Fusicoccin binding to its plasma membrane receptor and the activation of the plasma membrane proton atpase ii. stimulation of the proton atpase in a plasma membrane fraction purified by phase partitioning

De-Michelis, M.I.; Rasi-Caldogno, F.; Pugliarello, M.C.; Olivari, C.

Botanica Acta 104(4): 265-271

1991


ISSN/ISBN: 0932-8629
Accession: 007366955

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Abstract
The effect of fusicoccin (FC) [phytotoxin produced by Fusicoccum amygdali] on the activity of the PM H+-ATPase was investigated in a plasma membrane (PM) fraction from radish [Raphanus sativus] seedlings purified by the phase-partitioning procedure. FC stimulated the PM H+-ATPase activity by up to 100%; the effect was essentially on Vmax with only a slight decrease of the apparent KM of the enzyme for ATP. FC-induced stimulation of the PM H+-ATPase was evident within the first minute and maximal within five minutes of membrane treatment with the toxin indicating that transmission of the signal from the activated receptor to the PM H+-ATPase is very rapid. Both FC-induced simulation of the PM H+-ATPase and FC binding to its receptor decreased dramatically upon incubation of the membranes in ATPase assay medium at 33.degree. C in the absence of FC, due to the lability of the free FC receptor. FC-induced stimulation of the PM H+-ATPase was strongly pH dependent: absolute increase of activity was maximal at pH 7, while percent stimulation increased with increase of pH up to pH 7.5; FC binding was scarcely influenced by pH in the pH range investigated. Taken as a whole, these results indicate that FC binding is a condition necessary, but not sufficient, for FC-induced stimulation of the PM H+ATPase.