Potassium as an Intrinsic Uncoupler of the Plasma Membrane H+-ATPase*

The plant plasma membrane proton pump (H+-ATPase) is stimulated by potassium, but it has remained unclear whether potassium is actually transported by the pump or whether it serves other roles. We now show that K+ is bound to the proton pump at a site involving Asp617 in the cytoplasmic phosphorylation domain, from where it is unlikely to be transported. Binding of K+ to this site can induce dephosphorylation of the phosphorylated E1P reaction cycle intermediate by a mechanism involving Glu184 in the conserved TGES motif of the pump actuator domain. Our data identify K+ as an intrinsic uncoupler of the proton pump and suggest a mechanism for control of the H+/ATP coupling ratio. K+-induced dephosphorylation of E1P may serve regulatory purposes and play a role in negative regulation of the transmembrane electrochemical gradient under cellular conditions where E1P is accumulating.