Bilayer caveolae

While the fluid mosaic model of membrane stmcture has stood up well to detailed scrutiny, additional features of membrane structure and function are constantly emerging. Two structures of particular current interest, located in surface membranes, are tipid rafts and caveolae. The former are dynamic areas of the exo-plasmic leaflet of the lipid bilayer enriched in cholesterol and sphingolipids they are involved in signal transduction and possibly other processes. Caveolae may derive from lipid rafts. Many if not all of them contain the protein caveolin-1, which may be involved in their formation from rafts. Caveolae are observable by electron microscopy as flask-shaped indentations of the cell membrane. Proteins detected in caveolae include various components of the signal-transduction system (eg, the insutin receptor and some G proteins), the folate receptor, and endothetial nitric oxide synthase (eNOS). Caveolae and lipid rafts are active areas of research, and ideas concerning them and their possible roles in various diseases are rapidly evolving. 

The water insolubility of Cers, combined with strong intermolecular interactions, account for their participation in the water barrier of skin, where Cers are about one-third of the total lipid. In cell membranes, Cers tend to associate with rafts and caveolae and can affect membrane curvature. These may be important contributors to bilayer organization during cell signaling, for example, when SM is hydrolyzed to Cer in response to agonist activation of SMase [18]. Cer is also involved in cell signaling (as discussed in Section 5), and under certain conditions, Cers can form channels and induce leakiness in membranes, such as mitochondria, which may contribute to the induction of apoptosis (L.R. Montes, 2002 L.J. Siskind, 2006). 

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