The presence and properties of a Mg2+-dependent HCO3− stimulated ATPase in the Malpighian tubules of Locusta migratoria

Effects of efrapeptin and destruxin, metabolites of entomogenous fungi, on the hydrolytic activity of a vacuolar type ATPase identified on the brush border membrane vesicles of Galleria mellonella midgut and on plant membrane bound hydrolytic enzymes

The brush border membrane of the insect midgut is an initial site for interaction of insecticidal proteins. We have investigated the possibility that it may contain a target site for two insecticidal fungal toxins, destruxin and efrapeptin, both of which are ATPase inhibitors. We have studied the effects of the toxins on the hydrolytic activity of a vacuolar type ATPase (V-ATPase) that we have identified from Galleria mellonella midgut columnar cell brush border membrane vesicles (BBMV) by its cation and pH dependence, sensitivity to proton pump inhibitors and K(m) (0.49 mM ATP). Efrapeptin strongly inhibited the BBMV V-ATPase but destruxin had little effect. We compared the effects of the inhibitors on known plant membrane hydrolytic enzymes, and although the vacuolar pyrophosphatase and plasma membrane ATPase were not inhibited by the toxins, the V-ATPase from mung bean, but not barley, was inhibited (50%) by 10 microM concentrations of both compounds. Different forms of the toxins were tested on the ATPases and destruxin B and efrapeptin F were the most effective. Kinetic analysis showed that the purified forms of both compounds inhibited the V-ATPases uncompetitively and modelling of data for inhibition of the BBMV V-ATPase by efrapeptin at concentrations of 0.06--12 microM yielded a K(i) of 0.125 microM.

Characterization of ATPases of apical membrane fractions from Locusta migratoria Malpighian tubules

Apical and basal membrane fractions from Locusta Malpighian tubules were prepared and were characterized by marker enzyme analysis. The apical membranes contained an azide- and orthovanadate-insensitive ATPase activity that was inhibited by bafilomycin A1 (IC50 = 0.44 nM) and NEM (IC50 = 2.15 microM), and thus was characterized as putative V-type ATPase. The enzyme was stimulated by a variety of monovalent cations (Tris > K = Na > choline > Li = Rb) maximal stimulation occurring at 30-40 mM. It was also stimulated by a variety of monovalent anions (maximal activation 30-40 mM), but was strongly inhibited by nitrate and thiocyanate. SDS-PAGE separation of proteins present in the various membrane fractions was carried out. The apical membrane fraction alone contained a 28 kDa protein band that bound a monoclonal antibody specific for a 28 kDa peptide which was a component of the V-type ATPase from midgut of Manduca sexta and, in native gels, possessed ATPase activity which was also sensitive to both bafilomycin and NEM but not to azide or orthovanadate. Binding of the fluorescent monoclonal antibody was located at the apical boundary of the tubule cells. It was concluded that a V-type ATPase is present at the apical surface of Locusta Malpighian tubule cells and that it is involved in their secretory functioning.