Exocytosis

Exocytosis is the process by which a cell releases material to the outside by fusing a membrane-bound vesicle with the plasma membrane. It is how cells secrete proteins and hormones, how neurons release neurotransmitters, and how new membrane is added to the cell surface.

Vesicles loaded in the Golgi apparatus travel along the cytoskeleton to the plasma membrane. Proteins called SNAREs on the vesicle and the target membrane wind together like a zipper, pulling the two lipid bilayers into contact so they merge and open a pore. The contents are released and the vesicle membrane becomes part of the cell surface. James Rothman, Randy Schekman and Thomas Südhof shared the 2013 Nobel Prize for work on this machinery.

In constitutive exocytosis, vesicles fuse continuously and deliver new proteins and lipids to the membrane, as part of the secretory pathway of the endomembrane system. In regulated exocytosis, vesicles wait near the membrane until a signal arrives, usually a rise in calcium ions. At a synapse, a nerve impulse opens calcium channels and neurotransmitter is released within a fraction of a millisecond from neurons. Pancreatic cells release insulin in the same way.

Exocytosis balances endocytosis, keeping the surface area of a cell constant. Botulinum and tetanus toxins act by cutting SNARE proteins, which blocks release at the nerve ending and causes paralysis. Defects in the machinery are involved in disorders of the immune system and in diabetes. The process requires energy from ATP and relies on the cell membrane.

• Endocytosis — the opposite process
• Golgi Apparatus — where most vesicles are packaged
• Neuron — a cell that relies on rapid exocytosis