Arginine 302 (helix IX) in the lactose permease of Escherichia coli is in close proximity to glutamate 269 (helix VIII) as well as glutamate 325 (helix X)

M. M. He, John C Voss, W. L. Hubbell, H. R. Kaback

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By using a variety of biochemical and biophysical approaches, a helix packing model for the lactose permease of Escherichia coli has been proposed in which the four residues that are irreplaceable with respect to coupling are paired-Glu269 (helix VIII) with His322 (helix X) and Arg302 (helix XI) with Glu325 (helix X). In addition, the substrate translocation pathway is located at the interface between helices V and VIII, which is in close vicinity to the four essential residues. Based on this structural information and functional studies of mutants in the four irreplaceable residues, a molecular mechanism for energy coupling in the permease has been proposed [Kaback, H. R. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 5539]. The principle idea of this model is that Arg302 interacts with either Glu325 or Glu269 during turnover. Evidence that Arg302 is in close proximity with Glu325 has been presented [Jung, K., Jung, H., Wu, J., Prive, G. G., and Kaback, H. R. (1993) Biochemistry 32, 12273; He, M. M., Voss, J., Hubbell, W. L., and Kaback, H. R. (1995) Biochemistry 34, 15667]; however, the proximity of Arg302 to Glu269 has not been examined. In this report, it is shown by two methods that Arg302 is also close to Glu269: (i) permease with Glu269→His, Arg302→His, and His322→Phe binds Mn2+ with high affinity at pH 7.5, but not at pH 5.5; and (ii) site-directed spin-labeling of the double Cys mutant Glu269→Cys/Arg302→Cys exhibits spin-spin interaction with an interspin distance of about 14-16 Å. In addition, the spin-spin interaction is stronger and interspin distance shorter after the permease is reconstituted into proteoliposomes. Taken as a whole, the data are consistent with the idea that Arg302 may interact with either Glu325 or Glu269 during turnover.

Original languageEnglish (US)
Pages (from-to)13682-13687
Number of pages6
Issue number44
StatePublished - 1997


ASJC Scopus subject areas

  • Biochemistry

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