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J. Biol. Chem., Vol. 268, Issue 27, 20007-20015, Sep, 1993
JI Lee, PP Hwang and TH Wilson
It is believed that there are several charged amino acid residues in
membrane-spanning alpha-helices of the lactose carrier of Escherichia coli.
Evidence has previously been presented for two different salt bridges in
membrane-spanning regions of the lactose carrier. One of these involves an
interaction between Asp-237 and Lys-358; another involves interaction
between Asp-240 and Lys-319. Additional studies of Lys-319 suggest that it
may interact with Glu-269 as well as Asp-240. A cell containing the LacY
gene with the mutation Lys-319-->Asn failed to ferment melibiose and
after several days melibiose-positive mutants arose on indicator plates.
These revertants showed second site mutations which replaced Asp-240 by
neutral amino acids (Val or Gly). In addition, a second site mutation
showed Glu-269 changed to Asn. Cells containing the mutation
Lys-319-->Leu also failed to ferment melibiose and melibiose-positive
revertants showed Asp-240-->Ala and Asp-240-->Tyr as well as
Tyr-236-->Phe and His-322-->Arg. Second site revertants were also
sought from the mutant Glu-269-->Asn which grew poorly on melibiose
minimal plates. Melibiose-positive revertants included the double mutant
Gln-269/Asn-319. All of the Glu-269-->Asn mutants were extremely
defective in transport. It was concluded that Lys-319 interacts with
Glu-269 and Asp-240 probably as salt bridges.
Lysine 319 interacts with both glutamic acid 269 and aspartic acid 240 in the lactose carrier of Escherichia coli
Department of Cellular and Molecular Physiology, Harvard Medical School, Boston, Massachusetts 02115.
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