Binding of EGF to cells expressing human EGF receptor stimulated rapid tyrosine phosphorylation of phospholipase C-II (PLC-II), as revealed by immunoblotting analysis with phosphotyrosine-specific antibodies. Tyrosine phosphorylation of PLC-II was stimulated by low physiological concentrations of EGF (1 nM), was quantitative, and was already maximal after a 30 sec incubation with 50 nM EGF at 37 degrees C. Interestingly, antibodies specific for PLC-II were able to coimmunoprecipitate the EGF receptor and antibodies against EGF receptor also coimmunoprecipitated PLC-II. According to this analysis, approximately 1% of EGF receptor molecules were associated with PLC-II molecules. The protein tyrosine kinase inhibitor tyrphostin RG50864, which blocks EGF-dependent cell proliferation, blocked EGF-induced tyrosine phosphorylation of PLC-II, its association with EGF receptor, and EGF-induced Ca2+ release. Hence, EGF-induced tyrosine phosphorylation of PLC-II may be a regulatory event linking the tyrosine kinase activity of EGF receptor to the PIP2 hydrolysis signaling pathway.

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Abstract. We have previously shown that an active epidermal growth factor receptor (EGF-R) kinase is necessary for efficient sorting of the EGF-R to the lysosome, and we have shown that this occurs in the multivesicular body (MVB), where EGF-R are sorted away from recycling receptors by being removed to the internal vesicles of the MVB. The aim of the present study was to identify substrates of the EGF-R kinase associated with MVBs which might play a role in this sorting process. We used a density shift technique to isolate MVBs and show that the major substrates pbosphorylated in vitro within MVBs which contain an active EGF-R kinase are the EGF-R itself and annexin I. Annexin I is associated with both plasma membrane and MVBs in a calcium-independent manner but can be phosphorylated in vitro only in MVBs. Phosphorylation of calcium-independent annexin I in isolated MVBs converts it to a form that requires calcium for membrane association. In cells with an active EGF-R kinase the amount of calcium-independent annexin I in MVBs is reduced, suggesting that a phosphorylationinduced conversion of the calcium independent to the calcium-dependent form also occurs in vivo. Our observations, together with the known properties of annexin I in mediating membrane fusion, suggest that inward vesiculation in MVBs is induced by the EGF-R and is mediated by phosphorylated annexin I.

Abstract. We have tested the effects of an mAb directed against the protein core of the extracellular domain of the human EGF receptor (mAbl08), on the binding of EGF, and on the early responses of cells to EGF presentation. We used NIH 3T3 cells devoid of murine EGF receptor, transfected with a eDNA encoding the full-length human EGF receptor gene, and fully responsive to EGE The binding to saturation of mAbl08 to the surface of these cells at 4°C and at other temperatures specifically reduced high-affinity binding of EGF, but did not change the dissociation constant or the estimated number of binding sites for low-affinity binding of EGE The kinetics of EGF binding to the transfected cells were measured to determine the effects of the mAb on the initial rate of EGF binding at 37°C. Interestingly, high-affinity EGF receptor bound EGF with an intrinsic on-rate constant 40-fold higher (9.8 x 106 M-~-s -~) than did low-affinity receptor (2.5 × 105 M-Ls-~), whereas the off-rate constants, measured at 4°C were similar. Cells treated with the mAb or with phorbol myristate acetate displayed single on-rate constants similar to that for the low-affinity receptors.At low doses of EGF ranging from 0.4 to 1.2 nM, pretreatment of cells with mAbl08 inhibited by 50-100% all of the early responses tested, including stimulation of tyrosine-specific phosphorylation of the EGF receptor, turnover of phosphatidyl inositol, elevation of cytoplasmic pH, and release of Ca 2+ from intracellular stores. At saturating doses of EGF (20 nM) the inhibition of these early responses by prebinding of mAbl08 was overcome. On the basis of these results, we propose that the high-affinity EGF receptors are necessary for EGF receptor signal transduction.

src homology 2 (SH2) domains of intracellular signaling molecules such as phospholipase C-y and phosphatidylinositol 3'-kinase-associated protein p85 represent recognition motifs for specific phosphotyrosinecontaining regions on activated growth factor receptors. The binding of SH2 domains to activated growth factor receptors controls the interaction with signaling molecules and the regulation of their activities. In this report, we describe the kinetic parameters and binding affinities of SH2 domains of p85 toward short phosphotyrosine-containing peptides with the amino acid sequence motif YMXM, derived from a major insulin receptor substrate, IRS-1, by using real time biospecific interaction analysis (BlAcore). Associations were specific and of very high affinity, with dissociation constants of 0.3 to 3 nM, between phosphopeptides and the two separate SH2 domains contained within p85. Nonphosphorylated peptides showed no measurable binding, and the interactions were specific for the primary sequence very close to the phosphotyrosine residue. Moreover, the interactions between phosphopeptides and SH2 domains of other signaling molecules were of much lower affinity. Interestingly, the binding of the SH2 domains to the tyrosine-phosphorylated peptides was of high affinity as a result of a very high on rate, of 3 X 107 to 40 x 107/M/s; at the same time, the rate of dissociation, of 0.11 to 0.19/s, was rapid, allowing for rapid exchange of associating proteins with the tyrosine phosphorylation sites.Activation of receptors with tyrosine kinase activity by growth factors promotes tyrosine autophosphorylation and associations with proteins containing src homology 2 (SH2) domains (3,9,19,22,39,43), such as phospholipase C-y (PLC--y) (24,26,44), GTPase-activating protein (GAP) of ras (1,16,23,29), phosphatidylinositol (PI) 3'-kinase-associated p85 (6, 13-15, 32, 41), and GRB2 (21). Specific regions containing phosphorylated tyrosine within platelet-derived growth factor (PDGF), epidermal growth factor (EGF), and fibroblast growth factor (FGF) receptors which are responsible for binding to SH2 domains of signaling molecules have been identified (13-15, 23, 28, 37). The functional significance of these interactions is not fully understood; however, the ubiquity of the interactions suggests that they serve to mediate phosphorylation events crucial for the control of activity of signaling molecules by both receptor-linked and cytoplasmic tyrosine kinases (3, 4, 9-12, 17, 20, 29-31, 36). Indeed, it has been recently shown that FGF-induced PI hydrolysis is eliminated by a point mutation of the FGF receptor at Y-766, which prevents the association with and tyrosine phosphorylation of PLC--y (27,28,33). Similarly, elimination of Y-740 and Y-751 of the PDGF receptor abolishes the binding of p85 and stimulation of PI 3'-kinase activity (13). Important in defining functional significance will be quantitative measures of the kinetics and strength of these interactions.We have used a new technique for real-time kinetics and affinit...

We have tested one aspect of the allosteric dimerization model for the activation of EGF receptor (EGFR) by EGF: whether EGF binding favors dimerization of the receptor. For this to be true, EGF molecules must bind with higher affinity to dimeric receptors than to monomeric receptors. We have tested this directly in a defined system using the soluble, extracellular ligand binding domain of EGFR monomers (sEGFR) and sEGFR dimers stabilized by treatment with a covalent cross-linking agent. We describe real-time kinetic measurements of EGF binding to receptor monomers and dimers employing the method of total internal reflection (surface plasmon resonance). Our data show that sEGFR dimers bound EGF with 30-40-fold higher affinity [KD = (2-3) x 10(-8) M] than did sEGFR monomers. The enhanced binding affinity of sEGFR dimers resulted mainly from a reduced off-rate with k(off) = 0.001 s-1 for sEGFR dimers as compared to k(off) = 0.06 s-1 for sEGFR monomers. These measurements indicate that dimerization of sEGFR increases its affinity for EGF by prolonging the amount of time that EGF remains bound to the receptor. This provides evidence that EGF binding stabilizes receptor dimerization and provides further support for the allosteric dimerization model as a mechanism for ligand induced receptor activation.

Combinatorial libraries employing the one-bead-one-compound technique are reviewed. Two distinguishing features characterize this technique. First, each compound is identified with a unique solid support, enabling facile segregation of active compounds. Second, the identity of a compound on a positively reacting bead is elucidated only after its biological relevance is established. Direct methods of structure identification (Edman degradation and mass spectroscopy) as well as indirect "coding" methods facilitating the synthesis and screening of nonpeptide libraries are discussed. Nonpeptide and "scaffold" libraries, together with a new approach for the discovery of a peptide binding motif using a "library of libraries," are also discussed. In addition, the ability to use combinatorial libraries to optimize initially discovered leads is illustrated with examples using peptide libraries.