Separation and partial characterization of three distinct intracellular GLUT4 compartments in rat adipocytes. Subcellular fractionation without homogenization

Insulin recruits GLUT4 from an intracellular location to the plasma membrane in rat adipocytes. The process involves multiple intracellular compartments and multiple protein functions, details of which are largely unknown partly due to our inability to separate individual GLUT4 compartments. Here, by hypotonic lysis, differential centrifugation, and glycerol density gradient sedimentation, we separated intracellular GLUT4 compartments in rat adipocytes into three fractions: plasma membrane-containing fraction T and plasma membrane-free fractions H and L. The GLUT4 contents in fractions T, H, and L were approximately 25, 56, and 18% of total GLUT4, respectively, in basal adipocytes and 55, 42, and 3-4% in insulin-stimulated adipocytes. The plasma membrane GLUT4 contents estimated separately further revealed that intracellular GLUT4 in fraction T amounts to approximately 20% in both basal and insulin-stimulated adipocytes. Organelle-specific marker and membrane traffic-related protein distribution data suggested that intracellular GLUT4 in fraction T represents sorting endosomes, whereas GLUT4 in fractions H and L represents storage endosomes and exocytic vesicles, respectively. The subcellular fractionation without homogenization described here should be useful in identifying the role of the individual GLUT4 compartments and the associated proteins in insulin-induced GLUT4 recruitment in rat adipocytes.