A Three-dimensional Model of the Human Facilitative Glucose Transporter Glut1

Abstract

The human facilitative transporter Glut1 is the major glucose transporter present in all human cells, has a central role in metabolism, and is an archetype of the superfamily of major protein facilitators. Here we describe a three-dimensional structure of Glut1 based on helical packing schemes proposed for lactose permease and Glut1 and predictions of secondary structure, and refined using energy minimization, molecular dynamics simulations, and quality and environmental scores. The Ramachandran scores and the stereochemical quality of the structure obtained were as good as those for the known structures of the KcsA K ؉ channel and aquaporin 1. We found two channels in Glut1. One of them traverses the structure completely, and is lined by many residues known to be solvent-accessible. Since it is delimited by the QLS motif and by several well conserved residues, it may serve as the substrate transport pathway. To validate our structure, we determined the distance between these channels and all the residues for which mutations are known. From the locations of sugar transporter signatures, motifs, and residues important to the transport function, we find that this Glut1 structure is consistent with mutagenesis and biochemical studies. It also accounts for functional deficits in seven pathogenic mutants.

Functional Studies of Threonine 310 Mutations in Glut1

“…The three-dimensional model for Glut1, derived from these experiments, had two channels in it (18), suggesting a relation between glucose and water transport based on a model that had two channels per Glut1 monomer. In the present study, the size and hydrophilicity of alanine and serine differ from isoleucine, yet all these mutants decrease glucose transport and increase water transport.…”

“…This hydrophilic amino acid is predicted to be near (5) or at (18) the exofacial end of helix 8, which, in turn, is predicted to be involved in the formation of the water-accessible glucose channel (17,18,35). The threonine hydrophilic side chain with its hydroxyl group may play an important role in forming hydrogen bonds with glucose (36).…”

“…The K m values for the mutants are significantly lower than those for the wild-type Glut1 under zero-trans influx and equilibrium exchange influx conditions (Table I). Hence, one presumes that these mutations do not disturb a main Glut1 glucose-binding site (perhaps in the QLS motif region) (18,47) but increase the affinity of glucose binding. On the other hand, the mutations significantly decrease V max (Table I), perhaps by decreasing the mobility of glucose along its channel.…”

“…Based on the suggested helical bundle arrangement of Lac permease, Zuniga et al recently proposed a threedimensional model for Glut1 that accounts for mutational and biochemical evidence (18).…”

Mutational Analysis of the Hexose Transporter of Plasmodium falciparum and Development of a Three-dimensional Model

“…Recently, a three-dimensional model of GLUT1 has been published describing a helical arrangement that is consistent with experimental observations as well as theoretical structural considerations (16). This model predicts two transport channels, both of them partially limited by the QLS motif in helix VII.…”