Potassium/Proton Antiport System of Escherichia coli

Martha V Radchenko,Kimihiro Tanaka,Rungaroon Waditee,Sawako Oshimi,Yasutomo Matsuzaki,Masahiro Fukuhara,Hiroshi Kobayashi,Teruhiro Takabe,Tatsunosuke Nakamura

doi:10.1074/jbc.m600333200

Abstract

The intracellular level of potassium (K(+)) in Escherichia coli is regulated through multiple K(+) transport systems. Recent data indicate that not all K(+) extrusion system(s) have been identified (15). Here we report that the E. coli Na(+) (Ca(2+))/H(+) antiporter ChaA functions as a K(+) extrusion system. Cells expressing ChaA mediated K(+) efflux against a K(+) concentration gradient. E. coli strains lacking the chaA gene were unable to extrude K(+) under conditions in which wild-type cells extruded K(+). The K(+)/H(+) antiporter activity of ChaA was detected by using inverted membrane vesicles produced using a French press. Physiological growth studies indicated that E. coli uses ChaA to discard excessive K(+), which is toxic for these cells. These results suggest that ChaA K(+)/H(+) antiporter activity enables E. coli to adapt to K(+) salinity stress and to maintain K(+) homeostasis.

Highlights

Intracellular Kϩ concentrations are regulated in E. coli through a variety of systems, including Kdp, Trk, and Kup-related uptake systems (4 –7), KefB- and KefC-related efflux systems [8], Kch Kϩ channels [9], MscL, MscS, and MscK-related mechanosensitive channels that transport small molecules across the membrane (10 –12), and MdfA multidrug-resistance transporter [13, 14]
KefC act to acidify the cytoplasm only when activated by toxins and are probably not involved in regulating intracellular pH
Recent study shows that MdfA participates in regulation of E. coli pH homeostasis at external alkaline pH by Naϩ/Hϩ and Kϩ/Hϩ exchange [13]

Summary

Introduction

Intracellular Kϩ concentrations are regulated in E. coli through a variety of systems, including Kdp-, Trk-, and Kup (formerly TrkD)-related uptake systems (4 –7), KefB- and KefC-related efflux systems [8], Kch Kϩ channels [9], MscL-, MscS (formerly YggB)-, and MscK (formerly KefA)-related mechanosensitive channels that transport small molecules (including Kϩ) across the membrane (10 –12), and MdfA multidrug-resistance transporter [13, 14]. TO113 cells, containing the chaA gene, released ϳ114 –120 nmol Kϩ/mg of protein upon addition of 50 mM DEA (Fig. 3C). Measurement of Kϩ Extrusion from E. coli by the DEA-dependent Method Using Kϩ Buffer—To determine whether ChaA can mediate Kϩ exchange against, as well as with, a Kϩ concentration gradient, we examined DEA-dependent Kϩ efflux from cells in Kϩ buffer.

Results

Conclusion