Probing the functional subunits of the tonoplast H/sup +/-ATPase

The tonoplast ATPase of oat roots is composed of at least three polypeptides of 72, 60, and 16 kDa. The 16 kDA polypeptide covalently binds N,N'-dicyclohexylcarbodiimide and is postulated to be a component of the proton channel. Initial studies to identify other subunits indicate that both the 72 and 60 kDa subunits covalently bind /sup 14/C)-7-chloro-4-nitrobenzo-2-oxa-1,3-diazole and (/sup 14/C)N-ethylamleimide, inhibitors of the tonoplast ATPase. ATP prevents binding of these inhibitors suggesting that both the 72 and 60 kDa subunits are involved in substrate binding. Polyclonal antibody has been made to the 72 kDa subunit. Western blot analysis of tonoplast vesicles reveals single reactive polypeptide (72 kDa). The antibody shows no cross-reactivity towards either the mitochondrial F/sub 1/-ATPase or the plasma membrane ATPase. This antibody specifically inhibits ATP hydrolysis and ATP-dependent H/sup +/ pumping in native tonoplast vesicles. The authors conclude that the 72 kDa subunit is intimately associated with the catalytic (or ATP-binding) site.