title: Possible salt bridges between transmembrane alpha-helices of the lactose carrier of Escherichia coli.
abstract: Although it is energetically extremely unfavorable to have charged amino acid residues of a polypeptide in the hydrophobic environment of the membrane phospholipid bilayer, a few such charged residues are found in membrane-spanning regions of membrane proteins. Ion pairs (salt bridges) would be much more stable in low dielectric media than single ionized residues. This paper provides indirect evidence for a salt bridge between Asp-240 and Lys-319 in the lactose carrier of Escherichia coli. When Asp-240 was changed to alanine by site-directed mutagenesis, there was a loss of the ability to accumulate methyl-beta-D-thiogalactopyranoside (TMG), melibiose, or lactose. Fast-growing revertants were isolated on melibiose minimal agar plates. Two second-site revertants were isolated: Asp-240-->Ala plus Gly-268-->Val and Asp-240-->Ala plus Lys-319-->Gln. These revertants showed extremely poor accumulation of TMG, melibiose, and lactose, but showed significant "downhill" lactose entry into beta-galactosidase-containing cells with sugar concentrations of 2 and 5 mM. It is concluded that there is some important interaction between Asp-240 and Lys-319, possibly a salt bridge.
authors.affiliation: Department of Cellular and Molecular Physiology, Harvard Medical School, Boston, Massachusetts 02115.
authors.forename: J I
authors.initials: JI
authors.lastname: Lee
authors.forename: P P
authors.initials: PP
authors.lastname: Hwang
authors.forename: C
authors.initials: C
authors.lastname: Hansen
authors.forename: T H
authors.initials: TH
authors.lastname: Wilson
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D001665:Binding Sites; D002241:Carbohydrates / Q000494:pharmacology; D002462:Cell Membrane / Q000187:drug effects / Q000201:enzymology; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D029968:Escherichia coli Proteins; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007785:Lactose / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008961:Models, Structural; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins; D016297:Mutagenesis, Site-Directed; D009838:Oligodeoxyribonucleotides; D011487:Protein Conformation; D027981:Symporters
chemical_list: D002241:Carbohydrates; D029968:Escherichia coli Proteins; C050221:LacY protein, E coli; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D009838:Oligodeoxyribonucleotides; D027981:Symporters; C021499:lactose permease; D007785:Lactose