Polarized sorting in epithelial cells: raft clustering and the biogenesis of the apical membrane

Abstract

5955Polarized cells establish and maintain functionally distinct surface domains by an elaborate sorting process, which ensures accurate delivery of biosynthetic cargo to different parts of the plasma membrane. This is particularly evident in polarized epithelial cells, which have been used as a model system for studies of sorting mechanisms. The clustering of lipid rafts through the oligomerization of raft components could be utilized for segregating apical from basolateral cargo and for the generation of intracellular transport carriers. Besides functioning in polarized sorting in differentiated cells, raft clustering might also play an important role in the biogenesis of apical membrane domains during development. Mechanisms of polarized sorting A polarized surface distribution of proteins and lipids can be achieved by targeted delivery or selective retention (Yeaman et al., 1999;Matter, 2000). Targeted delivery relies on the segregation of cargo molecules destined for different plasma membrane domains before they reach the cell surface. Selective retention works by trapping them once they arrivefor instance, by anchoring to a domain-specific cytoskeletal scaffold. Proteins delivered to the wrong surface domain are removed by endocytosis and are then degraded or undergo another round of delivery. Targeted delivery and selective retention can be combined such that selective retention enhances the accuracy of intracellular sorting (Mays et al., 1995).The basolateral as well as the apical sorting pathway has been suspected to involve bulk flow, but it is now clear that neither is a general default pathway for the surface delivery of proteins. Instead, apical and basolateral sorting are both governed by sorting determinants embedded in cargo proteins. Sorting determinantsSorting determinants in membrane proteins can reside in the cytoplasmic domain, membrane anchor or extracellular domain (Table 1). Cytoplasmic domain determinants include the basolateral-targeting, tyrosine-based and dileucine motifs (Matter and Mellman, 1994), PDZ-domain-binding motifs and a growing list of unrelated sequences (Altschuler et al., 2003;Muth and Caplan, 2003). Determinants in membrane anchors include the transmembrane domains of some apically targeted viral proteins (Kundu et al., 1996; Lin et al., 1998) and glycosylphosphatidylinositol (GPI) anchors. The latter usually confer localization to the apical membrane (Brown et al., 1989;Lisanti et al., 1989), but alone are not always sufficient (Brown and London, 1998;Benting et al., 1999a). Both N-and O-glycosylation of the extracellular domain have been implicated in apical targeting (Scheiffele et al., 1995;Yeaman et al., 1997;Gut et al., 1998;Spodsberg et al., 2001). Finally, oligomerization of membrane proteins may be an important sorting determinant, particularly for apical transport. Evidence in epithelial cells is lacking so far, but sorting of the voltagegated potassium channel Kv1 into the axonal pathway of neurons, which is related to the apical pathway of epithelial cells ...

Role of N-glycosylation in trafficking of apical membrane proteins in epithelia

“…Significant evidence has accumulated to suggest involvement of lipid rafts in sorting and delivery of proteins to the apical membrane. Lipid rafts are rigid membrane microdomains enriched in glycolipids, sphingolipids, and cholesterol embedded in the fluid membrane (81). In 1997, Simons and Ikonen (84) postulated that newly synthesized apical proteins accumulate in these microdomains in the TGN, while basolateral proteins are excluded from these lipid rafts.…”

Rafts in oligodendrocytes: Evidence and structure–function relationship

“…In apical sorting pathways, the formation of high-molecularweight complexes as part of the mechanism of DRM recruitment seems to be required (Helms and Zurzolo, 2004;Paladino et al, 2004;Schuck and Simons, 2004;Zurzolo et al, 2003). Oligomerization or association with a high-molecular-weight complex might lead to an exponential increase in raft affinity with increasing oligomer size (Simons and Vaz, 2004), thereby stabilizing raft association.…”

“…MAL is upregulated during active myelination (Kim et al, 1995;Schaeren-Wiemers et al, 1995). In addition, MAL can form oligomers, which might function to cluster rafts at the sites at which sorting takes place (Schuck and Simons, 2004). However, MAL is relatively late expressed in myelinogenesis, i.e., after expression of most major myelin proteins, …”

Clustering and Lateral Concentration of Raft Lipids by the MAL Protein

“…The apical plasma membrane (PM) domain is a robust yet sophisticated sphingolipid-and cholesterol-enriched protective barrier against harsh extracellular environments that maintains exchange and regulatory capacities (Schuck and Simons, 2004). Sphingolipid rafts have been postulated as lipid microdomains that serve as platforms for apical cargo sorting and targeting processes as well as transport-carrier formation (Simons and Ikonen, 1997).…”