Structural Determinants of the IF-OF Transition in Human Glucose Transporter GLUT1

Abstract

Glucose transporters (GLUTs) are membrane proteins that facilitate the diffusion of glucose across the plasma membrane. Recently, the crystal structures of human GLUTs (e.g., GLUT1 and GLUT3) have been resolved in the inward-facing (IF) and outward-facing (OF) states, enabling a detailed study of their transport mechanism.In this work, we aim to characterize the IF-OF transition of GLUT1, the major glucose transporter in the human body. Starting from the GLUT1 IF state, we performed an extensive search for an optimal reaction coordinate for the transition. Employing non-equilibrium simulations, we induced the reorientation of multiple combinations of transmembrane (TM) helices of GLUT1 to trigger the transition to the OF state for each TM combination. The relevance of the reaction coordinates was then assessed by the non-equilibrium work required for the transition, as well as the induced structural rearrangement of GLUT1. As a starting point for the transitions, an OF target model of GLUT1 was first generated based on available structures of its close homologue OF-GLUT3, and further refined once the optimal reaction coordinate was obtained. Based on the minimum non-equilibrium work required to drive the IF-OF transition, we identified a specific set of TM helices whose motion is required to reach the OF state. Using this reaction coordinate, we were able to obtain previously uncharacterized stable GLUT1 OF state, and characterize multiple intermediate states connecting the IF and OF states, allowing us to identify important interhelical interactions and the gating helices suggesting a local gating mechanism apart from global large scale transition involving a proline-rich TM segment that mediates the conformational transition in GLUT1. Given the structural similarity among GLUTs, we expect that the our detailed description of the GLUT1 transition will provide a basis to study other human GLUTs.