[Abstract unavailable

Partial resistance of cell membranes to solubilization with mild detergents and the analysis of isolated detergent-resistant membranes (DRMs) have been used operationally to define membrane domains. Given the multitude of detergents used for this purpose, we sought to investigate whether extraction with different detergents might reflect the same underlying principle of domain formation. We therefore compared the protein and lipid content of DRMs prepared with a variety of detergents from two cell lines. We found that the detergents differ considerably in their ability to selectively solubilize membrane proteins and to enrich sphingolipids and cholesterol over glycerophospholipids as well as saturated over unsaturated phosphatidylcholine. In addition, we observed cell type-dependent variations of the molecular characteristics of DRMs and the effectiveness of particular detergents. These results make it unlikely that different detergents reflect the same aspects of membrane organization and underscore both the structural complexity of cell membranes and the need for more sophisticated analytical tools to understand their architecture.

Increasing the size of the ER by lipid synthesis helps the cell deal with ER stress.

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 ...