The Src homology domains SH2 and SH3 are modular components present in many signal transduction proteins. They allow rapid formation of stable protein complexes and may also regulate protein function through intramolecular binding events. SH2 domains recognize phosphotyrosyl residues in a specific sequence context, while SH3 domains recognize a PxxP motif and additional residues that mediate binding specificity. the SH2 domain. Positively charged amino acids in this pocket bond with the negatively charged phosphate group and the tyrosyl ring. The charge interactions with the phosphate provide a significant portion of the total protein-protein binding energy, since identical unphosphorylated protein sequences bind very poorly to SH2 domains. Specificity is created through additional contacts with residues in close proximity to the phosphotyrosine. The analysis of naturally occurring SH2-binding sites, screening of phosphopeptide libraries with SH2 domains and structural studies by nuclear magnetic resonance spectroscopy and X-ray crystallography have given much insight into the fine details of SH2 interactions. Currently, two groups of SH2 domains can be distinguished. The first group, represented by the SH2 domains of the protein tyrosine kinase (PTK) Src and the adaptor protein Grb2 (Fig. 1), has an ad~.~'.'ional socket which primarily binds Nck a single amino acid and mediates much of the binding specificity. It is therefore possible to change the specificity of the Src SH2 domain to that of the Grb2 SH2 domain by changing a single amino acid in the Src

SH2 domain 6. The second group, represented by the carboxy-terrninal SF! 2 domain of phospholipase C~/t and the ainino-terininal SH2 domain of the protein tyrosine phosphatase (PTP) Syp 7, contains, in addition to the phosphotyrosine~ binding pocket, a shallow valley on the surface of the SH2 domain. Into this valley fits the polypeptide chain of the specific binding partner and multiple, mostly hydrophobic, sidechain