Fig. 6. XMog1 alone promotes release of GTP, but not GDP, from Ran. (A) Nucleotide release in the presence of 1 mM free GDP in exchange buffer containing 10 mM Mg²⁺. Ran was loaded with [³H]GTP (open squares) or [³H]GDP (closed squares). (B) Nucleotide release in the absence of free nucleotide in exchange buffer containing 10 mM Mg²⁺. Wild-type Ran (squares) or RanQ69L (triangles) were preloaded with [³H]GTP (open symbols) or [³H]GDP (closed symbols). (C) GTP release by XMog1 in buffers containing 1 mM Mg²⁺ (closed symbols) or with addition of 10 mM EDTA (open symbols) in the absence of free nucleotide. For each datum, Ran (50 pmol) was loaded with the specified radionucleotide and incubated at 21°C in a total volume of 50 µl with the indicated amount of XMog1 in exchange buffer (see Materials and Methods). In (C), Ran-[³H]GTP was prepared with the addition of a lower concentration of Mg²⁺ so that the reactions contained 1 mM Mg²⁺ with or without the addition of 10 mM EDTA. After incubation for 10 minutes, the radioactivity remaining bound to Ran was determined as described in Materials and Methods.