IDENTIFICATION OF RESIDUES OF THE H-RAS PROTEIN CRITICAL FOR FUNCTIONAL INTERACTION WITH GUANINE-NUCLEOTIDE EXCHANGE FACTORS

Ras proteins are activated in vivo by guanine nucleotide exchange fact ors encoded by genes homologous to the CDC25 gene of Saccharomyces cer evisiae. We have taken a combined genetic and biochemical approach to probe the sites on Ras proteins important for interaction with such ex change factors and to further probe the mechanism of CDC25-catalyzed G DP-GTP exchange. Random mutagenesis coupled with genetic selection in S. cerevisiae was used to generate second-site mutations within human H-ras-ala15 which could suppress the ability of the Ala-15 substitutio n to block CDC25 function. We transferred these second-site suppressor mutations to normal H-ras and oncogenic H-ras(Val-12) to test whether they induced a general loss of function or whether they selectively a ffected CDC25 interaction. Four highly selective mutations were discov ered, and they affected the surface-located amino acid residues 62, 63 , 67, and 69. Two lines of evidence suggested that these mutants canno t bind CDC25: (i) using the yeast two-hybrid system, we demonstrated t hat these mutants cannot bind CDC25 under conditions where the wild-ty pe H-Ras protein can: (ii) we demonstrated that the binding to H-Ras o f monoclonal antibody Y13-259, whose epitope has been mapped to residu es 63, 65, 66, 67, 70, and 73, is blocked by the mouse sos1 and yeast CDC25 gene products. We also present evidence that the mechanism by wh ich CDC25 catalyzes exchange is more involved than simply catalyzing t he release of bound nucleotide and passively allowing nucleotides to r ebind. Most critically, a complex of Ras and CDC25 protein, unlike fre e Ras protein, possesses significantly greater affinity for GTP than f or GDP. Furthermore, the Ras CDC25 complex is more readily dissociated into free subunits by GTP than for GDP. Furthermore, the Ras CDC25 co mplex is more readily dissociated into free subunits by GTP than it is by GDP. Both of these results suggest a function for CDC25 in promoti ng the selective exchange of GTP for GDP.