Identification of guanine exchange factor key residues involved in exchange activity and Ras interaction.

We have carried out a functional analysis of the human HGRF55 exchange factor in the yeast Saccharomyces cerevisiae. Twelve residues conserved among most of all known guanine exchange factors (GEFs) have been independently changed to alanine. Taking advantage of the ability of Hgrf55p to replace the yeast Cdc25p exchange factor, and using the two-hybrid system with RAS2ala22 allele, we have identified key residues for the interaction with Ras and/or its activation. Substitution of arginine 392 to alanine leads to a complete loss of interaction with Ras, though the protein remains stable. Substitution of Asp266 or Arg359 to alanine results in inactive proteins at 39 degrees C, still able however to interact with Ras. The other charged-to-alanine substitutions led to no detectable phenotype when present alone but most of them dramatically increased the temperature sensitive phenotype observed with [Asp266Ala] substitution. Surprisingly, the cysteine to alanine substitution in the highly conserved PCVPF/Y motif proved to be without effect, suggesting that the sulfhydryl group is not essential for stability or interaction with Ras.