Wrapping the package

The membranes of living cells are in a state of dizzying flux. Lipid vesicles, often containing receptors and wrapped generally in protein coats, shuttle between cellular compartments at such a rate that, for example, the substance of the plasma membrane is typically turned over in less than an hour. Of the known classes of vesicles, the most extensively studied, by reason of their abundance and ease of preparation, are the clathrin-coated vesicles. These are the vehicles through which cell-surface receptors are internalized and delivered to the endosomal compartment, and they also transport lysosomal enzymes from the trans-Golgi network to the endosomes (1).