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A more recent version of this article appeared on November 19, 2004
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M407408200v1
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Papers In Press, published online ahead of print September 13, 2004
J. Biol. Chem, 10.1074/jbc.M407408200
Submitted on July 2, 2004
Revised on August 25, 2004
Accepted on September 13, 2004

Sugar recognition by the lactose permease of Escherichia coli

Jose-Luis Vazquez-Ibar, Lan Guan, Adam B. Weinglass, Gill Verner, Ruth Gordillo, and H. Ronald Kaback

Physiology Dept., UCLA, Howard Hughes Medical Institute, Los Angeles, CA 90095-1662

Corresponding Author: RonaldK{at}HHMI.UCLA.edu

Biochemical, luminescence and mass spectroscopy approaches indicate that Trp151 (helix V) plays an important role in hydrophobic stacking with the galactopyranosyl ring of substrate and that Glu269 (helix VIII) is essential for substrate affinity and specificity. The x-ray structure of the lactose permease (LacY) with bound substrate is consistent with these conclusions and suggests that a possible H-bond between Glu269 and Trp151 may play a critical role in the architecture of the binding site. We have now probed this relationship by exploiting the intrinsic luminescence of single-Trp151 LacY with various replacements for Glu269. Mutations at position 269 dramatically alter the environment of Trp151 in a manner that correlates with binding affinity of LacY substrates. Furthermore, chemical modification of Trp151 with N-bromosuccinimide (NBS) indicates that Glu269 forms an H-bond with the indole N. It is concluded that: (1) an H-bond between the indole N and Glu269 optimizes the formation of the substrate binding site in the inward-facing conformation of LacY; (2) the disposition of the residues implicated in sugar binding in different conformers suggests that sugar binding by LacY involves induced fit.


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