ABSCISIC-ACID INDUCTION OF GTP HYDROLYSIS IN MAIZE COLEOPTILE PLASMA-MEMBRANES

Since receptor-coupled G proteins increase GTP hydrolysis (GTPase) act ivity upon ligands binding to the receptor, a study was undertaken to determine if abscisic acid (ABA) induced such an effect. Plasma membra nes isolated from etiolated maize (Zea mays L.) coleoptiles were enric hed in GTPase activity relative to microsomal fractions. Vanadate was included in the assay to inhibit the high levels of vanadate sensitive low affinity GTPases present. Under these conditions, GTPase activity was enhanced by Mg2+, stimulated by mastoparan, and inhibited by GTP gamma S indicating the presence of either monomeric or heterotrimeric G proteins. The combination of NaF and AlCl3 is expected to inhibit he terotrimeric G protein activity but had little effect on GTPase activi ty in maize coleoptile membranes. Cholera toxin enhanced basal GTPase activity, confirming the presence of heterotrimeric G proteins in maiz e plasma membranes. Pertussis toxin also slightly enhanced basal GTPas e activity in maize membranes. Abscisic acid enhanced GTPase activity optimally at 5 mmol/L Mg2+ in a concentration dependent manner by 1.5- fold at 10 mu mol/L and up to three-fold at 100 mu mol/LABA. Abscisic acid induced GTPase activity was inhibited by GTP gamma S, the combina tion of NaF and AlCl3, and pertussis toxin. Overall, these results are typical of a receptor-coupled G protein responding to its ligand.