Fig. 4. Export of GPI-FP from the ER requires GTP hydrolysis by the Sar1p GTPase. (A,B) Cells co-injected with plasmids encoding Sar1p^(H79G) and either lumFP (A) or GPI-FP (B). Arrowhead shows localization to the nuclear envelope. (C,D) Cells expressing lumFP (C) or GPI-FP (D) were incubated in BFA for 2 hours. Subsequently, cells were microinjected with GTP--S, and BFA was washed-out for 1 hour (E,F). LumFP exits the ER in the absence of GTP hydrolysis by Sar1p (compare A and E) but GPI-FP remains exclusively within the ER (compare B and F). In addition to clear labelling of the ER, lumFP showed accumulation into punctate structures distributed throughout the cells but concentrated in the juxtanuclear region (C, enlargement, arrowheads). Bars, 10 µm.