Role of the plasma membrane H⁺-ATPase in K⁺ transport

The role of the plant plasma membrane H⁺-ATPase in K⁺ uptake was examined using red beet (Beta vulgaris L.) plasma membrane vesicles and a partially purified preparation of the red beet plasma membrane H⁺ -ATPase reconstituted in proteoliposomes and planar bilayers. For plasma membrane vesicles, ATP-dependent K⁺ efflux was only partially inhibited by 100 μM vanadate or 10 μM carbonyl cyanide-p-trifluoromethoxyphenylhydrazone. However, full inhibition of ATP-dependent K⁺ efflux by these reagents occurred when the red beet plasma membrane H⁺-ATPase was partially purified and reconstituted in proteoliposomes. When reconstituted in a planar bilayer membrane, the current/voltage relationship for the plasma membrane H⁺-ATPase showed little effect of K⁺ gradients imposed 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. Rather, this enzyme provides a driving force for cellular K⁺ uptake by secondary mechanisms, such as K⁺ channels or H⁺/K⁺ symporters. Although the presence of a small, protonophoreinsensitive component of ATP-dependent 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.