title: Conformational changes represent the rate-limiting step in the transport cycle of maize sucrose transporter1.
abstract: Proton-driven Suc transporters allow phloem cells of higher plants to accumulate Suc to more than 1 M, which is up to ~1000-fold higher than in the surrounding extracellular space. The carrier protein can accomplish this task only because proton and Suc transport are tightly coupled. This study provides insights into this coupling by resolving the first step in the transport cycle of the Suc transporter SUT1 from maize (Zea mays). Voltage clamp fluorometry measurements combining electrophysiological techniques with fluorescence-based methods enable the visualization of conformational changes of SUT1 expressed in Xenopus laevis oocytes. Using the Suc derivate sucralose, binding of which hinders conformational changes of SUT1, the association of protons to the carrier could be dissected from transport-associated movements of the protein. These combined approaches enabled us to resolve the binding of protons to the carrier and its interrelationship with the alternating movement of the protein. The data indicate that the rate-limiting step of the reaction cycle is determined by the accessibility of the proton binding site. This, in turn, is determined by the conformational change of the SUT1 protein, alternately exposing the binding pockets to the inward and to the outward face of the membrane.
authors: Julius-von-Sachs-Institute, Molecular Plant Physiology and Biophysics, University Würzburg, D-97082 Wuerzburg, Germany.
authors: Carmen
authors: C
authors: Derrer
authors: Anke
authors: A
authors: Wittek
authors: Ernst
authors: E
authors: Bamberg
authors: Armando
authors: A
authors: Carpaneto
authors: Ingo
authors: I
authors: Dreyer
authors: Dietmar
authors: D
authors: Geiger
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D005453:Fluorescence; D005470:Fluorometry; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating / Q000187:drug effects; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000378:metabolism*; D008954:Models, Biological; D008969:Molecular Sequence Data; D050505:Mutant Proteins / Q000737:chemistry / Q000378:metabolism; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism*; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D013395:Sucrose / Q000031:analogs & derivatives / Q000494:pharmacology; D014982:Xenopus laevis; D003313:Zea mays / Q000187:drug effects / Q000378:metabolism*
chemical_list: D026901:Membrane Transport Proteins; D050505:Mutant Proteins; D010940:Plant Proteins; C078729:sucrose transport protein, plant; D013395:Sucrose; C026285:trichlorosucrose
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