SH2 and SH3 domains: From structure to function

Abstract

Tony Pawson and Gerald D. Gish Division of Molecular and Developmental Biology Samuel Lunenfeld Research Institute Mt. Sinai Hospital Toronto, Ontario M5G 1X5 Canada Growth factors that function by activating receptors with protein-tyrosine kinase activity can elicit a wide range of biochemical responses, leading ultimately to changes in the proliferation, architecture, differentiation, and metabo- lism of their target cells. The means by which receptor tyrosine kinases select their targets and thereby stimulate specific intracellular signaling pathways has been illumi- nated by the identification of a conserved protein module of approximately 100 amino acids, the Src homology 2 (SH2) domain, which is found in a remarkably diverse group of cytoplasmic signaling proteins (Figure 1) and plays a pivotal role in their interactions with receptor tyro- sine kinases (Koch et al., 1991). Proteins with SH2 do- mains frequently possess a distinct sequence of about 50 residues, the SH3 domain, which is also implicated in the regulation of protein-protein interactions during signal transduction (see Clark et al., 1992, and references therein). Upon binding of an external ligand, growth factor recep- tors undergo dimerization, followed by the autophosphory- lation of specific tyrosine residues. Receptor autophos- phorylation acts as a molecular switch to create binding sites for the SH2 domains of cytoplasmic signaling pro- teins (Anderson et al., 1990), which thereby become tar- gets for activation. SH2 domains directly recognize phos- photyrosine (Matsuda et al., 1990). However, high affinity binding of an SH2 domain requires that the phosphotyro- sine be embedded within a specific amino acid sequence, as originally suggested by an examination of SH2-binding sites (Cantley et al., 1991). For example, the SH2-con- taining proteins phosphatidyfinositol (PI) 3’-kinase, the Ras GTP-ase-activating protein, and phospholipase Cy each bind to different autophosphorylated sites on the f3 recep- tor for platelet-derived growth factor (Kashishian et al., 1992; Fantl et al., 1992). Inhibition studies using small phosphotyrosine-contain- ing peptides have shown that molecules containing only 5 residues can effectively block SHBmediated binding to activated receptors (Fantl et al., 1992). The residues im- mediately C-terminal to the phosphotyrosine, especially those at the +l and +3 positions, appear to provide selec- tivity for specific SH2 domains. The dissociation constants for the interactions of SH2 domains with activated recep- tors or phosphotyrosine-containing peptides of optimal se- quence are generally estimated to be in the low nanomolar range, whereas binding to phosphotyrosine alone is much weaker (e.g., Mayer et al., 1992). In principle then, the ability of a given receptor to bind specific SHP-containing proteins (and thereby activate the corresponding signaling