Most of our recent growth regulation research has been concerned with the ras oncogene and its positive regulators, the Ras-specific guanine nucleotide exchange factors (Ras-GNEFs). These molecules play a key role in signal transduction and ras is mutationally activated in many human and animal tumors. The analysis of two closely related, widely expressed Ras-GNEFs, sos1 and sos2, has identified important differences in the stability of their encoded proteins and in the duration of their signaling properties. The Sos2 protein carries out long-term signaling less efficiently than Sos1 to a degree that physiologic levels of Sos2 cannot support transformation by oncoproteins such as tyrosine kinases, which rely on Sos-dependent activation of Ras for their transforming activity. In addition, the Sos2 protein is a physiologic substrate for degradation by the ubiquitin-proteasome system, while Sos1 is not subject to such regulation. Other studies on signaling through Sos indicate that coordinate signals at the N-terminus and C-terminus of the protein are required for its activation. We are also studying GRF1 and GRF2, two closely related brain-specific Ras-GNEFs, that are activated by calcium. We have found that the calcium-dependent activation of GRF stimulates Raf via two signals, one that is Ras-dependent and another that is Ras-independent. Also, GRF1 and GRF2 form homo- and hetero-dimers through their Dbl domains. Dimerization may be required for propagating the Ras-independent signal from GRF that stimulates Raf. Further studies are planned to elucidate the mechanisms underlying the regulation of Sos and GRF signaling.
