Voltage sensitivity of H+/Ca2+ antiport in higher plant tonoplast suggests a role in vacuolar calcium accumulation.
The electrogenicity of H+/Ca2+ exchange in vacuolar membrane (tonoplast) vesicles from Beta was studied to elucidate the role of this transport system in vacuolar Ca2+ accumulation. To overcome the inherently high proton permeability of tonoplast vesicles, the pH difference established by the primary H(+)-ATPase was titrated to a uniform value by variation of the concentration either of ATP or of a permanent anion (Cl-). This enabled manipulation of membrane potential independently of the transmembrane pH difference, with a higher inside-positive membrane potential produced at lower Cl- concentrations. The rate and the extent of uncoupler-sensitive Ca2+ uptake are both stimulated about 2-fold in conditions of more positive membrane potential, suggesting that the transport system translocates positive charge outward during Ca2+ uptake. A minimum integral H+:Ca2+ stoichiometry of 3 results in a driving force for Ca2+ accumulation in the vacuole amounting to -140 mV in typical physiological conditions. It is concluded that the antiporter is thermodynamically competent to account for Ca2+ accumulation in plant vacuoles and that its reversal in vivo is unlikely.
Blackford
S
S
Biology Department, University of York, Great Britain.
Rea
P A
PA
Sanders
D
D
D000165:Acridine Orange; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002458:Cell Fractionation; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008564:Membrane Potentials; D015388:Organelles / Q000378:metabolism* / Q000502:physiology; D010944:Plants / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D014617:Vacuoles / Q000378:metabolism* / Q000502:physiology
D002352:Carrier Proteins; D000165:Acridine Orange; D002118:Calcium