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Published on February 17, 2006; 10.1104/pp.105.073635

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Received November 4, 2005
Returned for revision January 8, 2006
Accepted February 6, 2006

A Reassessment of Substrate Specificity and Activation of Phytochelatin Synthases from Model Plants by Physiologically-Relevant Metals

Jorge Loscos , Loreto Naya , Javier Ramos , Maria R. Clemente , Manuel A. Matamoros , and Manuel Becana *

Departamento de Nutrición Vegetal, Estación Experimental de Aula Dei, Consejo Superior de Investigaciones Científicas, Apartado 202, 50080 Zaragoza, Spain

* Corresponding author; email: becana{at}eead.csic.es.

Phytochelatin synthases (PCS) catalyze phytochelatin (PC) synthesis from glutathione (GSH) in the presence of certain metals. The resulting PC-metal complexes are transported into the vacuole, avoiding toxic effects on metabolism. Legumes have the unique capacity to replace partially or completely GSH and PCs by homoglutathione (hGSH) and homophytochelatins (hPCs), respectively. However, the synthesis of hPCs has received little attention. A search for pcs genes in the model legume Lotus (Lotus japonicus) resulted in the isolation of a cDNA clone encoding a protein (LjPCS1) highly homologous to a previously reported homophytochelatin synthase (hPCS) of Glycine max (GmhPCS1). Recombinant LjPCS1 and Arabidopsis PCS1 (AtPCS1) were affinity purified and their polyHis-tags removed. AtPCS1 catalyzed hPC synthesis from hGSH alone at even higher rates than did LjPCS1, indicating that GmhPCS1 is not a genuine hPCS and that a low ratio of hPC to PC synthesis is an inherent feature of PCS1 enzymes. For both enzymes, hGSH is a good acceptor, but a poor donor, of {gamma}Glu-Cys units. Purified AtPCS1 and LjPCS1 were activated (in decreasing order) by Cd2+, Zn2+, Cu2+ and Fe3+, but not by Co2+ or Ni2+, in the presence of 5 mM GSH and 50 µM metal ions. Activation of both enzymes by Fe3+ was proven by the complete inhibition of PC synthesis by the Fe-specific chelator desferrioxamine. Plants of Arabidopsis and Lotus accumulated (h)PCs only in response to a large excess of Cu2+ and Zn2+, but to a much lower extent than did with Cd2+, indicating that (h)PC synthesis does not significantly contribute in vivo to Cu, Zn, and Fe detoxification.




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