Coated vesicles occur in most eukaryotic cells. They are characterised by the polygonal coats they have on the cytoplasmic surfaces of their lipid membranes; their diameters vary between 50 and 150 nm. Of their various functions, one of the most striking examples is seen in developing chicken oocytes, where coated vesicles can pinocytose up to 1 g of yolk proteins a day in a single cell. Another example is the recycling of membrane after transmitter release in excised frog neuromuscular junction preparations. These terminals can secrete hundreds of times more quanta of neurotransmitter than the number of vesicles they initially contain; yet they still look full of vesicles. Here the replenishment of these secretory vesicles from the presynaptic membrane is mediated by coated vesicles. In fibroblasts, coated vesicles are responsible for the uptake of several macromolecules, including low-density lipoprotein and epidermal growth factor, which ultimately reach lysosomes and are degraded. In other cells coated vesicles appear to bud from the Golgi apparatus and seem to be involved in secretion. In general, the function of coated vesicles apparently entails the transfer of membrane and selected macromolecules between different membranous sites within the cell and in a defined direction. However, much has still to be learned about their full role.

Biochemically, coated vesicles are characterised by the major structural protein of their coats – clathrin, a polypeptide of molecular weight 180,000. They can be disrupted in vitro to give defined subunits containing several molecules of clathrin which, on suitable treatment, reassemble to form coatlike particles.