Effect of Ca2+ and Calmodulin on ΔpH Formation in Tonoplast Vesicles from Corn Roots

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Abstract
The effects of calmodulin (CaM) on ATPaes activity and ATP-dependent formation of a proton gradient (.DELTA.pH) were studied in tonoplast membrane vesicles from corn (Zea mays L.) roots. At 0.6 micromolar, CaM stimulated ATPase activity by abount 20% in the absence of an uncoupler, but by only 4% in its presence. Thus, the uncoupler-dependent increment of activity was decreased 30 to 45% by CaM. The formation of a proton gradient across the membrane vesicle, measured by quinacrine fluorescence quench, was inhibited about 20% by CaM. Its effect was additive to the effect of Ca2+ and was completely abolished by EGTA. These effects of CaM could be due to stimulation of H+ efflux or due to inhibition of the H+-ATPase. To distinguish between these possibilities, we examined the effect of CaM on dissipation of preformed .DELTA.pH after the ATPase was inhibited. CaM stimulated the dissipation of a prefound .DELTA.pH by 40% after the H+-ATPase was inhibited with NO3-. This indicates that CaM facilitates the recycling of protons across the tonoplast membranes and does not regulate the H+-ATPase by direct inhibition.