GLUT4

GLUT4 is the doorway that insulin opens to let sugar into muscle and fat cells. Most of the time these doorways are kept hidden inside the cell in storage pouches; when insulin signals, they are rushed to the cell surface so glucose can flood in. This recruitment is the main reason muscle and fat clear so much glucose after a meal.

Mechanistically, insulin binding triggers a signaling cascade (through PI3-kinase and Akt) that causes GLUT4-containing vesicles to fuse with the plasma membrane, a process called translocation. When insulin falls, GLUT4 is recycled back inside, and glucose uptake slows. GLUT4 is thus the effector that turns the insulin signal into actual sugar movement.

Importantly, muscle contraction can recruit GLUT4 to the surface independently of insulin. This is why exercise lowers blood sugar even in people with insulin resistance, and it explains the value of physical activity in type 2 diabetes. Defective GLUT4 translocation in muscle and fat is a core feature of insulin resistance.