" To elucidate the structure and molecular mechanism of GLUT1–4, we launched a systematic effort to screen for well-behaved GLUT1–4 protein variants as well as their homologues. Among the tested proteins, the E. coli D-xylose:H+ symporter XylE29,30,31,32,33 showed excellent solution behaviour. XylE, an archetypal member of the SP subfamily19,32, shares sequence identities of 29%, 30%, 29% and 27%, and similarities of 49%, 51%, 48% and 47%, with GLUT1, GLUT2, GLUT3 and GLUT4, respectively (Supplementary Fig. 1). Here we report the crystal structures of XylE bound separately to three different ligands: D-xylose, D-glucose and 6-bromo-6-deoxy-D-glucose (6-BrGlc). We also performed an extensive, structure-guided, in vivo and in vitrofunctional analysis of XylE. These studies reveal significant insights into the structure and function of XylE and its human homologues GLUT1–4."