Polarized sorting and trafficking in epithelial cells

Abstract

The polarized distribution of proteins and lipids at the surface membrane of epithelial cells results in the formation of an apical and a basolateral domain, which are separated by tight junctions. The generation and maintenance of epithelial polarity require elaborate mechanisms that guarantee correct sorting and vectorial delivery of cargo molecules. This dynamic process involves the interaction of sorting signals with sorting machineries and the formation of transport carriers. Here we review the recent advances in the field of polarized sorting in epithelial cells. We especially highlight the role of lipid rafts in apical sorting.

Figure 1

A scheme for apical transport carrier formation by domain-induced budding. (A) Nanoscale dynamic rafts surrounded by non-raft membrane. (B) Growing rafts are selectively induced by galectin–glycolipid–glycoprotein interactions into a budding domain, while non-raft components are excluded. Raft clustering results in increased line tension. (C) Insertion of hydrophobic or amphipathic protein domains (red) promotes membrane bending. FAPP2 could play this role for membrane deformation. (D) Fission at the domain boundary (possibly aided by fission proteins) results in the release of an apical transport carrier. For simplicity GPI-APs, cholesterol and other (e.g., palmitoylated proteins) proteins are not shown and cytoskeletal elements are omitted as well.