Target Synonyms and Classification: The K+ uptake ATPase (Kdp ATPase) belongs to the p-type ATPase Superfamily (TCDB: 3.A.3), which catalyze cation transport driven by ATP hydrolysis. It is rather a unique member of this superfamily, since it contains four subunits (KdpFABC).
Function and Mechanism: Kdp ATPase is an inducible high-affinity K+ transporter that is synthesized under conditions of severe K+ limitation or osmotic upshift. It maintains the desired concentration of internal K+ required for various physiological processes like turgor homeostasis and pH regulation. Kdp is composed of four protein subunits forming the KdpFABC complex. Here KdpA acts as a K+ transporter with high affinity (KD = 2 µM). It can maintain cytoplasmic potassium concentrations against gradients up to 10.000 fold. KdpB contains the ATPase domain and KdpC and KdpF may facilitate complex assembly and stabilize the complex. There is a very special mechanism in place which guarantees coupling between KdpA and KdpB: KdpA has a protein embedded tunnel and the cytoplasmic gate within KdpA is linked to the phosphorylation domain of KdpB. Here, a channel architecture was repurposed for active transport. Furthermore, KdpFABC acts as a functional dimer.
Organism and Localization: The Kdp system is widely dispersed among the different classes of bacteria including the cyanobacteria and was studied most in Escherichia coli. It localizes to the plasma membrane. Kdp activity is essential for cell growth at low potassium concentrations.
Substrates and Inhibitors: KdpA has a very high specificity for potassium: Rb+, with a Pauling radius (1.49 Å) comparable to that of K+ (1.33 Å) is transported with much lower affinity (KM = 8 mM vs. KM = 2 µM). It was shown that KdpA depends on pH, showing highest K+ turnover rates at pH 7.4 – likely due to a proton leak pathway. The G232D mutant of KdpA instead transports various monovalent cations (K+ > Rb+ > Na+ > Li+ > H+). A common inhibitor for inhibition of p-type ATPase is orthovanadate. It inhibits the Kdp complex at 1 – 10 µM by trapping the protein in a transition state of phosphoenzyme hydrolysis.
Related Transporters: While KdpB is homologues to p-type ATPase a-subunits, KdpA is homologous to other K+ transporters like KcsA and KtrB. It contains a selectivity filter descenced from that of the bacterial channel KcsA.