Recent work has given rise to a fundamentally new way of thinking about biological membranes. Contrary to earlier views, we now know that lipids do not always mix homogeneously in membranes, but can be organized into domains in the bilayer. The best-characterized membrane domains, called rafts, are rich in cholesterol and sphingolipids. The ability of certain proteins to associate with these domains has profound effects on their function. This new realization has implications in fields as diverse as signal transduction, protein and lipid sorting, and cell adhesion and motility. For instance, following antigen stimulation of T lymphocytes, the T cell receptor and its signaling partners must cluster together in rafts to initiate the signal transduction cascade.
Our lab has developed a model for raft structure, and for how proteins and lipids associate with rafts, that is now generally accepted in the field. This model provides a conceptual foundation for further exploration of raft structure and function. We use a combination of cell biological, biochemical, and biophysical methods to study rafts in cells and model membranes.
The acyl chains of raft lipids are highly extended and tightly packed. This arrangement gives raft lipids a high degree of order. This contrasts with the disordered state of most lipids in biological membranes. In fact, rafts probably exist in a separate phase from the rest of the membrane, that has properties similar to those of the liquid-ordered (lo) phase described in model membranes. Lipids such as sphingolipids, which have long, saturated acyl chains, partition preferentially into these ordered domains and are enriched in rafts.
A number of proteins that are modified with saturated acyl chains (including glycosyl phosphatidylinositol (GPI)-anchored proteins, myristoylated, and palmitoylated proteins) are enriched in rafts. These include important signaling proteins, such as Src-family kinases and heterotrimeric G protein alpha subunits. Association of these proteins with rafts is likely to be important in function, as has already been shown for the Src-family kinase Lck in T cells.
Questions of raft structure are a major focus of our lab. How big are rafts? How are they distributed in membranes? How does clustering of raft proteins (as can occur during signaling through cell-surface receptors) affect raft structure and function? How are transmembrane proteins, which might not be expected to fit well into an ordered lipid environment, targeted to rafts in the absence of acylation? A close collaboration with the lab of Dr. Erwin London, a membrane biophysical chemist, allows us to combine complementary cell biological, biochemical, and biophysical methods. This powerful approach gives us a unique advantage in studying rafts in both cells and model membranes.
Caveolae are 50-100 nm pits in the plasma membrane of many mammalian cells, that contain the 22 kDa protein caveolin as a major structural component. Though caveolae were first described more than 30 years ago, their function(s) in endocytosis, cholesterol trafficking, and/or as signal transduction centers are just now being defined. Rafts have an affinity for caveolae, although the precise relationship between the two remains an intriguing mystery.
Caveolin is an unusual protein. Cytoplasmic N- and C-terminal domains flank a central 33 amino acid hydrophobic domain, which is presumed to form a tight alpha-helical hairpin that anchors the protein in the membrane. The protein forms large homo-oligomers, that can be up to 600 kDa in size. Caveolin may bind directly to a number of signaling proteins, modulating their function. It also binds tightly to cholesterol, and has been reported to cycle between the plasma membrane and intracellular compartments in an unusual manner. Caveolin is also the only protein known to associate directly with rafts without lipid modification. These and other unusual properties focus attention on this protein in linking the functions of rafts and caveolae.
We have purified 100-microgram amounts of caveolin from recombinant-baculovirus infected insect cells, and reconstituted it into liposomes for structural studies. We are also examining the behavior of over-expressed caveolin and of a number of caveolin mutant proteins in cells. Together, these experiments will shed new light on this intriguing protein, and how it may function to organize rafts in cells.