Implications of Aberrant Temperature-Sensitive Glucose Transport Via the Glucose Transporter Deficiency Mutant (GLUT1DS) T295M for the Alternate-Access and Fixed-Site Transport Models

In silico glucose docking to the transporter GLUT1 templated to the crystal structure of Escherichia coli XylE, a bacterial homolog of GLUT1-4 (4GBZ.pdb), reveals multiple docking sites. One site in the external vestibule in the exofacial linker between TM7 and -8 is adjacent to a missense T295M and a 4-mer insertion mutation. Glucose docking to the adjacent site is occluded in these mutants. These mutants cause an atypical form of glucose transport deficiency syndrome (GLUT1DS), where transport into the brain is deficient, although unusually transport into erythrocytes at 4 A degrees C appears normal. A model in which glucose traverses the transporter via a network of saturable fixed sites simulates the temperature sensitivity of normal and mutant glucose influx and the mutation-dependent alterations of influx and efflux asymmetry when expressed in Xenopus oocytes at 37 A degrees C. The explanation for the temperature sensitivity is that at 4 A degrees C glucose influx between the external and internal vestibules is slow and causes glucose to accumulate in the external vestibule. This retards net glucose uptake from the external solution via two parallel sites into the external vestibule, consequently masking any transport defect at either one of these sites. At 37 A degrees C glucose transit between the external and internal vestibules is rapid, with no significant glucose buildup in the external vestibule, and thereby unmasks any transport defect at one of the parallel input sites. Monitoring glucose transport in patients' erythrocytes at higher temperatures may improve the diagnostic accuracy of the functional test of GLUT1DS.