Clathrin Coated Vesicles

Clathrin forms a three-legged triskelion structure composed of three large and three small polypeptide chains. These triskelions assemble into a basket-like framework that determines the geometry of the clathrin cage. The triskelion arms extend up to 50 nanometers and intertwine with other triskelions to create a symmetrical protein lattice that encases the cargo during transport.

Adaptor proteins form the inner layer of the clathrin coat and select specific transmembrane receptors that bind cargo molecules for transport. These proteins, such as AP1 and AP2, link the membrane-embedded cargo receptors to the outer clathrin cage. Adaptor complexes vary by membrane location, with AP2 localizing to the plasma membrane and AP1 to the trans-Golgi network and endosomal membranes.

Dynamin, a GTP-binding protein, attaches around the neck of the budding vesicle and hydrolyzes GTP to release energy. This energy drives a conformational change in dynamin that stretches the vesicle neck until it pinches off from the cell membrane. This mechanism is central to the pinching off of coated vesicles during endocytosis.

Clathrin-coated vesicles transport proteins from the Golgi to the plasma membrane and mediate endocytosis to internalize receptors and cargo from the cell surface. They also transport lysosomal hydrolases to lysosomes as part of the delivery pathways to the lysosome. These vesicles are the most well-studied transport vesicles and involve over 50 coordinated proteins in their formation.

After a clathrin-coated vesicle buds off from the membrane, the clathrin coat is rapidly removed through uncoating. The uncoated vesicle then travels to its destination organelle, where it fuses with the target membrane to deliver its cargo. This uncoating step is essential for preparing the vesicle for membrane fusion and cargo delivery.

Clathrin-coated pits form when adaptor proteins and clathrin cage complexes are recruited to indented sites on the membrane. Cell surface receptors concentrate in these clathrin pits and selectively bind macromolecules for internalization. Once cargo is loaded, the coated pits invaginate into the cytoplasm and prepare to pinch off as complete vesicles.

The flexibility of triskelion arms enables the clathrin cage to achieve size variation required for transporting different cargo molecules. The bent rod-like structure of clathrin heavy chains allows the arms to bend and intertwine with other triskelions, creating a versatile protein lattice. This adaptability ensures that vesicles can accommodate diverse cargo while maintaining structural integrity.