Inhibition of SDC25 C-domain-induced guanine-nucleotide exchange by guanine ring binding domain mutants of v-H-ras.

Guanine-nucleotide exchange is the reaction that controls the activation of H-ras p21. This reaction is stimulated by the guanine-nucleotide exchange factor. In this study we show that H-ras p21 harboring guanine ring binding domain (the conserved NKXD motif) mutations, such as N116I or K117E, are potent inhibitors of H-ras p21 guanine-nucleotide exchange reaction promoted by SDC25C (Saccharomyces cerevisiae SDC25 C-domain gene product), a guanine-nucleotide exchange factor. The inhibition is due to the formation of a stable but catalytically inactive complex consisting of the H-ras mutant and SDC25C. By examining the interaction of v-H-ras(N116I) or v-H-ras(K117E) with SDC25C at different concentrations of guanine-nucleotide, we demonstrate that the mechanism of SDC25C-promoted guanine-nucleotide exchange proceeds through the following pathway. First, SDC25C binds to H-ras and forms an intermediate H-ras.SDC25C complex; subsequently, an incoming guanine-nucleotide dissociates the complex, releasing SDC25C from H-ras and causes guanine-nucleotide exchange. This mechanism is similar to the one proposed for Escherichia coli elongation factor Ts-catalyzed guanine-nucleotide exchange.