Clathrin helps vesicles form by assembling into a protein coat on the cytoplasmic surface of a membrane. This coat produces membrane curvature, allowing a bud to develop and separate as a transport vesicle.
The mechanism of vesicle formation
This is Additional Higher Level (AHL) content in B2.2 Organelles and compartmentalization. Vesicle formation occurs in a sequence:
- Molecules intended for transport bind to specific cargo receptors in the membrane. This concentrates the cargo within one region rather than enclosing membrane contents randomly.
- Adaptor proteins bind to the cytoplasmic portions of the cargo receptors. They connect the receptors to clathrin because clathrin does not usually bind directly to the membrane.
- Clathrin molecules assemble into a lattice-like coat on the membrane’s cytoplasmic surface. The shape of this coat bends the membrane, producing a clathrin-coated pit.
- Continued coat assembly increases the membrane’s curvature, forming a bud connected to the original membrane by a narrow neck.
- The neck is constricted and separated, releasing a clathrin-coated vesicle into the cytoplasm.
- The clathrin coat is removed and its components are recycled. The uncoated vesicle can then be directed to the correct target membrane, where membrane fusion allows delivery of its contents.
Clathrin-coated vesicles are involved in processes such as endocytosis at the plasma membrane and transport between parts of the endomembrane system, including the Golgi apparatus.
A common misconception is that clathrin directly cuts the vesicle away from the membrane. Clathrin’s central role is to form the coat that selects cargo and curves the membrane; other proteins complete separation of the vesicle.
Exam technique
For an IB explain question, give a linked sequence: cargo selection → adaptor binding → clathrin coat assembly → membrane curvature and budding → vesicle release → uncoating. Do not merely state that clathrin “makes vesicles”; connect its protein-coat structure to membrane bending.
