Effect of cytohesin-1 on [3H]GDP binding to ARF3. Samples of ARF3 (15 pmol) were incubated with zero (□), 0.06 (•), or 0.2 μg (▴) of cytohesin-1; 2 μM [3H]GDP (7.5 × 105 cpm); and 4 mM MgCl2 in 100 μl of the medium described in Fig. 2, at 36°C for the indicated time before proteins with bound [3H]GDP were collected on nitrocellulose for radioassay. Samples without ARF3, with or without cytohesin-1, bound 0.06–0.08 pmol; these control values have been subtracted from the data presented.