The C-terminus of GLUT4 targets the transporter to the perinuclear compartment but not to the insulin-responsive vesicles

Abstract

Postprandial blood glucose clearance is mediated by GLUT4 (glucose transporter 4) which is translocated from an intracellular storage pool to the plasma membrane in response to insulin. The nature of the intracellular storage pool of GLUT4 is not well understood. Immunofluorescence staining shows that, under basal conditions, the major population of GLUT4 resides in the perinuclear compartment. At the same time, biochemical fractionation reveals that GLUT4 is localized in IRVs (insulin-responsive vesicles). The relationship between the perinuclear GLUT4 compartment and the IRVs is not known. In the present study, we have exchanged the C-termini of GLUT4 and cellugyrin, another vesicular protein that is not localized in the IRVs and has no insulin response. We have found that GLUT4 with the cellugyrin C-terminus loses its specific perinuclear localization, whereas cellugyrin with the GLUT4 C-terminus acquires perinuclear localization and becomes co-localized with GLUT4. This, however, is not sufficient for the effective entry of the latter chimaera into the IRVs as only a small fraction of cellugyrin with the GLUT4 C-terminus is targeted to the IRVs and is translocated to the plasma membrane in response to insulin stimulation. We suggest that the perinuclear GLUT4 storage compartment comprises the IRVs and the donor membranes from which the IRVs originate. The C-terminus of GLUT4 is required for protein targeting to the perinuclear donor membranes, but not to the IRVs.

Molecular Basis of Insulin‐Responsive GLUT4 Trafficking Systems Revealed by Single Molecule Imaging

“…the stationary storage compartment. In this regard, since multiple studies have demonstrated that both the N‐ and the C‐terminus of GLUT4 play important roles in its intracellular trafficking routes and destinations, with the C‐terminus of GLUT4 being especially important for its perinuclear TGN accumulation (16,57–59), further studies are needed to clarify the interrelationships among these GLUT4 targeting sequences and the sortilin‐mediated retrograde trafficking processes revealed by this investigation. In addition, substantial amounts of sortilin are detected in GLUT4‐containing vesicles obtained by subcellular fractionation experiments (9,12,13).…”

The Sugar Is sIRVed: Sorting Glut4 and Its Fellow Travelers

“…Considering that on the way to and from the plasma membrane, Glut4 passes through several intracellular compartments, interfering with the targeting sequences within the cytoplasmic termini of the transporter may inhibit not only entry into the IRVs, but upstream trafficking steps as well. Indeed, in the ‘gain of function’ experiments, it was recently demonstrated that the C‐terminus of Glut4 targets the chimera reporter protein to the perinuclear region of the cell, that is, the IRV precursor compartment, either recycling endosomes and/or TGN, but is not sufficient for its entry into the IRVs per se (51).…”

“…By the same token, the cytoplasmic tail of IRAP targets this protein primarily to the IRV donor membranes but not to the IRVs (52). Still, the cytoplasmic tail of IRAP (53) as well as the C‐terminus of Glut4 (51) can confer some degree of insulin responsiveness to the reporter molecules, which, however, is less than the insulin responsiveness of wild‐type Glut4 and IRAP (51,52,54). One explanation of this phenomenon is that once a protein enters the IRV donor compartment(s), it can be captured by budding vesicles by, for example, mass action (10).…”

Biogenesis and regulation of insulin-responsive vesicles containing GLUT4

“…GLUT4, IRAP and sortilin are targeted to the perinuclear donor membranes by their cytoplasmic domains [14,15,19,20]. Upon arriving in this compartment, these proteins can redistribute to the IRVs by mass-action [21].…”