page 481). The vesicles occasionally touch the plasma membrane and, when stimulated by insulin, quickly fuse with it.
GLUT4 is sequestered in vesicles in resting cells. Insulin exposure induces the vesicles to fuse to the plasma membrane. But how these vesicles are stored in the resting cell was unclear.
To find out, the team used total internal reflection fluorescence (TIRF) microscopy on freshly isolated adipose cells transfected with GLUT4-GFP. Whereas confocal microscopy illuminated vesicles scattered throughout the cytoplasm, TIRF movies showed the vesicles just under the plasma membrane. These sub-membrane vesicles were moving on microtubules in unstimulated cells. Within ten minutes of insulin exposure, 50% of the GLUT4 had reached the surface—and most of the vesicles on the microtubule network were clustered at sites on the membrane.
The authors aim to find out what the vesicles are doing as they move around the resting cell, sampling the plasma membrane. They speculate that the moving vesicles might allow the cell to rapidly respond to a narrow area of insulin exposure.