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Lessons from lactose permease.

Lan Guan1, H Ronald Kaback

  • 1Department of Physiology, University of California, Los Angeles, California 90095-1662, USA. LanGuan@mednet.ucla.edu

Annual Review of Biophysics and Biomolecular Structure
|May 13, 2006
PubMed
Summary

Researchers solved the X-ray structure of lactose permease (LacY) from Escherichia coli, revealing its inward-facing conformation. This structure explains how LacY facilitates bidirectional sugar and proton transport using the same binding sites.

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Area of Science:

  • Structural Biology
  • Membrane Transport
  • Biochemistry

Background:

  • Lactose permease (LacY) from Escherichia coli is a crucial transporter for lactose uptake.
  • Understanding LacY's mechanism is key to deciphering secondary active transport.

Purpose of the Study:

  • To determine the high-resolution X-ray structure of LacY in an inward-facing conformation.
  • To elucidate the structural basis for LacY's bidirectional galactoside/H+ symport mechanism.

Main Methods:

  • X-ray crystallography
  • Protein structure determination

Main Results:

  • The X-ray structure of LacY in an inward-facing state was solved, revealing pseudosymmetrical N- and C-terminal domains.
  • Ligand binding occurs at the apex of a central hydrophilic cavity.
  • Key residues for substrate and proton binding are positioned for access to internal or external cavities in both conformations.

Conclusions:

  • The solved structure provides insights into the alternating access mechanism of LacY.
  • Structural features explain the bidirectional transport of galactosides and protons via the same residues.
  • A working model for LacY's symport mechanism involving alternating access is proposed.

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