ORD^ORP10 mediates countertransport of P4P and PS between liposomes. (A) Coomassie Brilliant Blue–stained SDS-PAGE gel of 3xFlag-ORD^ORP10 proteins purified from Expi293F cells. (B) Phosphoinositide (PI) species detected by MS of 3xFlag-ORD^ORP10 proteins purified from Expi293F cells. N.D., not detected. (C and D) Schematic representation for in vitro lipid transport assay for PS (C) or PI4P (D). A PH domain from FAPP1 (NBD-PH^FAPP; Balla et al., 2005) or a C2 domain from lactadherin (NBD-C2^Lact; Yeung et al., 2008) was used as a sensor for PI4P or PS, respectively. (E) The amount of PS (µM) transported to acceptor liposomes containing only PC (gray) or PC + PI4P (red) by ORD^ORP8 (left) or ORD^ORP10 (right; mean ± SD; n = 3 independent experiments). (F) The amount of PI4P (µM) transported to acceptor liposomes containing only PC (gray) or PC + PS (blue) by ORD^ORP8 (left) or ORD^ORP10 (right; mean ± SD; n = 3 independent experiments). ****, P < 0.0001.