Stress brings out the best: Potassium transport by KdpFABC.

Abstract

The kdp operon is used by bacteria to combat osmotic stress, such as the loss of K+ in the media or exposure to fresh water in the environment. Like all organisms across the kingdoms of life, elevated K+ in the cytosol is essential for maintaining the membrane potential, driving a variety of transport processes, and in bacteria, for maintenance of pH, cell growth and division. KdpD and KdpE comprise a constitutively expressed two-component system for detecting osmotic stress and inducing the expression of the K+ transport system KdpFABC. This system represents an unusual partnership between a K+ channel (KdpA) and a P-type ATPase (KdpB). Recent work has produced a series of structures in a variety of different states corresponding to major intermediates in the Post-Albers transport cycle that is characteristic of the P-type ATPase superfamily. Based on this work, a detailed reaction mechanism has been proposed in which KdpA selects K+ from the periplasm, a unique intramembrane tunnel delivers the K+ to canonical ion binding sites in KdpB where ATP-driven conformational changes deliver the ion to the cytosol. This mechanism highlights fascinating evolutionary adaptations of a K+ channel, which appears to be static and no longer provides a conduit across the membrane, and a P-type ATPase, which does not directly bind ions from the periplasm and may not have innate ion selectivity. Despite tremendous progress in characterizing this system, questions persist about the nature of the intramembrane tunnel, the release site of K+ from KdpB and the energetics of the transport process.