Three putative cation/proton antiporters from the soda lake alkaliphile Alkalimonas amylolytica N10 complement an alkali-sensitive Escherichia coli mutant

Microbiology. 2007 Jul;153(Pt 7):2168-2179. doi: 10.1099/mic.0.2007/007450-0.

Abstract

Attempts to identify members of the antiporter complement of the alkali- and saline-adapted soda lake alkaliphile Alkalimonas amylolytica N10 have used screens of DNA libraries in antiporter-deficient Escherichia coli KNabc. Earlier screens used Na(+) or Li(+) for selection but only identified one NhaD-type antiporter whose properties were inconsistent with a robust role in pH homeostasis. Here, new screens using elevated pH for selection identified three other putative antiporter genes that conferred resistance to pH >or=8.5 as well as Na(+) resistance. The three predicted gene products were in the calcium/cation antiporter (CaCA), cation/proton antiporter-2 (CPA2) and cation/proton antiporter-1 (CPA1) families of membrane transporters, and were designated Aa-CaxA, Aa-KefB and Aa-NhaP respectively, reflecting homology within those families. Aa-CaxA conferred the poorest Na(+) resistance and also conferred modest Ca(2+) resistance. Aa-KefB and Aa-NhaP inhibited growth of a K(+) uptake-deficient E. coli mutant (TK2420), suggesting that they catalysed K(+) efflux. For Aa-NhaP, the reversibility of the growth inhibition by high K(+) concentrations depended upon an organic nitrogen source, e.g. glutamine, rather than ammonium. This suggests that as well as K(+) efflux is catalysed by Aa-NhaP. Vesicles of E. coli KNabc expressing Aa-NhaP, which conferred the strongest alkali resistance, exhibited K(+)/H(+) antiport activity in a pH range from 7.5 to 9.5, and with an apparent K(m) for K(+) of 0.5 mM at pH 8.0. The properties of this antiporter are consistent with the possibility that this soda lake alkaliphile uses K(+)( )/H(+) antiport as part of its alkaline pH homeostasis mechanism and part of its capacity to reduce potentially toxic accumulation of cytoplasmic K(+) or respectively, under conditions of high osmolarity or active amino acid catabolism.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alkalies / pharmacology*
  • Antiporters
  • Bacterial Proteins*
  • Cations
  • Escherichia coli / genetics
  • Escherichia coli / metabolism*
  • Gammaproteobacteria / metabolism*
  • Molecular Sequence Data
  • Mutation
  • Protons
  • Sodium Chloride
  • Sodium-Hydrogen Exchangers / metabolism*

Substances

  • Alkalies
  • Antiporters
  • Bacterial Proteins
  • Cations
  • Protons
  • Sodium-Hydrogen Exchangers
  • Sodium Chloride

Associated data

  • GENBANK/ABG37980
  • GENBANK/ABG37986
  • GENBANK/ABG37987
  • GENBANK/DG649017
  • GENBANK/DG649019
  • GENBANK/DG649020
  • GENBANK/DQ649018