PATIAL asymmetry is fundamental to the structure and function ofmost eukaryotic cells. A basic aspect of this polarity is that the cell's plasma membrane is divided into discrete domains. Examples include the membranes of axons and dendrites in neurons, the growing bud and mating projection in the yeast Saccharomyces cerevisiae, and lamellipodia in fibroblasts. However, the best-studied type of polarity is found in epithelia, which is the most common type of tissue in animals (52, 56). In simple epithelia the cells form a monolayer where one surface of the cell-the apical-faces the outside world or a topologically equivalent space, such as the lumen of the intestines. This is separated by tight junctions from the basolateral surface, which contacts adjacent cells and the underlying connective tissue. Apical and basolateral surfaces have very different protein and lipid compositions (4, 10, 57). For example, the basolateral surface contains receptors for adhering to other cells and the basement membrane, and for uptake of nutrients from the circulation. The apical surface contains transporters for exchanging small molecules with the outside world. As epithelial cells divide and differentiate they must send proteins to the correct surface to establish their polarity (25). Moreover, in a differentiated epithelial cell, polarity is maintained despite an enormous flux of membrane components traveling to and from each surface.