DIFFERENTIAL STIMULATION BY CL- OF H+ TRANSPORT AND ATPASE ACTIVITY IN PURIFIED PLASMA-MEMBRANE VESICLES FROM MAIZE (ZEA-MAYS L) ROOTS

Maize (Zea mays L.) root plasma membranes purified by the aqueous polymer two-phase technique have previously been shown to be very low in tonoplast H+-ATPase and H+-PPase activities. Western blots of a similar preparation showed that, compared to a microsomal fraction, there was practically no reaction with antibodies to the tonoplast enzymes, but a strong reaction with an antibody to the plasma membrane H+-ATPase. Freeze/thaw treatment of the plasma membrane vesicles increased the proportion with an inside-out orientation to about 40%. This preparation was used to demonstrate that substitution of KCl for K2SO4 resulted in a 14-fold stimulation of H+ transport, but an increase in ATPase activity of less than 10%. In contrast to its effect on tonoplast vesicles, Cl- had only a small effect on the membrane potential of plasma membrane vesicles, assayed by oxonol V fluorescence quench recovery. To account for the apparent variability in the H+/ATP coupling ratio, it may be necessary to devise a model that takes into consideration the possibility of non-linear behaviour with respect to the membrane potential of the proton leak and/or of slip in the ATPase.