Lipid rafts, which are cholesterol- and sphingolipid-rich microdomains in the plasma membrane, have been proposed to form `signalling platforms' by fusing in response to extracellular signals. The epidermal growth factor receptor (EGFR) localises to lipid rafts; however, limitations of fluorescence microscopy have made it difficult to determine whether EGFR-containing rafts coalesce when EGFR is activated. Now, Paul van Bergen en Henegouwen and colleagues () overcome this problem by using fluorescence lifetime imaging microscopy (FLIM) to show nanometre-scale colocalisation of EGFR with other lipid-raft components. The authors detect EGFR by using a single-domain antibody from llama, which can label the protein without activating it. They show that EGFR colocalises with the ganglioside GM1 whether cholesterol is present or not; by contrast, GM1 and another lipid-raft marker, glycosylphosphatidylinositol-anchored GFP (GPI-GFP), colocalise only in the presence of cholesterol. Notably, EGFR does not colocalise with GPI-GFP in the resting state, but does when cells are stimulated with EGF. Thus, two classes of GM1-containing microdomain coalesce into larger lipid microdomains in response to EGF. These results provide evidence for the existence of activatable plasma-membrane signalling platforms.
