The modular SH2 domain is found embedded in a wide variety of metazoan proteins that regulate functionally diverse processes. The figure below indicates the domain organization of representative members from various SH2 domain containing protein families. (Reference: Pawson, T., Gish, G., and Nash, P. Trends in Cell Biology Vol.11 No.12 December 2001)

Conventional SH2 domains must achieve something of a balancing act. Their affinity for an unphosphorylated site must not be too high, or binding will not be regulated by phosphorylation. At the same time, the SH2 domain must obtain sufficient binding energy from the recognition of adjacent residues to allow discrimination between different phosphorylated sites, and thus a degree of specificity. Furthermore, SH2 domains must exhibit sufficiently high off-rates for rapid and reversible signal transduction. A structural basis for the specificity of SH2 domain-mediated interactions has been provided by numerous crystal and solution structures of SH2 domains bound to specific phosphotyrosine-containing peptides (sKuriyan J, Cowburn D. Annu Rev Biophys Biomol Struct 1997. 26, 259-288).

The SH2 domain fold, which is composed of a central anti-parallel b-sheet sandwiched between two a-helices, provides a positively charged pocket on one side of the b-sheet for binding of the ligands phosphotyrosine moiety, and an extended surface on the other for binding to ligand residues C-terminal to the phosphotyrosine.

Ribbons diagram of the SH2-C domain of phospholipase-C( bound to a specific phosphotyrosine-containing peptide. The peptide (gold) is derived from the cytoplasmic domain of the PDGF receptor (DNDpYIPLPDPK) derived from PTB:2PLD (Pascal SM, Singer AU, Gish G et al. Nuclear magnetic resonance structure of an SH2 domain of phospholipase C-gamma1 complexed with a high affinity binding peptide. Cell 1994. 77, 461-472.).

Subtle differences in the molecular architecture of this extended surface can have very significant effects on ligand-binding specificity. For example, mutation of a threonine residue in this region of the Src SH2 domain to tryptophan, found at the corresponding position of the Grb2 SH2 domain, converted ligand-binding specificity from the Src-like pTyr-Glu-Glu-Ile, to the signature Grb2 binding motif pTyr-X-Asn (Kimber MS, Nachman J, Gish G, Pawson T, Pai E. Molecular Cell 2000. 5, 1043-1049). This mutant Src SH2 domain behaves biologically like a Grb2 SH2 domain, and not like a Src SH2 domain, indicating that biological activity tracks with biochemical binding specificity.

Structural analysis provides a basis for understanding SH2 domain binding specificity. Crystal structures of the Grb2 and Src SH2 domains bound to optimal phosphopeptides illustrate the different modes of ligand binding. A switch in the ligand preference of the Src SH2 domain to pTyr-Val-Asn-Val is achieved through mutation of a threonine residue, highlighted in blue on the surface of the Src SH2, to the tryptophan shown as red in the Src T215W SH2 domain. In a similar fashion, the molecular architecture of the Grb2 SH2 domain uses a tryptophan (red) to define its binding to pTyr-X-Asn ligands. (For more details see Pawson, T., Gish, G., and Nash, P. Trends in Cell Biology Vol.11 No.12 December 2001)