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.

Ouabain-sensitive (OS) and -resistant (OR) Rb⁺ influx was examined in three sublines of U937 cells to compare alterations of K⁺ channel permeability and the Na⁺,K⁺-ATPase pump leading to the shift in ion and water balance during apoptosis induced by 0.2 and 1µM staurosporine (STS) for 4-5 h. Cell K⁺, Rb⁺, Na⁺ and Cl^- content was determined by flame photometry and ³⁶Cl distribution. Changes in cell water content were monitored by measurement of buoyant cell density and distribution of [³H]-glycerol or 3-O-methyl-D-[³H]glucose. Apoptosis was detected by DNA flow cytometry and light microscopy of the native cells stained with acridine orange. Treatment with 0.2 µM STS for 5 hours led to mild apoptosis with 10-13 % cell dehydration and either moderate increase of channel mediated Rb⁺ influx without significant changes in the pump activity or moderate decrease of pump Rb⁺ influx without significant change of channel influx, depending on the cell line used. Treatment with 1 µM STS was followed by 18-23 % cell dehydration, a decrease of the pump activity and a small or insignificant increase in the OR Rb⁺ influx in all studied sublines. It is concluded that moderate apoptotic cell shrinkage may be associated with both an increase in K⁺ channel permeability and inhibition of the pump whereas more remarkable shrinkage occurs presumably due to inhibition of the pump.