Structure Determination and Dynamics of Peptides Overlapping the Catalytic Hairpin of the Ras-Specific GEF Cdc25Mm

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 Cdc25Mm.We designed and synthesized Cdc25Mm-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 Cdc25Mm 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 Cdc25Mm 1178-1222 peptide is able to bind nucleotidefree 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.