MgATP-Dependent Transport of Phytochelatins Across the Tonoplast of Oat Roots

In Cd-exposed oat (Avena sativa) roots Cd was found to be associated primarily with the phytochelatin (gamma-glutamylcysteinyl)3-glutamic acid ([gammaEC)3G], with a peptide to Cd ratio of 1:3 (cysteine to Cd ratio of 1:1), even though both (gammaEC)2G and (gammaEC)3G were present in the roots. Phytochelatins are known to accumulate in the vacuoles of plant cells on exposure to Cd, but the mechanism is not clear. Here we present evidence for the transport of the phytochelatins (gammaEC)3G and (gammaEC)2G as well as the Cd complex Cd-(gammaEC)3G across the tonoplast of oat roots. Transport of (gammaEC)3G had a Km, for MgATP of 0.18 mM and a Vmax of 0.7 to 1 nmol mg-1 protein min-1. Transport of (gammaEC)3G was also energized by MgGTP and to a lesser extent MgUTP and was highly sensitive to orthovanadate, with a 50%-inhibitory concentration of 0.9 muM. The Cd complex Cd-(gammaEC)3G and (gammaEC)2G were also transported in a MgATP-dependent, vanadate-sensitive manner. Therefore, this process is a candidate for the transport of both phytochelatins, and Cd as its peptide complex, from the cytoplasm into the vacuole.