A Loose Relationship: Incomplete H + /Sugar Coupling in the MFS Sugar Transporter GlcP.

The glucose transporter from Staphylococcus epidermidis, GlcP _Se , is a homolog of the human GLUT sugar transporters of the major facilitator superfamily. Together with the xylose transporter from Escherichia coli, XylE _Ec , the other prominent prokaryotic GLUT homolog, GlcP _Se , is equipped with a conserved proton-binding site arguing for an electrogenic transport mode. However, the electrophysiological analysis of GlcP _Se presented here reveals important differences between the two GLUT homologs. GlcP _Se , unlike XylE _Ec , does not perform steady-state electrogenic transport at symmetrical pH conditions. Furthermore, when a pH gradient is applied, partially uncoupled transport modes can be generated. In contrast to other bacterial sugar transporters analyzed so far, in GlcP _Se sugar binding, translocation and release are also accomplished by the deprotonated transporter. Based on these experimental results, we conclude that coupling of sugar and H ⁺ transport is incomplete in GlcP _Se . To verify the viability of the observed partially coupled GlcP _Se transport modes, we propose a universal eight-state kinetic model in which any degree of coupling is realized and H ⁺ /sugar symport represents only a specific instance. Furthermore, using sequence comparison with strictly coupled XylE _Ec and similar sugar transporters, we identify an additional charged residue that may be essential for effective H ⁺ /sugar symport.
(Copyright © 2017 Biophysical Society. Published by Elsevier Inc. All rights reserved.)