Figure 1. GLUT4 sequestration and exocytosis.

The schematic diagram represents an extension of the 3-compartment sequestration model [42] and the 4-compartment dynamic-retention model [54,55]). The compartments considered in modelling of kinetics of GLUT4 traffic are the plasma membrane (PM), a sequestered GLUT4 vesicle (GSV) compartment, sorting endosomes (SE) and perinuclear compartments (PNC) and an endosome recycling compartment (ERC). The GSV reservoir is supplied by vesicles from SE and PNC, but the proportion of vesicles supplied by these compartments will vary in different cell types. In basal 3T3-L1 cells ∼80% of the GLUT4 is present in GSV, while 20% of GLUT4 is recycled from SE through ERC to the PM [54]. We suggest here that a retention-catalyst C is responsible for the partitioning and saturation of the sequestered GLUT4 compartment. It is proposed that C catalyses formation of GSV. In addition, C is released from the GSV compartment when insulin signalling leads to vesicle fusion with the PM. The rate constants linking these compartments are: kseq for movements at the saturable step and requiring C; kf for fusion of docked vesicles with the plasma membrane — and associated with desaturation of the GSV compartment and release or recycling of C; krd for reversal of docking without fusion — this step involves vesicles that sample docking sites but do not fuse; krc for recycling of GLUT4 from SE through ERC and to the PM.