Ras proteins are small G proteins
playing a major role in eukaryotic signal transduction. Guanine nucleotide
exchange factors (GEF) stimulate GDP/GTP exchange, resulting in the formation
of the active Ras-GTP complex. In mammalian cells,
two major Ras-specific GEF exist: Sos-like and
Cdc25-like. To date, structural data are available only for Cdc25(Mm).
We designed and synthesized Cdc25(Mm)-derived peptides
spanning residues corresponding to the hSos1 HI helical hairpin that has been
implicated in the GEF catalytic mechanism. NMR experiments on a chemically
synthesized Cdc25(Mm)(1178)(-)(1222) peptide proved
that helix I readily reaches a conformation very similar to the corresponding
helix in hSos1, while residues corresponding to helix H in hSos1 show higher
conformational flexibility. Molecular dynamics studies with the appropriate
solvent model showed that different conformational spaces are available for the
peptide. Since helix H is making several contacts with Ras
and a Cdc25(Mm)(1178)(-)(1222) peptide is able to bind
nucleotide-free Ras in a BIAcore
assay, the peptide must be able to obtain the proper Ras-interacting
conformation, at least transiently. These results indicate that rational design
and improvement of the Ras-interacting peptides
should take into account conformational and flexibility features to obtain
molecules with the appropriate biochemical properties.