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.