Sphingolipids dynamics

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. 

Because membranes components participate in nearly every cell activity their structures are also dynamic and far from the equilibrium states that are most readily understood in biophysical terms. Newly synthesized bilayer lipids are initially associated with endoplasmic reticulum (Ch.3) whereas phospholipids initially insert into the cytoplasmic leaflet while cholesterol and sphingolipids insert into the luminal endoplasmic reticulum (ER) leaflet. Glycosylation of ceramides occurs as they transit the Golgi compartments, forming cerebrosides and gangliosides in the luminal leaflet. Thus, unlike model systems, the leaflets of ER membranes are asymmetric by virtue of their mode of biosynthesis. 

Harder T, Simons K. Caveolae, DIGs, and the dynamics of sphingolipid-cholesterol microdomains. Curr Opin Cell Biol 1997 9 534-542. 

Apart of forming the bilayer, membrane lipids exhibit dynamic structures within the lamellas, forming microdomains with specific functionalities. The so called membrane rafts are sphingolipid-cholesterol domains that contribute to signal transduction, as well as to lipid and protein sorting and transport [18]. 

A number of people who were attending the Symposium for the first time commented on the open and friendly atmosphere. There is a family feeling since the community of people who work on plant lipids is still fairly small. At this Symposium however, there were several friends of the family, people accustomed to reporting their research in other areas who have much to contribute to the field of plant lipids. We think in particular of the contribution of Leo Parks on the function of sterols in yeast, the presentation of Joe Kuc on the importance of lipids in host-pathogen interactions, and the paper of Rudy Demel on the dynamics of glycerolipid, sphingolipid and sterol interaction in membranes. 

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