The carboxy-terminal part of the Saccharomyces cerevisiae SDC25 gene product (SDC25 C domain) can elicit activation of mammalian Ras proteins. Specifically, SDC25 C domain functions as an exchange factor for cellular Ras proteins in CHO cells. In this study, we used the dominant inhibitory Ha-Ras Asn-17 mutant and SDC25 C domain to further investigate the interaction between cellular Ras proteins and their putative endogenous guanine nucleotide-releasing factors. Transcription from the polyomavirus thymidine kinase gene (Py tk) promoter is strongly inhibited by the expression of Ha-Ras Asn-17 in NIH 3T3 cells. Coexpression of SDC25 C domain overcomes the negative effect of the Ras mutant on the Py tk promoter. On the other hand, transactivation of the Ras-responsive element of the Py tk promoter induced by SDC25 C domain is lost upon coexpression of increasing amounts of Ha-Ras Asn-17. In addition, coexpression of SDC25 C domain overcomes the inhibition of proliferation of NIH 3T3 cells caused by Ha-Ras Asn-17. These results are consistent with the idea that the Ha-Ras Asn-17 mutant functions by titrating an upstream activator of cellular Ras proteins.

Controversy exists as to whether the interaction of a guanosine triphosphatase activating protein (GAP) with Ras proteins functions both to initiate and to terminate Ras-dependent signaling events or only to terminate them. GAP-C, a carboxyl-terminal fragment of GAP that is sufficient to stimulate GTPase activity, inhibited the stimulation of transcription produced by some oncoproteins (v-Src, polyoma middle T, wild-type Ras, and oncogenic Ras) but not that produced by v-Mos. Wild-type GAP did not affect transcription induced by oncogenic Ras but reversed the inhibitory effect of GAP-C on transcription induced by oncogenic Ras. These results indicate that GAP is a negative regulator of wild-type Ras and elicits a downstream signal by interacting with Ras-GTP (guanosine triphosphate).

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