Galactoside-dependent proton transport by mutants of the Escherichia coli lactose carrier: substitution of tyrosine for histidine-322 and of leucine for serine-306

Steven King, T. Hastings Wilson

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Abstract

The lacY genes from two Escherichia coli mutants, MAB20 and AA22, have been cloned in a multicopy plasmid by a novel 'sucrose marker exchange' method. Characterization showed that the plasmids express a lactose carrier with poor affinity for lactose. Neither mutant carried out concentrative uptake with methyl ß-d-galactopyranoside, lactose, or melibiose as the substrate. Nor did the mutants catalyze counterflow or exchange with methyl ß-d-galactopyranoside. Both mutants did, however, retain the capacity to carry out facilitated diffusion with lactose or melibiose. DNA sequencing revealed that MAB20 (histidine-322 to tyrosine) and AA22 (serine-306 to leucine) have amino acid substitutions within the putative 'charge-relay' domain thought to be responsible for proton transport. Galactoside-dependent H+ transport was readily measured in both mutants. We conclude, therefore, that the presence of a histidine residue at position 322 of the lactose carrier is not obligatory for H+ transport per se.

Original languageEnglish (US)
Pages (from-to)253-264
Number of pages12
JournalBBA - Biomembranes
Volume982
Issue number2
DOIs
Publication statusPublished - Jul 10 1989
Externally publishedYes

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Keywords

  • Cotransport
  • Galactoside dependence
  • Lactose carrier
  • lacY
  • Proton transport
  • Transport energetics

ASJC Scopus subject areas

  • Biochemistry
  • Biophysics
  • Cell Biology

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