The formation of specific protein—protein interactions is one of the key mechanisms for signal transduction mediated by tyrosme phosphorylation. These intermolecular interactions target signaling proteins to particular cellular locations and modulate the enzymatic activities that further propagate the signal. A distinctive characteristic of the pathways that are mitiated by tyrosme phosphorylation is that target recognition and catalytic activity are usually functions of separate domains within the signaling molecules that participate in these pathways. Each of the signaling molecules contains one or more of a set of modular peptide-binding domains that are responsible for generating protein-protein interactions. Such peptide-recognition domains are modular in both structural and functional respects: They are capable of folding correctly when removed from the parent protein, and they can usually recognize their targets even when isolated.
