Abstract
Although Glut4 traffic is routinely described as translocation from an "intracellular storage pool" to the plasma membrane, it has been long realized that Glut4 travels through at least two functionally distinct intracellular membrane compartments on the way to and from the cell surface. Biochemical separation and systematic studies of the individual Glut4-containing compartments have been limited by the lack of appropriate reagents. We have prepared a monoclonal antibody against a novel component protein of Glut4 vesicles and have identified this protein as cellugyrin, a ubiquitously expressed homologue of a major synaptic vesicle protein, synaptogyrin. By means of sucrose gradient centrifugation, immunoadsorption, and confocal microscopy, we have shown that virtually all cellugyrin is co-localized with Glut4 in the same vesicles. However, unlike Glut4, cellugyrin is not re-distributed to the plasma membrane in response to insulin stimulation, and at least 40-50% of the total population of Glut4 vesicles do not contain this protein. We suggest that cellugyrin represents a specific marker of a functionally distinct population of Glut4 vesicles that permanently maintains its intracellular localization and is not recruited to the plasma membrane by insulin.
Highlights
The nature of these multiple compartments is not completely understood
It turned out that cellugyrin-positive vesicles are enriched in SCAMPs and VAMP2 but do not have VAMP3. In agreement with these data, VAMP3 is translocated to the cell surface in response to insulin to a greater extent than SCAMPs and VAMP2 (Fig. 9; see Refs. 33 and 20). These results are consistent with the earlier observation that VAMP2 and VAMP3 are localized in different vesicular populations (33) and may or may not indicate that in rat adipocytes, VAMP3 rather than VAMP2 plays the role of the v-SNARE in the process of Glut4 vesicle fusion with the plasma membrane
We have developed a monoclonal antibody against a previously unknown protein in Glut4 vesicles and have identified this protein as cellugyrin, a homologue of a major synaptic vesicle protein, synaptogyrin
Summary
The nature of these multiple compartments is not completely understood. One of them, has been identified as early/ sorting endosomes (7, 11, 12), and another may represent specialized post-endosomal insulin-responsive vesicles, or IRV (10 –14). Unlike Glut4, cellugyrin is not re-distributed to the plasma membrane in response to insulin stimulation, and at least 40 –50% of the total population of Glut4 vesicles do not contain this protein.
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