Separation of tonoplast and plasma membrane H+-ATPase from Zucchini hypocotyls by consecutive sucrose and glycerol gradient centrifugation

Abstract

In order to isolate tonoplast and plasma membrane vesicles involved in ATP-dependent proton transport we devised a preparative procedure with two consecutive centrifugations. Three fractions were obtained on a sucrose step gradient: light microsomes, heavy microsomes, and a mitochondria-rich fraction. The light and heavy microsomal fractions were each recentrifuged on an isopycnic glycerol density gradient. Recentrifugation of light microsomes resulted in two fractions with H+-ATPase activity, one equilibrating at a density less than 1.11 g/cm3 and one equilibrating at a density of about 1.17g/cm3. Comparison with marker enzyme activities suggests that the upper fraction was enriched in tonoplast, and the dense fraction with plasma membrane. In addition to marker enzyme content, H+ transport in the H+-ATPase-containing fractions was further characterized with respect to pH dependence, cation and anion dependence, and uncouplers and inhibitors. H+ transport in all fractions was strongly dependent on the presence of halides but no specific stimulation by potassium or any other monovalent cation was found. Of the anions tested, malate and fumarate preferentially stimulated H+ transport in the tonoplast-enriched fraction. It is suggested that a Ca2+/H+ antiporter is present in all fractions. Only H+-ATPase in the plasma membrane-enriched fractions was sensitive to nystatin, an uncoupler, and to orthovanadate, an inhibitor. The tonoplast fraction was more sensitive to nitrate than the plasma membrane-enriched fraction, and all fractions showed some sensitivity to high concentrations of oligomycin. Oligomycin sensitivity was not due to the presence of mitochondria.