We have tested the binding specificities of the pleckstrin homology (PH) domains of protein kinase B (PKB) and GRP1 (general receptor for phosphoinositides-1), expressed as green fluorescent protein (GFP) fusion proteins [PH(PKB)GFP and PH(GRP1)GFP respectively] in HEK 293 cells and Swiss 3T3 cells, using confocal microscopy. Stimulation of HEK 293 cells with insulin caused a small, but sustained, increase in PtdIns(3,4,5)P3 levels, detected using a radioligand displacement assay, which was mirrored by the translocation of PH(PKB)GFP and PH(GRP1)GFP from the cytosol to the plasma membrane of live, transfected cells. Similar results were obtained using Swiss 3T3 cells stimulated with platelet-derived growth factor (PDGF) and expressing either PH(PKB)GFP or PH(GRP1)GFP. Biochemical analyses confirmed the accumulation of both PtdIns(3,4,5)P3 and PtdIns(3,4)P2 in response to PDGF, but only the latter was present at increased levels in Swiss 3T3 cells 30 min after an oxidative stress (1 mM H2O2). Concomitantly, only PH(PKB)GFP, and not PH(GRP1)GFP, was localized at plasma membranes after 30 min of treatment with H2O2. The fusion proteins appear accurately to report the spatial and temporal distribution of PtdIns(3,4,5)P3 and PtdIns(3,4)P2 in intact cells. These results establish the lipid selectivity of these PH domains in vivo, and further emphasize the overlapping, but distinct, second messenger roles of PtdIns(3,4,5)P3 and PtdIns(3,4)P2.

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