Directing Traffic: How Vesicles Transport Cargo
Vesicles Carry Cargo
Most molecules, including proteins, are too large to pass directly through membranes. Instead, large molecules are loaded into small membrane-wrapped containers called vesicles. Vesicles are constantly forming - especially at the plasma membrane, the ER, and the Golgi. Once formed, vesicles deliver their contents to destinations within or outside of the cell.
A vesicle forms when the membrane bulges out and pinches off. It travels to its destination then merges with another membrane to release its cargo. In this way proteins and other large molecules are transported without ever having to cross a membrane.
Exploding Transport Containers
Fluorescent dots (transport containers) explode in a burst of light as they fuse with the plasma membrane and expel their contents out of the cell.
Vesicles Travel Cellular Highways
Busy cells are often filled with thousands of traveling vesicles. To help organize these vesicles and get them pointed in the right direciton, the cell uses the rigid filaments and tubes of the cytoskeleton. Special motor proteins attach to cargo-filled vesicles and carry them along the cytoskeleton like trucks on a highway.
Different motor proteins are specialized for carrying certain types of cargo and for traveling along the cytoskeleton in one direction or the other. Careful matching of motors with their cargo helps vesicles reach their targets.
Motor proteins attach to vesicles and walk along a microtubule of the cytoskeleton. Dyneins walk toward the microtubule organizing center (MTOC, or centrosome) and kinesins walk away from the MTOC.
Real-time video of vesicles traveling along neurons inside a living fruitfly embryo. The three columns of moving vesicles show the locations of three different nerves. Each nerve is a bundle of axons from many neurons (a specialized cell type of the nervous system). The vesicles are visible because they contain Green Fluorescent Protein.
A Different Kind of Motor
Some vesicles have unusual ways of getting around the cell. The ones shown here can be seen rocketing through the cytoplasm. To do this they build up actin proteins (in red) at their rear. The polymerization of actin into short filamets acts as a molecular jet pack.