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Chemo-enzymatic Synthesis of N-glycans for Array Development and HIV Antibody Profiling

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Structure of sugar-bound LacY.

Hemant Kumar¹, Vladimir Kasho, Irina Smirnova

¹Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94158.

Proceedings of the National Academy of Sciences of the United States of America

|January 24, 2014

Summary

The lactose permease (LacY) crystal structure reveals a near-occluded state, not open-outward as expected. This structure clarifies how LacY binds D-galactopyranosides and transitions between membrane-facing states.

Keywords:

X-ray structure conformational change induced fit membrane protein transport

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Area of Science:

Structural Biology
Membrane Transport Proteins
Biochemistry

Background:

Lactose permease (LacY) from Escherichia coli is a model secondary active transporter.
Understanding LacY's conformational changes is crucial for elucidating sugar transport mechanisms.
Previous studies suggested open-outward conformations, but direct structural evidence was limited.

Purpose of the Study:

To determine the X-ray crystal structure of a double-Trp mutant of E. coli lactose permease (LacY).
To visualize the binding of a high-affinity lactose analog within the permease.
To elucidate the conformational state and substrate binding mechanism of LacY.

Main Methods:

X-ray crystallography of a double-Trp mutant of E. coli lactose permease (LacY).
Co-crystallization with a high-affinity lactose analog.
Analysis of electron density maps to determine atomic structure and ligand interactions.

Main Results:

The crystal structure revealed a near-occluded conformation, partially open to the periplasmic side but sealed on the cytoplasmic side.
A bound lactose analog was observed at the apex of the binding cavity, ligated by specific side chains.
The periplasmic opening was too narrow for direct sugar passage, suggesting a gating mechanism.

Conclusions:

Protonated LacY specifically binds D-galactopyranosides.
Substrate binding induces an occluded conformation, which is a prerequisite for transport.
LacY likely transitions between states that open to either side of the membrane to facilitate transport.