title: Functional role of arginine 302 within the lactose permease of Escherichia coli.
abstract: Within the lactose permease, an arginine residue is found on a transmembrane segment at position 302. Based upon the effects of mutations at or in the vicinity of Arg-302, this residue has been implicated to be involved with H+ and/or sugar recognition. To further elucidate the role of this residue, we have substituted Arg-302 with serine, histidine, and leucine via site-directed mutagenesis. All three of these substitutions result in an impaired ability to transport galactosides as evidenced by their poor growth on minimal plates supplemented with lactose or melibiose. Furthermore, in vitro transport assays revealed substantial alterations in the kinetic constants for downhill lactose transport. The wild-type strain exhibited a Km for lactose transport of 0.30 mM and a Vmax of 267 nmol of lactose/min.mg of protein. The Ser-302, His-302, and Leu-302 were observed to have Km values of 0.18, 2.3, and 2.8 mM, and Vmax values of 11.6, 56.4, and 22.0 nmol of lactose/min.mg of protein, respectively. In uphill transport assays, all three mutants were unable to accumulate beta-methyl-D-thiogalactoside. However, both the Ser-302 and His-302 mutants were able to accumulate lactose against a concentration gradient. During H+ transport assays, all three mutants were shown to transport H+ in conjunction with thiodigalactoside. In addition, the Ser-302 and His-302 strains exhibited small alkalinizations upon the addition of lactose. However, for the Leu-302 mutant, the addition of lactose did not result in a significant level of H+ transport. Finally, experiments were conducted which were aimed at measuring the ability of the mutant permeases to catalyze an H+ leak. In this regard, a comparison was made between the wild-type and mutant strains concerning their steady state pH gradient and their rates of H+ influx following oxygen pulses. The results of these experiments suggest that mutations at position 302 cause a sugar-dependent H+ leak.
authors.affiliation: Department of Genetics and Cell Biology, University of Minnesota, St. Paul 55108.
authors.forename: E A
authors.initials: EA
authors.lastname: Matzke
authors.forename: L J
authors.initials: LJ
authors.lastname: Stephenson
authors.forename: R J
authors.initials: RJ
authors.lastname: Brooker
mesh_terms: D001120:Arginine / Q000737:chemistry; D001483:Base Sequence; D001693:Biological Transport, Active; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins; D006863:Hydrogen-Ion Concentration; D007785:Lactose / Q000378:metabolism*; D008564:Membrane Potentials; D026901:Membrane Transport Proteins / Q000737:chemistry*; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins; D016297:Mutagenesis, Site-Directed; D009838:Oligodeoxyribonucleotides / Q000737:chemistry; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D027981:Symporters
chemical_list: D029968:Escherichia coli Proteins; C050221:LacY protein, E coli; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D009838:Oligodeoxyribonucleotides; D027981:Symporters; C021499:lactose permease; D001120:Arginine; D007785:Lactose