C T I O N
Src Homology 2 (SH2) domains are protein modules of about 100 amino acids in size which are found in a large number of proteins involved in signal transduction. The function of SH2 domains is to specifically recognize the phosphorylated state of tyrosine residues, thereby allowing SH2 domain-containing proteins to localize to tyrosine-phosphorylated sites. An example of signal transduction mediated by SH2 domains is shown in Figure . In this example, a receptor is activated by the binding of an extracellular ligand. This binding event induces activation of kinase activities on the other side of the membrane, resulting in specific phosphorylation on tyrosine residues located in the intracellular domains of the receptor. This process constitutes the fundamental event of signal transduction through a membrane, in which a signal in the extracellular compartment is "sensed" by a receptor and is converted in the intracellular compartment to a different chemical form, that is, that of a phosphorylated tyrosine. Tyrosine phosphorylation leads to activation of a cascade of protein-protein interactions whereby SH2 domain-containing proteins are recruited to tyrosine-phosphorylated sites. This process initiates a series of events which eventually result in altered patterns of gene expression or other cellular responses.
Receptor activation usually results in several phosphorylation events, which in turn leads to activation of several signal transduction pathways. Misreading of phosphorylated sites by SH2 domains would lead to recruitment of inappropriate SH2 domain-containing proteins to the receptor and hence to undesirable activation of pathways. It was established several years ago that specificity is conferred by the sequence context of the phosphotyrosine within the tyrosine-phosphorylated site, and, more specifically, by the three residues immediately C-terminal to the phosphotyrosine. The peptide-binding specificity of a large number of SH2 domains was then investigated using libraries of peptides phosphorylated on a tyrosine residue and randomized at the ϩ1, ϩ2, and ϩ3 positions C-273