We have used a two-hybrid approach to test various forms of Saccharomy
ces cerevisiae Ras2p for their ability to interact with the human guan
ine nucleotide exchange factor HGRF55. We have previously shown that a
strong two-hybrid interaction is found between the HGRF55p and the do
minant negative Ras2p(G22A) form of ras [Camus et al, (1995) Oncogene
11, 951-959], We show here that the substitution N123I which weakens t
he guanine nucleotide binding also promotes ras-GEF interaction, We de
monstrate that the R80D substitution alone completely abolishes the in
teraction of Ras2p(G22A) with GEF, whereas substitutions at positions
81, 82 and 73 have only small effects, Since residue 73 is involved in
the response of ras to GEF, we propose that it plays a role in the co
nformational change induced by the GEF rather than in its binding, Tho
se results emphasize the role of the alpha 2 helix of the switch II re
gion in the recognition of the GEF family. (C) 1997 Federation of Euro
pean Biochemical Societies.