NONPHOSPHORYLATED PEPTIDE LIGANDS FOR THE GRB2 SRC HOMOLOGY-2 DOMAIN

Critical intracellular signals in normal and malignant cells are trans mitted by the adaptor protein Grb2 by means of its Src homology 2 (SH2 ) domain, which binds to phosphotyrosyl (pTyr) residues generated by t he activation of tyrosine kinases, To understand this important contro l point and to design inhibitors, previous investigations have focused on the molecular mechanisms by which the Grb2 SH2 domain selectively binds pTyr containing peptides, In the current study, we demonstrate t hat the Grb2 SH2 domain can also bind in a pTyr independent manner, Us ing phage display, an 11-amino acid cyclic peptide, G1, has been ident ified that binds to the Grb2 SH2 domain but not the src SH2 domain, Sy nthetic G1 peptide blocks GrbP SH2 domain association (IC50 10-25 mu M ) with a g-amino acid pTyr-containing peptide derived from the SHC pro tein (pTyr317), These data and amino acid substitution analysis indica te that G1 interacts in the phosphopeptide binding site, G1 peptide re quires a YXN sequence similar to that found in natural pTyr-containing ligands, and phosphorylation of the tyrosine increases G1 inhibitory activity. G1 also requires an internal disulfide bond to maintain the active binding conformation, Since the G1 peptide does not contain pTy r, it defines a new type of SH2 domain binding motif that may advance the design of Grb2 antagonists.