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Updated: Jun 5, 2026

Introduction to Solid Supported Membrane Based Electrophysiology
Published on: May 11, 2013
Site-directed alkylation studies with LacY provide evidence for the alternating access model of transport
Xiaoxu Jiang1, Yiling Nie, H Ronald Kaback
1Department of Physiology, University of California, Los Angeles, California 90095, United States.
Abstract:
In total, 59 single Cys-replacement mutants in helix VII and helix X of the lactose permease of Escherichia coli were subjected to site-directed fluorescence labeling in right-side-out membrane vesicles to complete the testing of Cys accessibility or reactivity. For both helices, accessibility/reactivity is relatively low at the level of the sugar-binding site where the helices are tightly packed. However, labeling of Cys substitutions in helix VII with tetramethylrhodamine-5-maleimide decreases from the middle toward the cytoplasmic end and increases toward the periplasmic end. Helix X is labeled mainly on the side facing the central hydrophilic cavity with relatively small or no changes in the presence of ligand. In contrast, sugar binding causes a significant increase in accessibility/reactivity at the periplasmic end of helix VII. When considered with similar findings from N-ethylmaleimide alkylation studies, the results confirm and extend support for the alternating access model.
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