The first luminal loop confers insulin responsiveness to the glucose transporter 4
Glucose transporter isoform 4, or GLUT4, is expressed in insulin-sensitive tissues and is responsible for postprandial blood glucose clearance. The mechanistic dissection of GLUT4 regulation is crucial for our understanding of the molecular nature of insulin resistance and diabetes mellitus. Unlike most other glucose transporters, GLUT 4 is compartmentalized inside the cell primarily in small insulin-responsive-vesicles, or IRVs, that are translocated to the cell surface upon insulin stimulation. From the plasma membrane, GLUT4 is internalized and is rapidly delivered to a sub-domain of either the trans-Golgi network (TGN) or recycling endosomes that represent the donor compartment for the formation of the IRVs. Such a complicated pattern of intracellular trafficking is apparently defined by unique signals in the GLUT4 molecule. Previous studies have suggested that sequences in the cytoplasmic tails of GLUT4 are important for its faithful intracellular localization. However, the definitive nature of signals that target GLUT4 specifically to the IRVs is still not clear. In this study, the targeting role of the first luminal loop of GLUT4 was explored by exchanging the first luminal loop of GLUT4 with the corresponding region of cellugyrin, a 4-transmembrane protein that is absent from the IRVs. Using fluorescence microscopy and biochemical fractionation, it was determined that the first luminal loop of GLUT4 is sufficient to confer insulin responsiveness to cellugyrin. Mechanistically, the first luminal loop of GLUT4 targets the reporter protein to the IRVs by interacting with the sorting receptor sortilin. A model is proposed in which targeting of GLUT4 into the IRVs requires at least two distinct steps: targeting into the donor compartment (TGN and/or recycling endosomes) provided by sequences in the cytoplasmic tail and targeting from the donor compartment into the IRVs that depends on the first luminal loop of the transporter.
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