title: Regulation of the lemon-fruit V-ATPase by variable stoichiometry and organic acids.
abstract: The lemon-fruit V-ATPase can exist in two forms: nitrate-sensitive and nitrate-insensitive. Here we report the results of measurements of H+ /ATP stoichiometries using two kinetic methods: one based on steady-state DpH and one based on initial rates of H+-pumping. Our findings indicate that the nitrate-insensitive fruit V-ATPase has an H+ /ATP stoichiometry of ~1, while both the nitrate-sensitive fruit V-ATPase and the epicotyl V-ATPase have stoichiometries of 2, under zero-load conditions. As DpH increases, the stoichiometry of the nitrate-sensitive fruit V-ATPase decreases to 1. Under similar conditions, the stoichiometry of the epicotyl enzyme remains 2. Thus, the pH-dependent variable stoichiometry of the lemon-fruit V-ATPase may represent a key factor in juice sac vacuolar hyperacidification. On the other hand, the H+ /ATP stoichiometry of the epicotyl V-ATPase can decrease from 2 to 1 in the presence of a membrane potential. The low pH of the fruit vacuole is not due solely to the lower H+/ATP stoichiometry of its pump. We show that lumenal citrate and malate improve the coupling of both the epicotyl and fruit V-ATPases and enhance their ability to generate a pH gradient. Since citrate accumulation is restricted to fruit vacuoles, it may be another important determinant of vacuolar pH.
authors.affiliation: Biology Department, Sinsheimer Laboratories, University of California, Santa Cruz, CA 95064, USA. mathias_muller@maxygen.com
authors.forename: M L
authors.initials: ML
authors.lastname: Müller
authors.forename: L
authors.initials: L
authors.lastname: Taiz
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D019343:Citric Acid / Q000378:metabolism*; D002957:Citrus / Q000201:enzymology*; D005638:Fruit / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D008293:Malates / Q000378:metabolism*; D008564:Membrane Potentials / Q000502:physiology; D018520:Plant Shoots / Q000378:metabolism; D017494:Proton Pumps / Q000378:metabolism; D011522:Protons; D025262:Vacuolar Proton-Translocating ATPases / Q000737:chemistry / Q000378:metabolism*; D014617:Vacuoles / Q000201:enzymology
chemical_list: D008293:Malates; D017494:Proton Pumps; D011522:Protons; D019343:Citric Acid; C030298:malic acid; D000255:Adenosine Triphosphate; D025262:Vacuolar Proton-Translocating ATPases