Kup is the major K⁺ uptake system in Escherichia coli upon hyper‐osmotic stress at a low pH

Abstract

The K + uptake was observed in washed cells of Escherichia coli, wild-type, upon hyper-osmotic stress at pH 5.5 when glucose was supplemented. This uptake had apparent a K m of 0.58 mM and V mx of 0.10 W Wmol K + /min/mg protein. Such a K + uptake was investigated using a mutant defective in Kdp and TrkA but with a functional Kup and a mutant defective in Kdp and Kup but having an active TrkA. The K + uptake to reach the steady state level as well as the initial K + influx rate in the first mutant were at least 3.5-fold greater than these values with the second mutant and similar to those of the wild-type. Such differences in the K + uptake activity were correlated with K + requirements for growth of these mutants. Moreover, the K + uptake in the wild-type was blocked by a protonophore (carbonyl cyanide m-chlorophenylhydrazone). Valinomycin, arsenate and N,NP-dicyclohexylcarbodiimide were not effective in changing the K + uptake. It is suggested that Kup is the major K + uptake system in E. coli upon hyper-osmotic stress at a low pH.z 1999 Federation of European Biochemical Societies.

Tansley Review No. 113

“…Evidence suggests that KUP can facilitate K + \Rb + uptake against its electrochemical gradient and therefore must couple K + \Rb + transport either directly to ATP hydrolysis or to the electrochemical gradient of another ion (for example Na + or H + ). The homologous bacterial KUP and fungal HAK transporters seem to function as H + \K + symports (Haro et al, 1999 ;Trchounian & Kobayashi, 1999) ; this is consistent with electrophysiological evidence (Maathuis & Sanders, 1994) and physiological evidence for a H + \K + symport in plant roots. The E. coli KUP transporter apparently obeys Michaelis-Menten kinetics and has a K m of 5 mM for Cs + transport, a K m of 0.37 mM for K + transport and a V max for Cs + transport two-thirds that for K + transport (Bossemeyer et al, 1989).…”

“…Thus, it is likely that their transport mechanism is also conserved. Both the bacterial KUP and fungal HAK transporters function as K + \H + symporters (Haro et al, 1999 ;Trchounian & Kobayashi, 1999) and the KUP transporter of E. coli mediates Cs + uptake (Bossemeyer et al, 1989).…”

Bacterial strategies to inhabit acidic environments.

“…The activity of the major system, TrkA, declines rapidly at acid pH. In contrast, Kup has an optimum activity at acid pH and is the main system for potassium ion accumulation under hyperosmotic stress at pH 5.5 (Trchounian and Kobayashi, 1999). Therefore Kup seems to work to compensate for the decrease in the activity of TrkA at acid pH.…”

Expression and Activity of Kdp under Acidic Conditions in Escherichia coli

“…8) In contrast to near neutral pH, few reports have examined the expression and activity of K ϩ transport systems under acidic conditions. 9,10) It was reported that the activity of Trk was decreased at low pH and that Kup was active at acidic pH to compensate for the decrease in the Trk function.…”