Purification and Characterization of Tonoplast ATPase from Etiolated Mung Bean Seedlings

The tonoplast ATPase from etiolated seedlings of Vigna radiata L. (mung bean) was isolated using a two-step detergent solubilization modified from Mandala and Taiz (S Mandala, L Taiz [1985] Plant Physiol 78: 327-333). After ultracentrifugation on 10 to 28% sucrose gradient, the ATPase showed a 31.6-fold purification over the initial specific activity of the starting tonoplast-enriched membranes. The purified ATPase used Mg(2+)-ATP as the preferred substrate. The tonoplast ATPase was isolated in a form with characteristics similar to that on its native membrane environment. Analysis by SDS-PAGE revealed two prominent bands with molecular weights of 78,000 (alpha subunit) and 64,000 (beta subunit). The intensity of Coomassie blue staining showed a 1:1 stoichiometry for alpha and beta subunits. The amino acid composition of alpha and beta subunits also confirmed the suggested stoichiometry of the subunit composition of the tonoplast ATPase. Moreover, radiation inactivation analysis yielded a functional size of 414 +/- 24 and 405 +/- 25 kilodaltons for soluble and membrane bound tonoplast ATPases, respectively. It is possible that the functioning tonoplast ATPase may be in a form of alphabeta-heteromultimer.