing 500 mM NaCl, and that it was not detected in protoplasts, indicated that bound GGT is likely localized to the cell wall. GGT activity analysis indicated that AtGGT1 codes for a bound GGT, whereas AtGGT2 and AtGGT3 code for soluble GGTs. In order to carefully analysis the intracellular localization of GGT proteins, AtGGT1, AtGGT2, AtGGT3 cDNAs fused to GFP and expressed under the control of their corresponding promoter were introduced in Arabidopsis.