RESUMO
KdpFABC is an oligomeric K+ transport complex in prokaryotes that maintains ionic homeostasis under stress conditions. The complex comprises a channel-like subunit (KdpA) from the superfamily of K+ transporters and a pump-like subunit (KdpB) from the superfamily of P-type ATPases. Recent structural work has defined the architecture and generated contradictory hypotheses for the transport mechanism. Here, we use substrate analogs to stabilize four key intermediates in the reaction cycle and determine the corresponding structures by cryogenic electron microscopy. We find that KdpB undergoes conformational changes consistent with other representatives from the P-type superfamily, whereas KdpA, KdpC, and KdpF remain static. We observe a series of spherical densities that we assign as K+ or water and which define a pathway for K+ transport. This pathway runs through an intramembrane tunnel in KdpA and delivers ions to sites in the membrane domain of KdpB. Our structures suggest a mechanism where ATP hydrolysis is coupled to K+ transfer between alternative sites in KdpB, ultimately reaching a low-affinity site where a water-filled pathway allows release of K+ to the cytoplasm.
Assuntos
Adenosina Trifosfatases/química , Adenosina Trifosfatases/metabolismo , Proteínas de Transporte de Cátions/química , Proteínas de Transporte de Cátions/metabolismo , Proteínas de Escherichia coli/química , Proteínas de Escherichia coli/metabolismo , Escherichia coli/enzimologia , Proteínas de Membrana/química , Proteínas de Membrana/metabolismo , Adenosina Trifosfatases/genética , Sítios de Ligação , Proteínas de Transporte de Cátions/genética , Escherichia coli/química , Escherichia coli/genética , Escherichia coli/metabolismo , Proteínas de Escherichia coli/genética , Transporte de Íons , Proteínas de Membrana/genética , Modelos Moleculares , Óperon , Potássio/metabolismoRESUMO
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 abolishes the K+-dependence of ATP hydrolysis and blocks the catalytic cycle after formation of the aspartyl phosphate intermediate (E1~P). This regulatory mechanism is unique amongst P-type pumps and this study furthers our understanding of how bacteria control potassium homeostasis to maintain cell volume and osmotic potential.