INTERACTION BETWEEN POLY(L-LYSINE) AND MEMBRANES INHIBITS PROTON-PUMPING BY CORN ROOT TONOPLAST H-ATPASE()

The influence of poly(L-lysine) binding on the coupled activities of n itrate-sensitive Ht-ATPase in isolated corn (Zea mays L. cv. FRB73) ro ot tonoplast vesicles was investigated. The addition of membrane-imper meable poly(L-lysine) caused a slow increase in light scattering of th e tonoplast suspension. Electron microscopy showed that the increase w as the result of an aggregation of the vesicles. In the presence of 75 mM KCl, a concentration sufficient to sustain near optimal ATP hydrol ysis, poly(L-lysine) slightly enhanced the hydrolysis activity but sig nificantly inhibited proton pumping of the H+-ATPase. Inhibition incre ased with the average molecular mass of poly(L-lysine) and reached a m aximum at 58 kDa. When total osmolarity was kept constant, the replace ment of sucrose by KCl enhanced both ATP hydrolysis and proton pumping activities. However, enhancement of proton pumping was significantly greater than that of ATP hydrolysis. An increase in KCl, but not K2SO4 , significantly relieved poly(L-lysine)-induced inhibition of proton p umping. Kinetic analysis indicated that poly(L-lysine) did not signifi cantly affect the proton leakage of the tonoplast membranes under diff erent energetic conditions. These results suggest that the electrostat ic interaction between poly(L-lysine) and the negative charges on the exterior surface of tonoplast vesicles could change the coupling ratio of ATP hydrolysis to proton pumping. Thus, the surface charge of the tonoplast membrane may be involved in the regulation of these two acti vities.