CALMODULIN STIMULATION OF CA-2+ NH+ ANTIPORT ACROSS THE VACUOLAR MEMBRANE OF SUGAR-BEET TAPROOT

Using preparations of vacuoles and vacuolar membranes isolated from storage tissue of sugar beet (Beta vulgaris L.), it was shown that Ca2+ ions caused the dissipation of an ATP-generated or artificially formed transtonoplast pH gradient monitored by the absorption change of acridine orange. Ca2+-dependent Δ pH decreasing at the tonoplast is blocked by La3+ and is due to the action of the Ca2+/nH+ antiporter associated with this membrane and characterized by an apparent Km of 10 μM for Ca2+ at pH 8.0. It was found that calmodulin greatly accelerated transtonoplast Δ pH dissipation in the presence of Ca2+. This effect is independent of ATP and blocked by La3+ or removing Ca2+ from the medium using EGTA. Calmodulin stimulation of Ca2+/H+ exchange across the tonoplast was shown to be due to a dramatic drop (almost 1000-fold) in apparent Km of Ca2+/nH+ antiporter for Ca2+. This result led us to suggest that calmodulin directly interacts with the carrier in question and such interaction takes place even at Ca2+ concentrations lower than the Kd of the Ca2+-calmodulin complex. © 1990, Gustav Fischer Verlag, Stuttgart. All rights reserved.