MODULATION OF H-ATPASE ACTIVITY BY FUSICOCCIN IN PLASMA-MEMBRANE VESICLES FROM OAT (AVENA-SATIVA L) ROOTS - A COMPARISON OF MODULATION BY FUSICOCCIN, TRYPSIN, AND LYSOPHOSPHATIDYLCHOLINE()

The fungal phytotoxin fusicoccin affects various transport processes i n the plasma membrane of plant cells. The plasma membrane (PM) H+-ATPa se (EC 3.6.1.35) seems to be the primary target of fusicoccin action. The kinetics of the stimulation of the PM H+-ATPase by fusicoccin was studied in PM vesicles isolated from oat (Avena sativa cv Adamo) roots by aqueous two-phase partitioning. Considerable stimulation of activi ty was observed only when roots were treated with fusicoccin prior to the PM isolation. Fusicoccin treatment shifted the pH optimum of the A TPase toward more alkaline values and increased V-max. No effects on K -m were observed. Treatment with trypsin resulted in stimulation of AT Pase activity in control vesicles but not in the fusicoccin-treated ve sicles. The characteristics of stimulation by trypsin in control vesic les were comparable with those of stimulation by fusicoccin. This resu lt and the change of the polypeptide pattern on western blots suggest the involvement of the C-terminal inhibitory domain in the fusicoccin signal transduction chain. On the other hand, stimulation by lyso-PC d emonstrated other characteristics than stimulation by fusicoccin. Lyso -PC was able to stimulate ATPase activity at both acidic and alkaline pH values. Kinetic analysis of the pH dependency curves revealed diffe rent mechanisms for activation by fusicoccin and by lyso-PC. Whereas f usicoccin shifted the pH dependency of formation of phosphorylated int ermediate to more alkaline values, lyso-PC seemed to increase dephosph orylation independently of pH.