Lipid rafts and apical membrane traffic.

Lipid rafts are dynamic assemblies floating freely in the surrounding membranes of living cells. This membrane heterogeneity provides a useful concept for understanding processes as diverse as cell polarity, signal transduction, and membrane sorting. Individual rafts are small entities containing thousands of lipids but only a few proteins. Regulation of raft association and size is an elementary feature of interactions at the molecular level. By clustering small rafts into a bigger platform, proteins are brought together for modification. Oligomerization might transform a monomeric weakly raft-associated protein into an assembly with higher raft affinity. Lectins are multivalent glycoprotein-binding proteins and are likely to be key players in mediating the clustering of rafts in vivo. Glycosylation-dependent surface delivery in a polarized fashion is a feature conserved across evolution, and we expect lectins to be at the heart of the molecular machinery responsible for lipid raft delivery to the cell surface. Currently, we are evaluating candidate proteins by affinity chromatography, proteomics, and RNA interference.