About ten years ago, a self-avowed “lipid raft” skeptic referred to the membrane subcompartments as “now you see them, now you don’t” entities. Actually, he was trying to draw attention to the various shifts in the definitions of lipid rafts, or raft domains, sections of the cell membrane that are said to consist of special groups of molecules. He complained that raft domains were of uncertain composition, size, and duration. He was concerned, above all, that raft domains had never been directly observed in living cells.
Well, raft domains have finally been glimpsed, and they have shown themselves worthy of the “now you see them, now you don’t” epithet—though not in the way the author may have imagined. Raft domains are not illusions that come and go on the basis of semantic or definitional whims. They are real. They just happen to be highly dynamic, lasting just tens of milliseconds. Raft domains, we now know, are hard to see because they are so transient.
This finding appeared April 4 in the journal Nature Chemical Biology, in an article entitled, “Raft-based interactions of gangliosides with a GPI-anchored receptor.” The article describes how researchers based at Kyoto University developed methods for systematically synthesizing ganglioside analogs that “behave like their native counterparts in regard to partitioning into raft-related membrane domains or preparations.”
The researchers focused on gangliosides because these lipid molecules have been presumed to play key roles in numerous biological processes, including the entry of toxins into cells and chemical signal reception—and the formation of raft domains. The problem with gangliosides, however, is that they are reluctant to cooperate with the usual tagging and tracking methods. As the researchers explained, gangliosides are poorly understood primarily because of the scarcity of suitable fluorescent ganglioside analogs.
Yet the researcher team persisted. Instead of just attaching a fluorescent marker to a ganglioside, the team chemically synthesized four whole gangliosides with fluorescent markers attached at specific locations. Using these new fluorescent analogs—combined with unique high-definition, single fluorescent-molecule imaging—the team was able to directly document specific ganglioside actions in a living cell for the first time.
“Single-fluorescent-molecule imaging in the live-cell plasma membrane revealed the clear but transient colocalization and codiffusion of fluorescent ganglioside analogs with a fluorescently labeled glycosylphosphatidylinisotol (GPI)-anchored protein, human CD59,” the researchers reported. They added that the raft-related membrane domains were short-lived, “with lifetimes of 12 ms for CD59 monomers, 40 ms for CD59's transient homodimer rafts in quiescent cells, and 48 ms for engaged-CD59-cluster rafts, in cholesterol- and GPI-anchoring-dependent manners.”
CD59, as the researchers indicated, belongs to a special class of receptor proteins, the GPI-anchored receptors. CD59 interacts with cholesterol and gangliosides for just 1/100th to 1/20th of a second to form a raft domain. Then the raft's molecular constituents quickly move to other tasks. That's why no one could observe raft domains in live cells before.
“Such dynamic behaviours were difficult to find using normal techniques,” said Kenichi Suzuki of Kyoto University’s Institute for Integrated Cell-Material Sciences and one of the paper's co-authors. “Our findings were made possible by single-molecule tracking of new fluorescent ganglioside probes.”
The authors of the study noted that the ganglioside molecules were always mobile in quiescent cells, and they concluded that gangliosides “continually and dynamically exchange between raft domains and the bulk domain, indicating that raft domains are dynamic entities.”
Now, researchers can begin to investigate further how toxins, bacteria, and viruses invade cells through these raft domains.