Use of Hexose Photolabels to Reveal the Structure and Function of Glucose Transporters

Abstract

Photoaffinity labelling probes which are hexose and bis-hexose derivatives have been expensively used to study the structure and physiological functions of the glucose transporters of the GLUT family. Substrate binding site locations have been probed using these derivatives. The glucose transport proteins are physiologically important in maintaining blood glucose levels, and they have a tissue-specific distribution that allows them to remove glucose from the bloodstream in a controlled manner. In heart, skeletal muscle and adipose tissue, the GLUT4 transporter is present both at the cell surface and in an intracellular membrane vesicle reservoir compartment. Insulin action leads to a translocation of this protein between these locations. Only the cell surface transporters are labelled by the impermeable photoaffinity probes, and these have therefore been used to determine the extent of response to insulin and to additional stimuli, including those that are due to changes in the cellular energy status that occurs during exercise. These responses can be monitored in human subjects with pathophysiological changes in glucose homeostasis as occurs in obesity and type 2 diabetes. It is also possible to carry out pulse-chase kinetic experiments in which the tagged GLUT4 movement between cellular compartments is followed over time. These studies have revealed that insulin action mainly leads to an acceleration of the exocytotic limb of the GLUT4 recycling pathway. For the use of photoprobes in kinetic experiments on intact cells, it was necessary to develop biotinylated photoaffinity probes that had very long spacer arms of approximately 20 Å.