Different functions of Smg GDP dissociation stimulator and mammalian counterpart of yeast Cdc25

We have previously shown that both Smg GDP dissociation stimulator (GDS) and mammalian Cdc25 (mCdc25) stimulate the GDP/GTP exchange reaction of Ki- Ras and that Smg GDS is active only on the post-translationally lipid- modified form of Ki-Ras, whereas mCdc25 is active on both the lipid-modified and unmodified forms but is more active on the lipid-modified form. In the present study, we compared more detailed kinetic properties of Smg GDS and mCdc25 by use of the lipid-modified form of Ki-Ras as a common substrate. Both Smg GDS and mCdc25 stimulated the dissociation of GDP from Ki-Ras and formed the stable binary complex with Ki-Ras. In the presence of guanosine 5'-(3-O-thio) triphosphate (GTPγS), the stable ternary complex of Smg GDS- GTPγS-Ki-Ras was produced, whereas GTPγS induced the dissociation of mCdc25 from mCdc25-Ki-Ras complex, yielding GTPγS-Ki-Ras. mCdc25 stimulated the dissociation of GDP from both the membrane-bound and soluble forms of Ki- Ras, whereas Smg GDS was far less active on the membrane-bound form than on the soluble form. Moreover, Smg GDS translocated the GTPγS-bound form of membrane-bound Ki-Ras to the soluble fraction as the stable ternary complex of Smg GDS-GTPγS-Ki-Ras, whereas mCdc25 did not show this activity. These results suggest that Smg GDS and mCdc25 play different roles in the regulation of Ki-Ras.