LIPID RAFTS INSIDE AND OUT:
© 2002 AAAS
In the outer leaflet (A), sphingolipids and cholesterol form less fluid microdomains (B) called lipid rafts, which are enriched for GPI-proteins. Microdomains may contain more rigid subdomains (C) enriched for the sphingolipid ganglioside GM1. The membrane inner leaflet contains microdomains (D) with unknown lipid composition enriched for prenylated proteins. In contrast, caveolin and proteins carrying the two saturated fatty acyl chains become concentrated in caveolae (F). (From G. van Meer, Science, 296: 855–7, 2002.)
Certain plasma membrane structures, such as clathrin-coated pits and caveolae, have been recognized for decades. More recently, scientists have identified other less-visible domains within the lipid bilayer, collectively termed lipid rafts. With new imaging techniques and new findings, membrane domain research has proceeded at a rate more befitting a rocket than a raft.
The Hot Papers featured herein both use green fluorescent protein (GFP) to visualize lipid rafts. Roger Tsien's group at the University of California, San Diego, tagged different lipid chains with GFP to track the movement to and clustering within the plasma membrane, that is, raft formation.1 A group at the University of Copenhagen's Panum Institute, led by Bo van Deurs, fused GFP to the marker protein caveolin to observe the mobility and turnover of caveolae in the membrane.2
The contributions of these papers are largely technical, but the techniques promise to reveal the molecular structure of rafts. The study from Tsien's laboratory demonstrated the presence of rafts on the inner leaflet of the plasma membrane. A surprise lay in the lipid type that formed this inner leaflet raft, signifying a greater variety of lipidraft classes than had been imagined. Van Deurs' work depicts caveolae as relatively immobile structures and argues against endocytosis as a function of these structures, at least under normal conditions.
Using fluorescence, scientists can observe membrane domains in intact cells. Previously, biochemists used cell fractionation to study lipid rafts. Marker proteins such as caveolin were found in a detergent-resistant fraction, as were lipids such as cholesterol. But molecules that intermingle in that fraction may not necessarily associate in live cells. "The methods used grouped them all together, put them into the same fraction, artificially" says Kai Simons, executive director of the Max Planck Institute of Cell Biology and Genetics in Dresden. Simons coined the term lipid rafts.
While some complain that more effort should be made to integrate the results with earlier findings, most researchers are captivated by the new and yet very basic questions raised by these findings. The sheer variety of domains being proposed as raft structures are sending scientists scrambling to find their defining features.
Data derived from the Science Watch/Hot Papers database and the Web of Science (Thomson Scientific) show that Hot Papers are cited 50 to 100 times more often that the average paper of the same type and age.
The Tsien study used fluorescence resonant energy transfer (FRET) to demonstrate protein interactions, by mapping energy transfer between labeled molecules. "What FRET measures is the distance between the so-called donor and acceptor," says Anne Kenworthy, assistant professor of molecular physiology and biophysics at Vanderbilt University in Nashville, who coauthored a seminal paper on FRET in 1998.3 "But it has a very sharp cutoff. So if you get to distances above around 100 Ångstroms, then there's no FRET signal anymore." By comparing experimental FRET values to those predicted for a random distribution, scientists can infer molecular organization.
To track lipid aggregation, the investigators needed a fluorescent molecule that didn't aggregate on its own. The GFP from
The methodology has "huge potential," although it has yet to advance knowledge of raft function, says Richard Anderson, professor of cell biology at the University of Texas Southwestern Medical Center at Dallas. "I don't think they're giving better data, just different data." Anderson laments that technology can obfuscate biology, especially when techniques require special expertise. "If I have one complaint about this field, it's that no one verifies that their technique meets the criteria that other techniques have already established." Kenworthy agrees, saying that it's not clear how clusters detected by FRET are correlated with molecules found in detergent-resistant membrane fractions.
By attaching GFP to lipids rather than proteins, Zacharias' experiments favor a primary role for lipids in raft formation. "It was the lipid modifications themselves that contained the targeting information," says Kenworthy. Simons says this should dispel a lingering contention that raft proteins are in the driver's seat.
MODIFYING FLUORESCENT PROTEINS:
© 2002 AAAS
Researchers created myristoylated and palmitoylated (MyrPalm), geranylgeranylated (GerGer), tandemly palmitoylated (PalmPalm), and caveolin fusion constructs and expressed them in MDCK cells. Arrows denote representative sites for data collection.
Zacharias, who was a postdoctoral fellow in Tsien's lab when he did the studies, confesses "a serious naïveté" regarding the probes he chose to study. Nonetheless, that innocence led to two major contributions with regard to characterizing lipid rafts. Because the lipid chains he used were inner-leaflet probes, "it was the first physical evidence in live cells that you get raft formation in the inner leaflet," he says.
More surprising was the finding that a prenyl group can target proteins to the plasma membrane. "This is perhaps the most contentious point of the whole paper," says Zacharias, because prenylation had been thought to target proteins exclusively to endomembranes, such as endoplasmic reticulum and Golgi.
This new role for prenylation jolted the field in the way that breeds doubt. But subsequent work has confirmed the existence of prenylated protein rafts, says Kenworthy. "Now we have a lot more information about that, in particular for the Ras family of proteins. So that's one area where new work has been done, using other techniques," including electron microscopy, functional assays of Ras signaling, and fluorescence recovery after photobleaching (FRAP).
Another unusual aspect of the prenylated protein clusters was that they were not disrupted by cholesterol depletion. Because classic lipid rafts are enriched in cholesterol, "If you deplete cholesterol, the clusters disperse," says Kenworthy. The prenylated proteins Zacharias found were in the cholesterol independent domains, she says, indicative of a truly different type of raft.
Criticisms of the Zacharias paper center on how well experimental data fit predictions for clustering because the variability was huge. But many researchers believe rafts themselves are transient, with lipid and protein components constantly moving in and out. "That would definitely explain some of that scatter," says Jennifer Lippincott-Schwartz, head of the section on organelle biology at the National Institute of Child Health and Human Development.
FROM FRET TO FRAP
As evidence accumulates for a wide variety of lipidraft structures, caveolae, once the granddaddy of membrane domains, may be "the exception to the rule," says Lippincott-Schwartz, because of their discrete size and shape. And still, after decades of study, caveolae function has yet to be sorted out.
Van Deurs wanted to address the longstanding question of whether caveolae are involved in endocytosis. It is a question, van Deurs says, that is inspired in part by the "nice shape that they have," referring to the flask-shaped indentation of the plasma membrane. "If caveolae are equivalent to clathrin-coated pits, then they must be dynamic structures," says van Deurs. The movement of clathrin-coated pits is evidenced by clathrin's short half-life on the plasma membrane, which is about one minute.
To study caveolae mobility, van Deurs tagged caveolin with GFP and then used FRAP. After photo-bleaching a small portion of the cell surface to wipe out the fluorescent signal, they monitored the area for renewed fluorescence. A large recovery would be consistent with the tagged caveolin moving into the bleached area, but this is not what occurred. "It turned out that they have very little mobility," says van Deurs. "So they cannot be involved in endocytosis in the normal definition of the word," although van Deurs acknowledges that special circumstances, such as stimulation by SP-40 virus, would cause caveolae to internalize.
So while the van Deurs study characterized caveolae as more anchored than previously thought, the Tsien study has pushed the boundaries of what lipid rafts can be in the other direction, that is, smaller and more transient. The enormous appeal of linking functional molecules across the plasma membrane initially propelled lipid raft research. But structural evidence, characterized in molecular detail with fluorescence technology, has failed most preconceived notions of these membrane organelles and forced a paradigm shift in thinking about function of these elusive structures.
Jill U. Adams