A basic aspect of this polarity is that the cell's plasma membrane is divided into discrete domains. The best studied and simplest example of this occurs in epithelial cells, which line exposed body surfaces. Epithelial cells have an apical surface facing the outside world and a basolateral surface contacting adjacent cells and the underlying connective tissue. These surfaces have completely different compositions.
Epithelial cells use two pathways to send proteins to the cell surface.
Alternatively, proteins can be sent to the basolateral surface and then endocytosed and transcytosed to the apical surface. We are studying the machinery that is responsible for the specificity and regulation of polarized membrane traffic in epithelial cells. Overexpression of a dominant negative form of the small GTPase, rac, retards lumen formation and leads to a partial reversal of polarity, with the apical surface oriented towards the outside of the cyst. Growth of the cysts laminin rescues this phenotype, indicating that interfering with rac function interferes with the ability of the cell to secrete, assemble, or sense laminin, which normally provides a spatial cue. The exocyst is an eight-subunit complex involved in targeting transport vesicles to specific regions of the plasma membrane.