Will the original glucose transporter isoform please stand up!

Abstract

Monosaccharides enter cells by slow translipid bilayer diffusion by rapid, protein-mediated, cation-dependent cotransport and by rapid, protein-mediated equilibrative transport. This review addresses protein-mediated, equilibrative glucose transport catalyzed by GLUT1, the first equilibrative glucose transporter to be identified, purified, and cloned. GLUT1 is a polytopic, membrane-spanning protein that is one of 13 members of the human equilibrative glucose transport protein family. We review GLUT1 catalytic and ligand-binding properties and interpret these behaviors in the context of several putative mechanisms for protein-mediated transport. We conclude that no single model satisfactorily explains GLUT1 behavior. We then review GLUT1 topology, subunit architecture, and oligomeric structure and examine a new model for sugar transport that combines structural and kinetic analyses to satisfactorily reproduce GLUT1 behavior in human erythrocytes. We next review GLUT1 cell biology and the transcriptional and posttranscriptional regulation of GLUT1 expression in the context of development and in response to glucose perturbations and hypoxia in blood-tissue barriers. Emphasis is placed on transgenic GLUT1 overexpression and null mutant model systems, the latter serving as surrogates for the human GLUT1 deficiency syndrome. Finally, we review the role of GLUT1 in the absence or deficiency of a related isoform, GLUT3, toward establishing the physiological significance of coordination between these two isoforms.

Fig. 1.

Glucose transporter 1 (GLUT1) structure. A and B: GLUT1 structure as modeled in Ref. 10 and drawn using VMD (version 1.8.5). The 12 transmembrane helices are shown in 2 forms, as a surface representation (left) or as a cartoon representation (right). Extracellular and cytoplasmic structures are omitted. The cartoon representations include the transmembrane helix numbers. A: transmembrane α-helices (TMs) are shown parallel to the membrane. B: the TMs are viewed along the membrane normal from the cytoplasmic side. C: GLUT1 sequence and putative topology. Amino acids are shown using the 1 letter code. The 12 TMs are colored as in A and B. Some amino acids are numbered. Individual amino acids that are not colored white: cyan, sites of trypsin cleavage or lysine modification by N-hydroxy-succinimide esters (11); red, amino acids, which when mutagenized to cysteine are reactive with PCMBS in the external solvent (94); green, amino acids, which when mutagenized to cysteine are reactive with p-chloromercuribenzylsulfonic (PCMBS) in the external solvent in a substrate-protected manner (94); yellow, cysteine residues that are accessible to iodoacetamide (11); purple, amino acids, which when mutagenized to cysteine result in ≥90% inhibition of GLUT1 (94); orange, putative substrate-binding sites predicted by docking studies (27, 109); blue, amino acids implicated in substrate discrimination (87); black, sites at which mutations cause GLUT1 deficiency syndrome (68, 124). Some amino acids fall into multiple categories.

Fig. 2.

Model for GLUT1-mediated sugar transport. A: schematic representation of the catalytic center of the transporter. Extracellular sugar (G_o) and intracellular sugar (G_i) react with exo- and endofacial sites, respectively, to form G₂ and G₁, respectively. Sugar dissociates from these sites into the intersite cavity to form G_c. When G₂ and G₁ are occupied, dissociation to G_c and reassociation are accelerated (red dissociation steps; adapted from Ref. 99). B: simulation of biphasic exchange transport by this carrier mechanism (adapted from Ref. 74). C: The GLUT1 model (adapted from Fig. 1B, with TMs 2 and 11 removed for clarity) showing G_i complexed to the G₁ site, a putative G₂ site, the small intersite cavity (109), the TM9 oligomerization surface (Levine KB, DeZutter J, and Carruthers A, unpublished observations), the 2 conformationally dynamic TMs (1 and 8) that are released into the aqueous solvent when the GLUT1 backbone is cleaved by trypsin (11), the 3 exofacial lysine residues exposed by d-glucose (11), and the putative cytochalasin B-binding hairpin (11, 109).