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Published on June 1, 2007; 10.1104/pp.106.094409

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Received December 11, 2006
Accepted May 29, 2007

Localization of Members of the g-Glutamyl Transpeptidase Family Identifies Sites of Glutathione and Glutathione S-Conjugate Hydrolysis

Melinda N. Martin *, Pilar H. Saladores , Elton Lambert , Andre O. Hudson , and Thomas Leustek

Rutgers, The State University of New Jersey, Biotechnology Center for Agriculture and the Environment, New Brunswick, New Jersey 08901-8520, USA

* Corresponding author; email: mnmartin{at}aesop.rutgers.edu.

{gamma}-Glutamyl transpeptidases (GGTs) are essential for hydrolysis of the tripeptide glutathione ({gamma}Glu-Cys-Gly) and glutathione S-conjugates since they are the only enzymes known to cleave the amide bond linking the {gamma}-carboxylate of glutamate to cysteine. In Arabidopsis thaliana, four GGT genes have been identified based on homology with animal GGTs. They are designated GGT1 (At4g39640), GGT2 (At4g39650), GGT3 (At1g69820), and GGT4 (At4g29210). By analyzing the expression of each GGT in plants containing GGT:{beta}-glucuronidase fusions, the temporal and spatial pattern of degradation of GSH and its metabolites was established revealing appreciable overlap among GGTs. GGT2 exhibited narrow temporal and spatial expression primarily in immature trichomes, developing seeds, and pollen. GGT1 and GGT3 were co-expressed in most organs/tissues. Their expression was highest at sites of rapid growth including the rosette apex, floral stem apex, and seeds and might pinpoint locations where GSH is delivered to sink tissues to supplement high demand for cysteine. In mature tissues, they were expressed only in vascular tissue. Knockout mutants of GGT2 and GGT4 showed no phenotype. The rosettes of GGT1 knockouts showed premature senescence after flowering. Knockouts of GGT3 showed reduced number of siliques and reduced seed yield. Knockouts were used to localize and assign catalytic activity to each GGT. In the standard GGT assay with {gamma}-glutamyl p-nitroanilide as substrate, GGT1 accounted for 80-99% of the activity in all tissues except seeds where GGT2 was 50% of the activity. Protoplasting experiments indicated that both GGT1 and GGT2 are localized extracellularly but have different physical or chemical associations.




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