ORIENTATION OF PEPTIDE-FRAGMENTS FROM SOS PROTEINS BOUND TO THE N-TERMINAL SH3 DOMAIN OF GRB2 DETERMINED BY NMR-SPECTROSCOPY

NMR spectroscopy has been used to characterize the protein-protein interactions between the mouse Grb2 (mGrb2) N-terminal SH3 domain complexed with a 15-residue peptide (SPLLPKLPP-KTYKRE) corresponding to residues 1264-1278 of the mouse Sos-2 (mSos-2) protein. Intermolecular interactions between the peptide and C-13-N-15-labeled SH3 domain were identified in half-reverse-filtered 2D and 3D NOESY experiments. Assignments for the protons involved in interactions between the peptide and the SH3 domain were confirmed in a series of NOESY experiments using a set of peptides in which different leucine positions were fully deuterated. The peptide ligand-binding site of the mGrb2 N-terminal SH3 domain is defined by the side chains of specific aromatic residues (Tyr(7), Phe(9), TrP(36), Tyr(52)) that form two hydrophobic subsites contacting the side chains of the peptide Leu(4) and Leu(7) residues. An adjacent negatively charged subsite on the SH3 surface is likely to interact with the side chain of a basic residue at peptide position 10 that we show to be involved in binding. The peptide-binding site of the SH3 is characterized by large perturbations of amide chemical shifts when the peptide is added to the SH3 domain. The mGrb2 N-terminal SH3 domain structure in the complex is well-defined (backbone RMSD of 0.56+/-0.21 calculated over the backbone N, C alpha, and C atoms of residues 1-54). The structure of the peptide in the complex is less well-defined but displays a distinct orientation. Relative to the SH3 domain binding site, the polypeptide backbone of the peptide in the complex is positioned in the opposite orientation compared to that recently described for the synthetic peptide (RKLPPRP)-PI3K SH3 domain complex [Yu et al. (1994) Cell 76, 933-945]. The mSos peptide-mGrb2 N-terminal SH3 complex has a dissociation constant of 54 mu M as determined by isothermal titration calorimetry. Another mSos peptide (VPPPVPPRRR) exhibits tighter binding (3.5 mu M dissociation constant) with the N-terminal Grb2 SH3 domain and appears to bind in a similar manner on the basis of NOESY experiments.