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

Abstract

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.