On the other hand, several important SNARE accessory proteins such as Munc18c, Synip, and NSF ( N-ethylmaleimide sensitive factor) may be involved in the control of Glut4 docking and fusion events and might be targets of insulin action ( 19– 22). Although these SNARE interactions are essential, none of the core proteins appear to be direct targets of insulin action. During Glut4 vesicle docking, VAMP2 interacts directly with its t-SNARE counterpart syntaxin 4 in the plasma membrane ( 3). Thus, it is likely that molecular motors move the Glut4 vesicles along tracks involving the microtubule and actin cytoskeletons, which may undergo dynamic remodeling in response to insulin.Īs mentioned above, Glut4-containing vesicles contain the v-SNARE protein VAMP2. In addition, microtubule motor proteins kinesin KIF5b and KIF3 have been shown to facilitate insulin-stimulated Glut4 transit to the plasma membrane ( 17, 18). Cortical actin is required for Glut4 translocation to the plasma membrane in response to insulin ( 13– 15), which is regulated by TC10 (see Insulin Signaling From Lipid Rafts, below) ( 14, 16).
The microtubule network and actin cytoskeleton play a role in Glut4 trafficking, either by linking signaling components or by directing movement of vesicles from the perinuclear region to the plasma membrane in response to insulin. Consistent with these data, ablation of transferrin receptor containing endosomes does not impair insulin-stimulated Glut4 translocation ( 12). The Glut4 compartment is enriched in the v-SNARE (soluble N-ethylmaleimide sensitive factor attachment protein) protein VAMP2 (vesicle-associated membrane protein 2) but not the related VAMP3/cellubrevin isoform that is present in recycling endosome ( 11). In adipocytes, these vesicles are retained in a perinuclear region in the cell via an unknown mechanism that might involve a tethering protein ( 9) or continuous futile recycling ( 10). Following internalization, Glut4 is localized into tubulovesicular and vesicular structures that are biochemically distinct from but possibly interacting with the recycling endosomal network ( 8).
There is substantial evidence that Glut4 exists in specialized vesicles sequestered within the cell, but the precise intracellular location and trafficking pathways of these vesicles are unclear.