Serine Phosphorylation Regulates the P-type Potassium pump KdpFABC

KdpFABC is an ATP-dependent K+ pump that ensures bacterial survival in K+-deficient environments. Whereas transcriptional activation of kdpFABC expression is well studied, a mechanism for down regulation when K+ levels are restored has not been described. Here we show that KdpFABC is inhibited when cells return to a K+-rich environment. The mechanism of inhibition involves phosphorylation of Ser162 on KdpB, which can be reversed in vitro by treatment with serine phosphatase. Mutating Ser162 to Alanine produces constitutive activity, whereas the phosphomimetic Ser162Asp mutation inactivates the pump. Analyses of the transport cycle show that serine phosphorylation uncouples the pump and blocks the cycle after formation of the catalytic aspartyl phosphate intermediate (E1[~]P). Molecular dynamics simulations show that serine phosphorylation has allosteric effects that potentially explain the uncoupling. This regulatory mechanism, unique amongst P-type pumps, furthers our understanding of how bacteria control potassium homeostasis to maintain cell volume and osmotic potential.