Selective anchoring in the specific plasma membrane domain: a role in epithelial cell polarity

T hE polarization of plasma membrane proteins into apical and basolateral domains is key to vectorial function of epithelia (60, 70) . Recent work with the MDCK cell system has demonstrated two mechanisms that contribute to this surface polarity: (a) sorting by the trans-Golgi into carrier vesicles which are then vectorially delivered to the correct domain (10, 41, 45, 55, 58, 64) , and (b) the tight junctional fence (16) . It seems now quite clear that tight junctions may segregate lipids in the outer leaflet of the bilayer (17, 81) but the extent of their contribution to protein segregation is still unknown. Thus, although dissociation of tight junctions with calcium chelating agents results in the invasion of the apical and basolateral regions by markers previously segregated (29, 56, 88) , other Ca+÷-dependent cell adhesion molecules, junction adhesion molecules, and substrate adhesion molecules are also disrupted by the same treatment (21, 52, 75) . In addition, some apical membrane proteins are polarized in MDCK cells with incomplete tight junctions (82) , which indicates that restrictions to lateral diffusion other than complete tight junctions are involved in the maintenance of epithelial surface polarity. This result raised the question of whether restrictions to lateral diffusion within the plane of the membrane or interdomain fences play a significant role in the maintenance of epithelial surface polarity.