a) QF-FRL of unilamellar liposomes containing 95 mol% of phosphatidylcholine and 5 mol% of a phosphoinositide, labelled with GST-PX. Only liposomes containing PtdIns(3) P were labelled intensely. GST-PXR58A, a PX-domain mutant that does not bind PtdIns(3) P, yielded virtually no labelling in PtdIns(3) P-containing liposomes. Scale bar, 0.2 μm. 