Direct Analysis of the Binding of Src-Homology 2 Domains of Phospholipase C to the Activated Epidermal Growth Factor Receptor

A number of proteins involved in intracellular signaling contain regions of homology to the product of the src oncogene that are termed Src-homology (SH) 2 domains. SH2 domains are believed to mediate the association of these proteins with various tyrosine-phosphorylated receptors in a growth factor-dependent manner. We have examined the kinetic characteristics of one of these interactions, the binding of the SH2 domains of phospholipase Cγ1 with the receptor for epidermal growth factor (EGF). Bacterial fusion proteins were prepared containing the two SH2 domains of PLCγ1 and labeled metabolically with [35S]methionine/cysteine. A fusion protein containing both SH2 domains bound to the purified EGF receptor from EGF-treated cells, whereas no binding to receptors from control cells was detected. Binding was rapid, reaching apparent equilibrium by 10 min. Dissociation of the complex occurred only in the presence of excess unlabeled SH2 protein and exhibited two kinetic components. Similarly, analysis of apparent equilibrium binding revealed a nonlinear Scatchard plot, further indicating complex binding kinetics that may reflect cooperative behavior. The binding of the fusion protein containing both SH2 domains was inhibited by a fusion protein containing only the amino-terminal SH2 domain, although at concentrations an order of magnitude higher than that observed with the complete fusion protein. Fusion proteins containing SH2 domains from the GTPase-activating protein, the p85 regulatory subunit of phosphatidylinositol 3'-kinase, or the Abl oncoprotein competed less effectively. Binding of the PLCγ1 SH2 fusion protein to a mutant EGF receptor lacking the two carboxyl-terminal tyrosine phosphorylation sites exhibited a significantly lower affinity than that observed with the wild type, suggesting that this region of the receptor may play an important role. This binding assay represents a means with which to evaluate the pleiotropic nature of growth factor action.