ROLE OF THE PLASMA-MEMBRANE H-ATPASE IN K+ TRANSPORT()

The role of the plant plasma membrane H+-ATPase in K+ uptake was exami ned using red beet (Beta vulgaris L.) plasma membrane vesicles and a p artially purified preparation of the red beet plasma membrane H+-ATPas e reconstituted in proteoliposomes and planar bilayers. For plasma mem brane vesicles, ATP-dependent K+ efflux was only partially inhibited b y 100 mu M vanadate or 10 mu M carbonyl cyanide-p-trifluoromethoxyphen ylhydrazon However, full inhibition of ATP-dependent K+ efflux by thes e reagents occurred when the red beet plasma membrane H+-ATPase was pa rtially purified and reconstituted in proteoliposomes. When reconstitu ted in a planar bilayer membrane, the current/voltage relationship for the plasma membrane H+-ATPase showed little effect of K+ gradients im posed across the bilayer membrane. When taken together, the results of this study demonstrate that the plant plasma membrane H+-ATPase does not mediate direct K+ transport chemically linked to ATP hydrolysis. R ather, this enzyme provides a driving force for cellular K+ uptake by secondary mechanisms, such as K+ channels or H+/K+ symporters. Althoug h the presence of a small, protonophore-insensitive component of ATP-d ependent K+ transport in a plasma membrane fraction might be mediated by an ATP-activated K+ channel, the possibility of direct K+ transport by other ATPases (i.e. K+-ATPases) associated with either the plasma membrane or other cellular membranes cannot be ruled out.