Structural comparative analysis of GLUT1 and GLUT3 reveal role of SP motif in transport regulation

The human glucose transporters GLUT1 and GLUT3 are canonical members of the Sugar Porter (SP) family and responsible for uptake of glucose in most human tissues. GLUT1 and GLUT3 share a fully conserved substrate-binding site with identical substrate coordination, but still differ significantly in transport affinity in accordance with their physiological function. Here we present a 2.4 crystal structure of GLUT1 and make a direct comparison between GLUT1 and GLUT3 using both structural and functional data. Our work shows that interactions between a cytosolic Sugar Porter and a conserved A motif stabilize the outward conformational state and increases substrate affinity, while a previously undescribed Cl- ion site and endofacial lipid/glucose binding site impede this interaction to modulate GLUT kinetics. The results provide an explanation for the difference between GLUT1 and GLUT3 affinity, imply a general model for kinetic regulation in GLUTs and suggest a physiological function of the defining SP sequence in the Sugar Porter family.