A Cytosolic Protein Catalyzes the Release of GDP from p21 ^<i>ras</i>

The rate of release of guanine nucleotides from the ras proteins (Ras) is extremely slow in the presence of Mg 2+ . It seemed likely, therefore that a factor might exist to accelerate the release of guanosine diphosphate (GDP), and hence the exchange of GDP for guanosine triphosphate (GTP). Such a factor has now been discovered in rat brain cytosol. Brain cytosol was found to catalyze, by orders of magnitude, the release of guanine nucleotides from recombinant v-H-Ras protein bound with [α- 32 P]GDP. This effect occurred even in the presence of a large excess of Mg 2+ , but was destroyed by heat or by incubation of the cytosol for an hour at 37°C in the absence of phosphatase inhibitors. The effect was observed with either v-H-Ras or c-H-Ras, but not with p25 rab3A , a small G protein with about 30% similarity to Ras. The effect could not be mimicked by addition of recombinant Ras-GAP or purified GEF, a guanine nucleotide exchange factor involved in the regulation of eukaryotic protein synthesis. By gel filtration chromatography, the factor appears to possess a molecular size between 100,000 and 160,000 daltons. This protein (Ras-guanine nucleotide-releasing factor, or Ras-GRF) may be involved in the activation of p21 ras .