1. Plant Sucrose Transporters from a Biophysical Point of View
Geiger Dietmar  
Molecular Plant 
Volume 4, Issue 3, Pages (May 2011)
DOI: /mp/ssr029
Copyright © 2011 The Author. All rights reserved. Terms and Conditions

2. Figure 1
Cytosolic Sucrose Feedback on Magnitude and Direction of ZmSUT1 Currents.
ZmSUT1 currents recorded in inside-out giant patches in the presence of 5 mM external sucrose. Schematic representations above the graph depict the proton and sucrose concentration; cytosolic and external pH was 7.5 and 5.6, respectively; cytosolic sucrose concentrations were elevated from 0 to 50, 100, 200, and 500 mM as indicated. The membrane potential was clamped to 0 mV. Cytosolic sucrose concentrations of 500 mM reversed the proton currents. Modified from Carpaneto et al. (2005).
Molecular Plant 2011 4, DOI: ( /mp/ssr029)
Copyright © 2011 The Author. All rights reserved. Terms and Conditions

3. Figure 2
Long-Distance Transport of Sucrose from Source to Sink in an Apoplastic Loader and Unloader.
The source site of the SE/CC complex is characterized by an outward-directed sucrose and inward-directed H+ gradient. The membrane potential is hyperpolarized due to the activity of the H+-ATPases localized in the companion cells. Under these conditions, sucrose is accumulated in the phloem cells by H+/sucrose symporters, like ZmSUT1. In the sink phloem, the apoplastic concentrations of sucrose are reduced by the cleavage of sucrose due to the activity of cell wall-bound invertases. In this region, the membrane potential mainly depends on the potassium conductance because of the reduced size (or even absence) of the energy-supplying companion cells. Thus, pmf is decreased. This regime directs ZmSUT1 into the inverse transport mode and sucrose is released. CC, companion cell; SE, sieve element; MC, mesophyll cell; SC, sink cell; Fruc, fructose; Gluc, glucose; Suc, sucrose.
Molecular Plant 2011 4, DOI: ( /mp/ssr029)
Copyright © 2011 The Author. All rights reserved. Terms and Conditions

4. Figure 3 Sucrose Dependence of ZmSUT1 Pre-Steady-State Currents.
(A) Pre-steady-state currents of a single oocyte at varying external sucrose concentrations are shown. These transient currents decreased with increasing sucrose concentrations.
(B) Isat – IX against the applied membrane potential. Isat is the current at saturating external sucrose concentration; IX are the currents in the presence of 0, 0.5, and 1 mM external sucrose. Empty and filled symbols refer to the measured and predicted currents, respectively. The following relationship was used to evaluate the predicted currents: Isat – IX= QX/τ, where X represents the external sucrose concentration and τ is the slow time constant at zero external sucrose. Measuring the pre-steady-state currents in the absence of sucrose was sufficient to predict the transport-associated currents in the presence of various sucrose concentrations. From Carpaneto et al. (2010).
Molecular Plant 2011 4, DOI: ( /mp/ssr029)
Copyright © 2011 The Author. All rights reserved. Terms and Conditions

5. Figure 4
Schematic Illustration of the Proposed Six-State Reaction Cycle of ZmSUT1.
States 1–3 and 4–6 face the external and internal membrane surfaces, respectively. The experimental evidences from pre-steady-state measurements with ZmSUT1 suggest that one proton binds to the transporter on the extracellular surface to form state 2 before sucrose binds to form the fully loaded transporter (state 3). The fully loaded transporter (state 3) undergoes a conformational change (states 3–4) resulting in H+/sucrose symport (transport-associated currents). In states 4 and 5, the proton and the sucrose molecule dissociate and the empty transporter re-orientates in the membrane to complete the transport cycle. The order of H+ and sucrose dissociation for ZmSUT1 is not experimentally determined. Zhou et al. (1997), however, proposed an ordered release with the sugar dissociating before the proton at the internal side of AtSUC1. In the ZmSUT1 model, the pre-steady-state currents (and, in turn, Cm changes) are produced by the back-and-forth movement of protons from an external side to a proton binding site of the empty transporter between states 1 and 2. The intramembrane charge movement (Ipre) is due to the in-and-out movement of protons that is strictly associated with the conformational change between states 6 and 1—the accessibility of the proton binding site. This conformational change is very likely the rate-limiting step of the overall reaction cycle and the pre-steady-state transients. Thus, the knowledge of the pre-steady-state currents in the absence of the substrate is sufficient to predict the sucrose-induced transport currents.
Molecular Plant 2011 4, DOI: ( /mp/ssr029)
Copyright © 2011 The Author. All rights reserved. Terms and Conditions