Distribution of lipid raft markers in live cells.

GPI (glycosylphosphatidylinositol)-anchored proteins are characteristic components of biochemically defined lipid rafts. Rafts may be involved in T-cell stimulation, but it is not clear whether molecules involved in TCR (T-cell receptor) signalling are partitioned to T-cell synapses through raft microdomains or through specific protein-protein interactions. We have used FRET (fluorescence resonance energy transfer) analysis to study the distribution of GPI-anchored fluorescent proteins in the plasma membrane of live cells. Multiple criteria suggested that FRET between different GPI-anchored fluorescent proteins in COS-7 or unstimulated Jurkat T-cells is generated by a random, unclustered distribution. Stimulation of TCR signalling in Jurkat T-cells by beads coated with antibodies against TCR subunits resulted in localized increases in fluorescence of raft markers. However, measurements of FRET and ratio imaging showed that there was no detectable clustering and no overall enrichment of raft markers in these regions.