Response of plasma membrane H+-ATPase to heavy metal stress in Cucumis sativus roots

The effect of heavy metals on the modification of plasma membrane H+-ATPase (EC 3.6.3.14) activity in cucumber roots was studied. In plants stressed for 2 h with 10 mu M or 100 mu M Cd, Cu or Ni the hydrolytic as well as the transporting activity of H+-ATPase in the plasma membranes of root cells was decreased. Transcript levels of Cucumis sativus plasma membrane H+-ATPase in roots treated with 10 mu M Cd, Cu, or Ni as well as with 100 mu M Cu or Ni were similar to the control, indicating that the action of metals did not involve the gene expression level. Only in roots exposed to 100 mu M Cd was the level of plasma membrane H+-ATPase mRNA markedly decreased. The inhibition of the plasma membrane proton pump caused by 100 mu M Cd, Cu and Ni was partially diminished in the presence of cantharidin, a specific inhibitor of protein phosphatases. Western blot analysis with the antibody against phosphothreonine confirmed that decreased activity of plasma membrane H+-ATPase under heavy metals resulted from dephosphorylation of the enzyme protein. Taken together, these data strongly indicated that alteration of the enzyme under heavy metal stresses was mainly due to the post-translational modification of its proteins in short-term experiments.