ORD^ORP10 mediates countertransport of P4P and PS at ER–endosome MCSs. (A) Cartoon illustrating the principle of lipid exchange assay in cultured cells. Addition of rapamycin acutely recruits FKBP-ORD^ORP10 to the ER–endosome MCSs tethered by ORP9-ΔORD-FRB. (B) Representative snapshot images of confocal live microscopy showing PI4P levels (GFP-OSBP-PH) or PS levels (mRFP-LactC2) before (0 min) and after (5 min) acute recruitment of ORD^ORP10 to ER–endosome MCSs. Note that the PI4P levels in the Golgi did not change after acute recruitment of ORD^ORP10. Scale bars, 5 µm (full size) and 1 µm (inset). (C) Quantification for (B; grand mean ±SEM; n = 11 cells). Note that rapamycin (Rapa.) was added at 1 min.