Transport function and subcellular distribution of purified human erythrocyte glucose transporter reconstituted into rat adipocytes

A synthetic peptide corresponding to the GLUT4 C-terminal cytoplasmic domain causes insulin-like glucose transport stimulation and GLUT4 recruitment in rat adipocytes

In rat epididymal adipocytes, practically all of the major glucose transporter isoform GLUT4 is constitutively sequestered in intracellular membranes and moves to the plasma membrane in response to insulin, whereas about half of GLUT1, the minor isoform, is constitutively functional at the plasma membrane and thus less affected by insulin. Transfection studies using cells whose glucose transport is normally not regulated by insulin have suggested that the C-terminal cytoplasmic domain of GLUT4 is responsible for its constitutive intracellular sequestration. To test if this was also the case in a classical insulin target cell, we introduced synthetic peptides corresponding to the C-terminal cytoplasmic domain of GLUT4 and GLUT1 (GLUT4C and GLUT1C, respectively) into rat adipocytes and studied their effects on the glucose transport activity and the steady state GLUT4 and GLUT1 distribution between the plasma membrane and intracellular membranes in host cells. GLUT4C introduced into basal adipocytes caused a large (up to 4.5-fold) and dose-dependent increase in the plasma membrane GLUT4, with a proportional reduction in microsomal GLUT4, without affecting GLUT1 distribution. GLUT4C incorporation also caused a large (up to 3-fold) dose-dependent stimulation of 3-O-methyl D-glucose (3OMG) flux in host cells. GLUT4C caused little if any GLUT4 or GLUT1 redistribution and changes in 3OMG flux in insulin-stimulated adipocytes. GLUT1C, on the other hand, did not affect GLUT1 or GLUT4 targeting and 3OMG flux in host cells. These findings not only underscore the importance of the C-terminal cytoplasmic domain of GLUT4 in its constitutive intracellular sequestration in a classical insulin target cell but also suggest the existence of a regulatory protein in adipocytes that interacts with GLUT4 at its cytoplasmic domain, thus participating in the constitutive intracellular sequestration of GLUT4.

Interaction of facilitative glucose transporter with glucokinase and its modulation by ADP and glucose-6-phosphate

Bacterial glucokinase (GK) binds to purified, human erythrocyte glucose transporter (GT) reconstituted in vesicles. The binding is largely abolished if GT is predigested with trypsin, indicating that GK binds to the cytoplasmic domain of GT. The binding is a saturable function of GK concentration showing two distinct affinities with apparent KD of 0.33 and 5.1 microM. The binding is stimulated by an increasing concentration of ADP with the 50% maximal effect at 5 mM. Glucose-6-phosphate (G6P) also stimulates the binding with a distinct optimum at 25 mM. The binding is stimulated only slightly by ATP. D-glucose has no affect on the binding. KCl enhances the binding with the maximal effect at physiological intracellular concentrations. The binding is sensitive to changes in pH with an optimum at pH 4. The binding causes no detectable functional change in GT. However, the enzymatic activity of GK measured at nanomolar concentrations of GK is significantly greater in the presence of GT vesicles than in its absence or in the presence of protein-free vesicles, indicating that GK interacts with GT at this low concentration range with an apparent KD of 10 mM. Although its physiological significance is not known, the GK-GT interaction in vitro described here suggests that these two proteins may also interact in the cell and regulate carbohydrate metabolism.