The Saccharomyces cerevisiae gene product SDC25 C-domain functions as an oncoprotein in NIH3T3 cells.

Ras proteins in mammalian cells cycle between a GTP-bound 'on' state and a GDP-bound 'off' state. Activation of Ras p21 results from the dissociation of tightly bound GDP and the exchange of bound GDP for GTP. A guanine nucleotide exchange factor is required for this activation. Activation promotes interaction with effector molecules and allows the signal to be transduced. In Saccharomyces cerevisiae, the function of guanine nucleotide exchange has been ascribed to the product of the CDC25 gene. The C-terminus domain of SDC25, a homologue of CDC25, can substitute for the CDC25 protein in yeast. We have demonstrated that the SDC25 C-terminus domain promotes GTP binding to Ras p21 in CHO cells. In the present study, we found that the stable expression of the SDC25 C-terminus domain induced transformation of NIH3T3 cells. Ras proteins in these tumorigenic cells were GTP bound. In addition, the coexpression of wild-type Ha-Ras protein with the SDC25 C-terminus was found to enhance the tumorigenic properties of the NIH3T3 cells. These results imply that, in subsets of human tumours, cellular Ras p21 might be found in its GTP-bound active form as a consequence of an oncogenic activation of a mammalian Ras guanine nucleotide exchange factor.