During the past few years, intensive research has focused on elucidating the function of two polypeptide domains known as SH2 and SH3 (for syc Lomology). These two domains were originally recognized in the non-catalytic regions of non-receptor tyrosine kinases('). It was then discovered that a class of cytoplasmic proteins that complex with ligand-stimulated rece tor tyrosine kinases contain SH2 and SH3 activating protein (GAP) of p21"", phospholipase C-y, and the 85 kD subunit of phosphatidylinositol-3'kinase. GAP functions to stimulate the intrinsic GTPase activity of p21"', returning ras to its inactive GDP-bound state. Phospholipase C-y catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate to diacylglycerol and inositol triphosphate, two second messengers involved in the activation of protein kinase C and in the mobilization of intracellular calcium, respectively. The function of the 85 kD subunit of phosphatidylinositol-3'-kinase is less clear, but one proposed role will be discussed later. It has since been discovered that some of the non-receptor tyrosine kinases that possess SH2 and SH3 domains also complex with receptor tyrosine kinasesc'). In many instances, these SH2jSH3-containing proteins serve as substrates of the ligand-stimulated receptor with which they associate. and are activated as a consequence of this interaction. Their enzymatic activities and their associations with receptor tyrosine kinases have implicated these molecules as key regulatory components of cell signalling pat1iways(2).
The association of these proteins with receptor tyrosine kinases occurs via their SH2 d o m a d 3 ) . Sitedirected mutagenesis and biochemical studies have revealed that these SH2-containing proteins bind phosphotyrosine residues in the context of autophosphorylated receptors. Non-tyrosine-phosphorylated receptors or receptors mutationally-altered at specific tyrosine residues bind these SH2-containin proteins that upon ligand binding, a receptor tyrosine kinase dimerizes and trans-phosphorylates each receptor half.