Protein Sorting and Glycolipid-Enriched Detergent-Insoluble Complexes in Epithelial Cells

Abstract

Epithelial cells form a permeability barrier between the external and internal environment of the body and are involved in physiological processes such as secretion, absorption and ion transport (Simons and Fuller, 1985; Rodriguez-Boulan and Nelson, 1989; Wandinger-Ness and Simons, 1991). Simple epithelial cells are organized into a cell monolayer with an apical and basolateral cell surface separated by tight junctions. These two plasma membrane domains do not only contain distinct proteins but they also have different compositions of lipids (van Meer et al., 1987; van Meer and Burger, 1992). The exoplasmic leaflet of the apical cell surface is highly enriched in glycosphingolipids (GSL), presumably providing protection against the external environment. The mechanisms involved in the generation and maintenance of epithelial cell polarity have been extensively studied using MDCK (Madin-Darby canine kidney) cells as a model system (McRoberts et al., 1981; Rodriguez-Boulan, 1983). Apically and basolaterally destined proteins and lipids have been found to be sorted from each other in the trans-Golgi network (TGN) and packaged into separate vesicular carriers followed by selective targeting to the cell surface (Griffiths and Simons, 1986; Kobayashi et al., 1992).

The work was supported by the Boehringer Ingelheim Fonds (K.F.), the Royal Society (P.D.) and SFB 352 of the DFG.