Fluorescent probes for superresolution imaging of lipid domains on the plasma membrane

Accumulating evidence indicates that membrane lipids are not randomly distributed but rather form specific domains. In particular, raft-like microdomains composed of cholesterol and sphingolipids are attracting a lot of attention. These microdomains are thought to serve as platforms for signal transduction and molecular trafficking, but it is difficult to elucidate their detailed structure since their reported size is smaller than the resolution of light microscopy. To circumvent this limitation, we designed probes for cholesterol- and sphingolipid-enriched microdomains dedicated for superresolution microscopy, PALM. The probes utilise the affinity of the toxins, θ-toxin and lysenin, for the cholesterol- and sphingomyelin-enriched membranes, respectively. The toxicity can be avoided by using non-toxic domains that retain the specific binding to the aforementioned membranes. The probes can easily be produced in E. coli as recombinant protein domains of toxins fused to a photoswitchable fluorescent protein, Dronpa. PALM imaging with these probes revealed two types of cholesterol-enriched microdomains, line-shaped ones with widths of around 150 nm and round ones with an average radius of 118 nm. All sphingomyelin-enriched microdomains were round with an average radius of 124 nm. Both the cholesterol- and sphingomyelin-enriched microdomains vanished by the depletion of cholesterol. The sphingomyelin-enriched microdomains also vanished by the depletion of sphingomyelin whereas the cholesterol-enriched microdomains were unaffected. We conclude that cholesterol- and sphingomyelin-enriched domains occupy different regions on the plasma membrane.