Opening and closing of the periplasmic gate in lactose permease |
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Authors: | Zhou Yonggang Guan Lan Freites J Alfredo Kaback H Ronald |
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Affiliation: | Departments of *Physiology and ;‡Microbiology, Immunology, and Molecular Genetics, ;§Molecular Biology Institute, University of California, Los Angeles, CA 90095-1662; and ;†Department of Physiology and Biophysics, University of California, Irvine, CA 92697-4560 |
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Abstract: | X-ray crystal structures of lactose permease (LacY) reveal pseudosymmetrically arranged N- and C-terminal six-transmembrane helix bundles surrounding a deep internal cavity open on the cytoplasmic side and completely closed on the periplasmic side. The residues essential for sugar recognition and H(+) translocation are located at the apex of the cavity and are inaccessible from the outside. On the periplasmic side, helices I/II and VII from the N- and C- six helix bundles, respectively, participate in sealing the cavity from the outside. Three paired double-Cys mutants-Ile-40 --> Cys/Asn-245 --> Cys, Thr-45 --> Cys/Asn-245 --> Cys, and Ile-32 --> Cys/Asn-245 --> Cys-located in the interface between helices I/II and VII on the periplasmic side of LacY were constructed. After cross-linking with homobifunctional reagents less than approximately 15 A in length, all three mutants lose the ability to catalyze lactose transport. Strikingly, however, full or partial activity is observed when cross-linking is mediated by flexible reagents greater than approximately 15 A in length. The results provide direct support for the argument that transport via LacY involves opening and closing of a large periplasmic cavity. |
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Keywords: | cross-linking membrane proteins protein dynamics transport structure/function |
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