A novel phosphotyrosine mimetic 4'-carboxymethyloxy-3'-phosphonophenylalanine (Cpp): exploitation in the design of nonpeptide inhibitors of pp60(Src) SH2 domain

The novel phosphotyrosine (pTyr) mimetic 4'-carboxymethyloxy-3'-phosphonophenylalanine (Cpp) has been designed and incorporated into a series of nonpeptide inhibitors of the SH2 domain of pp60(c-Src) (Src) tyrosine kinase. A 2.2 A X-ray crystal structure of 1a bound to a mutant form of Lck SH2 domain provides insight regarding the structure-activity relationships and supports the design concept of this new pTyr mimetic.     

Selective inhibition of Src SH2 by a novel thiol-targeting tricarbonyl-modified inhibitor and mechanistic analysis by (1)H/(13)C NMR spectroscopy.

Detailed analysis of Src SH2 binding by peptides containing a novel tricarbonyl-modified pTyr moiety is described. We envisaged that Src SH2 selectivity might be obtained by exploiting the thiol group of Cys188 present in the pTyr binding pocket of the protein at the betaC3 position. Peptidyl as well as non-peptidyl compounds 1-4 possessing a 4-alpha,beta-diketoester-modified pTyr mimic exhibited micromolar affinity to Src SH2. Furthermore, these tricarbonyl compounds were selective for Src SH2 to the extent they showed no significant affinity for either Cys188Ser or Cys188Ala Src SH2 mutants. Upon closer examination of the binding of these tricarbonyls to Src SH2 using NMR of 13C-labeled compounds (6a, 6b, and 6c), we found that after the initial binding event the molecule disproportionated in a 'retro-Claisen' fashion to provide benzoic acid 16 and, following hydrolysis of the methyl ester 17, the hemiketal adduct of glyoxalic acid 18.     

Investigation of phosphotyrosine recognition by the SH2 domain of the Src kinase.

The binding of tyrosine phosphorylated targets by SH2 domains is required for propagation of many cellular signals in higher eukaryotes; however, the determinants of phosphotyrosine (pTyr) recognition by SH2 domains are not well understood. In order to identify the attributes of pTyr required for high affinity interaction with SH2 domains, the binding of the SH2 domain of the Src kinase (Src SH2 domain) to a dephosphorylated peptide, a phosphoserine-containing peptide, and the amino acid pTyr was studied using titration calorimetry and compared with the binding of a high affinity tyrosyl phosphopeptide. The dephosphorylated peptide and the phosphoserine containing peptide both bind extremely weakly to the Src SH2 domain (DeltaGo (dephosphorylated)=-3.6 kcal/mol, DeltaGo (phosphoserine) >-3.7 kcal/mol); however, the DeltaGo value of pTyr binding is more favorable (-4.7 kcal/mol, or 50 % of the entire binding free energy of a high affinity tyrosyl phosphopeptide). These results indicate that both the phosphate and the tyrosine ring of the pTyr are critical determinants of high affinity binding. Alanine mutagenesis was also used to evaluate the energetic contribution to binding of ten residues located in the pTyr-binding site. Mutation of the strictly conserved Arg betaB5 resulted in a large increase in DeltaGo (DeltaDeltaGo=3.2 kcal/mol) while elimination of the other examined residues each resulted in a significantly smaller (DeltaDeltaGo<1.4 kcal/mol) reduction in affinity, indicating that Arg betaB5 is the single most important determinant of pTyr recognition. However, mutation of Cys betaC3, a residue unique to the Src SH2 domain, surprisingly increased affinity by eightfold (DeltaDeltaGo=-1.1 kcal/mol). Using a double mutant cycle analysis, it was revealed that residues of the pTyr-binding pocket are not coupled to the peptide residues C-terminal to the pTyr. In addition, comparison of each residue's DeltaDeltaGo value upon mutation with that residue's sequence conservation among SH2 domains revealed only a modest correlation between a residue's energetic contribution to pTyr recognition and its conservation throughout evolution. The results of this investigation highlight the importance of a single critical interaction, the buried ionic bond between the phosphate of the pTyr and Arg betaB5 of the SH2 domain, driving the binding of SH2 domains to tyrosine phosphorylated targets.     

A phosphotyrosine displacement mechanism for activation of Src by PTPalpha

Protein tyrosine phosphatase alpha (PTPalpha) is believed to dephosphorylate physiologically the Src proto-oncogene at phosphotyrosine (pTyr)527, a critical negative-regulatory residue. It thereby activates Src, and PTPalpha overexpression neoplastically transforms NIH 3T3 cells. pTyr789 in PTPalpha is constitutively phosphorylated and binds Grb2, an interaction that may inhibit PTPalpha activity. We show here that this phosphorylation also specifically enables PTPalpha to dephosphorylate pTyr527. Tyr789-->Phe mutation abrogates PTPalpha-Src binding, dephosphorylation of pTyr527 (although not of other substrates), and neoplastic transformation by overexpressed PTPalpha in vivo. We suggest that pTyr789 enables pTyr527 dephosphorylation by a pilot binding with the Src SH2 domain that displaces the intramolecular pTyr527-SH2 binding. Consistent with model predictions, we find that excess SH2 domains can disrupt PTPalpha-Src binding and can block PTPalpha-mediated dephosphorylation and activation in proportion to their affinity for pTyr789. Moreover, we show that, as predicted by the model, catalytically defective PTPalpha has reduced Src binding in vivo. The displacement mechanism provides another potential control point for physiological regulation of Src-family signal transduction pathways.     

Design of tetrapeptide ligands as inhibitors of the Src SH2 domain

Src homology-2 (SH2) domains are noncatalytic motifs containing approximately 100 amino acid residues that are involved in intracellular signal transduction. The phosphotyrosine-containing tetrapeptide pTyr-Glu-Glu-Ile (pYEEI) binds to Src SH2 domain with high affinity (K(d)=100 nM). The development of five classes of tetrapeptides as inhibitors for the Src SH2 domain is described. Peptides were prepared via solid-phase peptide synthesis and tested for affinity to Src SH2 domain using a fluorescence polarization based assay. All of the N-terminal substituted pYEEI derivatives (class II) presented binding affinity (IC(50)=of 2.7-8.6 microM) comparable to pYEEI (IC(50)=6.5 microM) in this assay. C-Terminal substituted pYEEI derivatives (class III) showed a lower binding affinity with IC(50) values of 34-41 microM. Amino-substituted phenylalanine derivatives (class IV) showed weak binding affinities (IC(50)=16-153 microM). Other substitutions on phenyl ring (class I) or the replacement of the phenyl ring with other cyclic groups (class V) dramatically decreased the binding of tetrapeptides to Src SH2 (IC(50)>100 microM). The ability of pYEEI and several of the tetrapeptides to inhibit the growth of cancer cells were assessed in a cell-based proliferation assay in human embryonic kidney (HEK) 293 tumor cells. The binding affinity of several of tested compounds against Src SH2 domain correlates with antiproliferative activity in 293T cells. None of the compounds showed any significant antifungal activity against Candida albicans ATCC 14053 at the maximum tested concentration of 10 microM. Overall, these results provided the structure-activity relationships for some FEEI and YEEI derivatives designed as Src SH2 domain inhibitors.     

Structural basis for specificity switching of the Src SH2 domain

The Src SH2 domain binds pYEEI-containing phosphopeptides in an extended conformation with a hydrophobic pocket, which includes ThrEF1, binding Ile(pY +3). Mutating ThrEF1 to tryptophan switches specificity to an Asn(pY +2) requirement, yielding a biological mimic of the Grb2 SH2 domain. Here we show that the Src ThrEF1Trp SH2 domain mutant binds pYVNV phosphopeptides in a beta turn conformation, which, despite differing conformations of the interacting tryptophan, closely resembles the native Grb2/pYVNV cognate peptide binding mode. The ThrEF1Trp substitution therefore switches specificity by physically occluding the pTyr +3 binding pocket and by providing additional interaction surface area for Asn(pY +2). This demonstrates structurally how novel SH2 domain specificities may rapidly evolve through single amino acid substitutions and suggests how new signaling pathways may develop.     

N-terminal carboxyl and tetrazole-containing amides as adjuvants to Grb2 SH2 domain ligand binding.

High affinity binding of peptides to Src homology 2 (SH2) domains, often requires the presence of phosphotyrosyl (pTyr) or pTyr-mimicking moieties in the N-terminal position of the binding ligand. Several reports have shown that N(alpha)-acylation of the critical pTyr residue can result in increased SH2 domain binding potency. For Grb2 SH2 domains which recognize pTyr-Xxx-Asn-NH(2) motifs, significant potency enhancement can be incurred by N(alpha)-(3-amino)Z derivatization of tripeptides such as pTyr-Ile-Asn-NH(2). Using ligands based on the high affinity pY-Ac(6)c-Asn-(naphthylpropylamide) motif, (where Ac(6)c=1-aminocyclohexanecarboxylic acid), additional reports have shown moderate potentiating effects of N(alpha)-oxalyl derivatization. The current study examined variations of the N(alpha)-oxalyl theme in the context of a Xxx-Ac(6)c-Asn-(naphthylpropylamide) platform, where Xxx=the hydrolytically stable pTyr mimetics phosphonomethyl phenylalanine (Pmp) or carboxymethyl phenylalanine (Cmf). The effects of N(alpha)-(3-amino)Z derivatization were also investigated for this platform, to ascertain whether the large binding enhancement reported for tripeptides such as pTyr-Ile-Asn-NH(2) could be observed. In ELISA-based extracellular Grb2 SH2 domain binding assays, it was found for the Pmp-based series, that extending the oxalyl carboxyl out by one methylene unit or replacing carboxyl functionality with a tetrazole isostere, resulted in binding potency greater than the parent N(alpha)-acetyl-containing compound, with enhancement approximating that observed for the N(alpha)-oxalyl derivative. When Cmf was used as the pTyr mimetic, only modest differences in IC(50) values were observed for the series. Examination of the N(alpha)-(3-amino)Z derivatized Pmp-Ac(6)c-Asn-(naphthylpropylamide), showed that binding affinity was reduced relative to the parent N(alpha)-acetyl analogue, in contrast to the reported significant enhancement of affinity observed with other peptide ligands. Treatment of MDA-453 tumor cells, which are mitogenically driven through erbB-2 tyrosine kinase-dependent pathways, with Pmp-containing inhibitors resulted in growth inhibition, with the N(alpha)-oxalyl and N(alpha)-malonyl-containing compounds exhibiting IC(50) values (4.3 and 4.6 microM, respectively) approximately five-fold lower than the parent N(alpha)-acetyl-containing compound. Tetrazole and N(alpha)-(3-amino)Z-containing inhibitors were from two- to four-fold less potent than these latter analogues in the growth inhibition assays.     

Solution structure of the SH2 domain of Grb2 complexed with the Shc-derived phosphotyrosine-containing peptide.

The solution structure of growth factor receptor-bound protein 2 (Grb2) SH2 complexed with a Shc-derived phosphotyrosine (pTyr)-containing peptide was determined by nuclear magnetic resonance (NMR) spectroscopy. The pTyr binding site of Grb2 SH2 was similar to those of other SH2 domains. In contrast, the amino acid residues C-terminal to pTyr did not form an extended structure because of steric hindrance caused by a bulky side-chain of Trp121 (EF1). As a result, the peptide formed a turn-structure on the surface of Grb2 SH2. The asparagine residue at the pTyr+2 position of the Shc-peptide interacted with the main-chain carbonyl groups of Lys109 and Leu120. The present solution structure was similar to the crystal structure reported for Grb2 SH2 complexed with a BCR-Abl-derived phosphotyrosine-containing peptide. Finally, the structure of Grb2 SH2 domain was compared with those of the complexes of Src and phospholipase C-gamma1 with their cognate peptides, showing that the specific conformation of the peptide was required for binding to the SH2 domains.     

Development of Grb2 SH2 Domain Signaling Antagonists: A Potential New Class of Antiproliferative Agents

Aberrant signaling through protein-tyrosine kinase (PTK)-dependent pathways is associated with several proliferative diseases. Accordingly, PTK inhibitors are being developed as new approaches for the treatment of certain cancers. Growth factor receptor bound protein 2 (Grb2) is an important downstream mediator of PTK signaling that serves obligatory roles in many pathogenic processes. One of the primary functions of Grb2 is to bind to specific phosphotyrosyl (pTyr)-containing sequences through its Src homology 2 (SH2) domain. Agents that bind to the Grb2 SH2 domain and prevent its normal function could disrupt associated PTK signaling and serve as alternatives to kinase-directed inhibitors. Starting from the X-ray crystal structure of a lead peptide bound to the Grb2 SH2 domain, this review will summarize important contributions to these efforts. The presentation will be thematically arranged according to the region of peptide modified, proceeding from the N-terminus to the C-terminus, with a special section devoted to aspects of conformational constraint.     

Novel mode of ligand binding by the SH2 domain of the human XLP disease gene product SAP/SH2D1A

BACKGROUND: The Src homology 2 (SH2) domains of cytoplasmic signaling proteins generally bind phosphotyrosine (pTyr) sites in the context of carboxy-terminal residues. SAP (also known as SH2D1A or DSHP), the product of the gene that is mutated in human X-linked lymphoproliferative (XLP) disease, comprises almost exclusively a single SH2 domain, which may modulate T-cell signaling by engaging T-cell co-activators such as SLAM, thereby blocking binding of other signaling proteins that contain SH2 domains. The SAP-SLAM interaction can occur in a phosphorylation-independent manner. RESULTS: To characterize the interaction between SAP and SLAM, we synthesized peptides corresponding to the SAP-binding site at residue Y281 in SLAM. Both phosphorylated and non-phosphorylated versions of an 11-residue SLAM peptide bound SAP, with dissociation constants of 150 nM and 330 nM, respectively. SLAM phosphopeptides that were truncated either at the amino or carboxyl terminus bound with high affinity to SAP, suggesting that the SAP SH2 domain recognizes both amino-terminal and carboxy-terminal sequences relative to the pTyr residue. These results were confirmed by nuclear magnetic resonance (NMR) studies on (15)N- and (13)C-labeled SAP complexed with three SLAM peptides: an amino-terminally truncated phosphopeptide, a carboxy-terminally truncated phosphopeptide and a non-phosphorylated Tyr-containing full-length peptide. CONCLUSIONS: The SAP SH2 domain has a unique specificity. Not only does it bind peptides in a phosphorylation-independent manner, it also recognizes a pTyr residue either preceded by amino-terminal residues or followed by carboxy-terminal residues. We propose that the three 'prongs' of a peptide ligand (the amino and carboxyl termini and the pTyr) can engage the SAP SH2 domain, accounting for its unusual properties. These data point to the flexibility of modular protein-interaction domains.     

ATP-phosphopeptide conjugates as inhibitors of Src tyrosine kinases

A number of Src SH2 domain inhibitors enhance the kinase catalytic activity by switching the closed inactive to the open active conformation. ATP-phosphopeptide conjugates were designed and synthesized as Src tyrosine kinase inhibitors based on a tetrapeptide sequence pTyr-Glu-Glu-Ile (pYEEI) and ATP to block the SH2 domain signaling and substrate phosphorylation by ATP, respectively. In general, ATP-phosphopeptide conjugates with optimal linkers such as compounds 5 and 7 (K(i) = 1.7-2.6 microM) showed higher binding affinities to the ATP-binding site relative to the other ATP-phosphopeptide conjugates having short or long linkers, 1-4 and 6, (K(i) = 10.1-16.1 microM) and ATP (K(m) = 74 microM). These ATP-phosphopeptide conjugates may serve as novel templates for designing protein tyrosine kinase inhibitors to block SH2 mediated protein-protein interactions and to counter the activation of enzyme that resulted from the SH2 inhibition.     

Discovery of the catechol structural moiety as a Stat3 SH2 domain inhibitor by virtual screening

The Stat3 SH2 domain is essential for its activation, and development of a potent SH2 inhibitor will be therapeutically valuable in treating cancers with constant Stat3 activation. We report here the identification of the catechol (1,2-dihydroxybenzene) structural moiety by virtual screening as a Stat3 SH2 inhibitor. The catechol compound docked to the Stat3 SH2 domain in computer modeling forms hydrogen bonds with the conserved pTyr-interacting amino acids. In the biochemical assay, a catechol-containing compound, but not the hydroxyl group-acetalized analogue, was able to inhibit Stat3 DNA-binding activity. Furthermore, the catechol compound was demonstrated to compete with pTyr peptides in binding to the Stat3 SH2 domain, suggesting that the catechol moiety is a pTyr bioisostere and may potentially be used for designing cell-permeable SH2 inhibitors. In our preliminary effort, we also demonstrated that the potency of catechol compound as Stat3 SH2 inhibitors could be improved by modifying the non-catechol part of the compound structure.     

Phosphoryltyrosyl mimetics in the design of peptide-based signal transduction inhibitors

The central roles played by protein-tyrosine kinase (PTK)-dependent signal transduction in normal cellular regulation and homeostasis have made inappropriate or aberrant functions of certain of these pathways contributing factors to a variety of diseases, including several cancers. For this reason, development of PTK signaling inhibitors has evolved into an important approach toward new therapeutics. Since in these pathways phosphotyrosyl (pTyr) residues provide unique and defining functions either by their creation under the catalysis of PTKs, their recognition and binding by protein modules such as SH2 and phosphotyrosyl binding (PTB) domains, or their destruction by protein-tyrosine phosphatases, pTyr mimetics provide useful general starting points for inhibitor design. Important considerations in the development of such pTyr mimetics include enzymatic stability (particularly toward PTPs), high affinity recognition by target pTyr binding proteins, and good cellular bioavailability. Although small molecule, nonpeptide inhibitors may be ultimate objectives of inhibitor development, peptides frequently serve as display platforms for pTyr mimetics, which afford useful and conceptually straightforward starting points in the development process. Reported herein is a limited overview of pTyr mimetic development as it relates to peptide-based agents. Of particular interest are recent findings that highlight potential limitations of peptides as display platforms for the identification of small molecule leads. One conclusion that results from this work is that while peptide-based approaches toward small molecule inhibitor design are often intellectually satisfying from a structure-based perspective, extrapolation of negative findings to small molecule, nonpeptide contexts should be undertaken with extreme caution.     

New syntheses of tetrazolylmethylphenylalanine and O-malonyltyrosine as pTyr mimetics for the design of STAT3 dimerization inhibitors

Investigation within the pTyr-binding pocket of the STAT3 SH2 domain led us to develop a novel synthesis of two pTyr mimetics, l-tetrazolylmethylphenylalanine (l-Tmp) and l-O-malonyltyrosine (l-OMT), that were next incorporated in a high affinity ligand of STAT3 SH2 domain. Biological evaluation of peptidomimetics on STAT3 dimerization identified l-OMT as the first non-phosphorus pTyr mimetic so far reported against STAT3 SH2 domain, harboring an activity similar to that of the Pmp-containing reference peptidomimetic.     

Monocarboxylic-based phosphotyrosyl mimetics in the design of GRB2 SH2 domain inhibitors

Three monocarboxylic-containing analogues, O-carboxymethyltyrosine (cmT, 5), 4-(carboxymethyl)phenylalanine (cmF, 6), and 4-(carboxydifluoromethyl)phenylalanine (F2cmF, 7) were utilized as phosphotyrosyl (pTyr) replacements in a high affinity B-bend mimicking platform, where they exhibited IC50 values of 2.5 microM, 65 microM and 28 microM, respectively, in a Grb2 SH2 domain Biacore binding assay. When a terminal N(alpha)-oxalyl axillary was utilized to enhance ligand interactions with a critical SH2 domain Arg67 residue (alphaA-helix), binding potencies increased from 4- to 10-fold, resulting in submicromolar affinity for cmF (IC50 = 0.6 microM) and low micromolar affinity for F2cmF (IC50 = 2 microM). Cell lysate binding studies also showed inhibition of cognate Grb2 binding to the p185erbB-2 phosphoprotein in the same rank order of potency as observed in the Biacore assay. These results indicate the potential value of cmF and F2cmF residues as pTyr mimetics for the study of Grb2 SH2 domains and suggest new strategies for improvements in inhibitor design.     

Src protein-tyrosine kinase structure and regulation

Src and Src-family protein kinases are proto-oncogenes that play key roles in cell morphology, motility, proliferation, and survival. v-Src (a viral protein) is encoded by the chicken oncogene of Rous sarcoma virus, and Src (the cellular homologue) is encoded by a physiological gene, the first of the proto-oncogenes. From the N- to C-terminus, Src contains an N-terminal 14-carbon myristoyl group, a unique segment, an SH3 domain, an SH2 domain, a protein-tyrosine kinase domain, and a C-terminal regulatory tail. The chief phosphorylation sites of Src include tyrosine 416 that results in activation from autophosphorylation and tyrosine 527 that results in inhibition from phosphorylation by C-terminal Src kinase. In the restrained state, the SH2 domain forms a salt bridge with phosphotyrosine 527, and the SH3 domain binds to the kinase domain via a polyproline type II left-handed helix. The SH2 and SH3 domains occur on the backside of the kinase domain away from the active site where they stabilize a dormant enzyme conformation. Protein-tyrosine phosphatases such as PTPalpha displace phosphotyrosine 527 from the Src SH2 domain and mediate its dephosphorylation leading to Src kinase activation. C-terminal Src kinase consists of an SH3, SH2, and kinase domain; it lacks an N-terminal myristoyl group and a C-terminal regulatory tail. Its X-ray structure has been determined, and the SH2 lobe occupies a position that is entirely different from that of Src. Unlike Src, the C-terminal Src kinase SH2 and SH3 domains stabilize an active enzyme conformation. Amino acid residues in the alphaD helix near the catalytic loop in the large lobe of C-terminal Src kinase serve as a docking site for the physiological substrate (Src) but not for an artificial substrate (polyGlu(4)Tyr).     

Dissection of the energetic coupling across the Src SH2 domain-tyrosyl phosphopeptide interface

Src Homology (SH2) domains play critical roles in signaling pathways by binding to phosphotyrosine (pTyr)-containing sequences, thereby recruiting SH2 domain-containing proteins to tyrosine-phosphorylated sites on receptor molecules. Investigations of the peptide binding specificity of the SH2 domain of the Src kinase (Src SH2 domain) have defined the EEI motif C-terminal to the phosphotyrosine as the preferential binding sequence. A subsequent study that probed the importance of eight specificity-determining residues of the Src SH2 domain found two residues which when mutated to Ala had significant effects on binding: Tyr beta D5 and Lys beta D3. The mutation of Lys beta D3 to Ala was particularly intriguing, since a Glu to Ala mutation at the first (+1) position of the EEI motif (the residue interacting with Lys beta D3) did not significantly affect binding. Hence, the interaction between Lys beta D3 and +1 Glu is energetically coupled. This study is focused on the dissection of the energetic coupling observed across the SH2 domain-phosphopeptide interface at and around the +1 position of the peptide. It was found that three residues of the SH2 domain, Lys beta D3, Asp beta C8 and AspCD2 (altogether forming the so-called +1 binding region) contribute to the selection of Glu at the +1 position of the ligand. A double (Asp beta C8Ala, AspCD2Ala) mutant does not exhibit energetic coupling between Lys beta D3 and +1 Glu, and binds to the pYEEI sequence 0.3 kcal/mol tighter than the wild-type Src SH2 domain. These results suggest that Lys beta D3 in the double mutant is now free to interact with the +1 Glu and that the role of Lys beta D3 in the wild-type is to neutralize the acidic patch formed by Asp beta C8 and AspCD2 rather than specifically select for a Glu at the +1 position as it had been hypothesized previously. A triple mutant (Lys beta D3Ala, Asp beta C8Ala, AspCD2Ala) has reduced binding affinity compared to the double (Asp beta C8Ala, AspCD2Ala) mutant, yet binds the pYEEI peptide as well as the wild-type Src SH2 domain. The structural basis for such high affinity interaction was investigated crystallographically by determining the structure of the triple (Lys beta D3Ala, Asp beta C8Ala, AspCD2Ala) mutant bound to the octapeptide PQpYEEIPI (where pY indicates a phosphotyrosine). This structure reveals for the first time contacts between the SH2 domain and the -1 and -2 positions of the peptide (i.e. the two residues N-terminal to pY). Thus, unexpectedly, mutations in the +1 binding region affect binding of other regions of the peptide. Such additional contacts may account for the high affinity interaction of the triple mutant for the pYEEI-containing peptide.     

Simultaneous assay of Src SH3 and SH2 domain binding using different wavelength fluorescence polarization probes.

pp60(c-src) is a prototypical nonreceptor tyrosine kinase and may play a role in diseases as diverse as cancer and osteoporosis. In Src, the SH3 domain (Src homology 3) binds proteins at specific, proline-rich sequences, while the SH2 domain (Src homology 2) binds phosphotyrosine-containing sequences. Inhibition of Src SH3 and SH2 domain function is of potential therapeutic value because of their importance in signaling pathways involved in disease states. We have developed dual-wavelength fluorescent peptide probes for both the Src SH3 and the Src SH2 domains, which allow the simultaneous measurement of compounds binding to each domain in assays based on the technique of fluorescence polarization. We demonstrate the utility of these probes in a dual-binding assay (suitable for high-throughput screening) to study the interactions of various peptides with these domains, including a sequence from the rat protein p130(CAS) which has been reported to bind simultaneously to both Src SH3 and SH2 domains. Utilizing this dual-binding assay, we confirm that sequences from p130(CAS) can simultaneously bind Src via both its SH3 and its SH2 domains. We also use the dual-binding assay as an internal control to identify substances which inhibit SH3 and SH2 binding via nonspecific mechanisms.     

Binding of the Src SH2 domain to phosphopeptides is determined by residues in both the SH2 domain and the phosphopeptides.

Src homology 2 (SH2) domains are found in a variety of signaling proteins and bind phosphotyrosine-containing peptide sequences. To explore the binding properties of the SH2 domain of the Src protein kinase, we used immobilized phosphopeptides to bind purified glutathione S-transferase-Src SH2 fusion proteins. With this assay, as well as a free-peptide competition assay, we have estimated the affinities of the Src SH2 domain for various phosphopeptides relative to a Src SH2-phosphopeptide interaction whose Kd has been determined previously (YEEI-P; Kd = 4 nM). Two Src-derived phosphopeptides, one containing the regulatory C-terminal Tyr-527 and another containing the autophosphorylation site Tyr-416, bind the Src SH2 domain in a specific though low-affinity manner (with about 10(4)-lower affinity than the YEEI-P peptide). A platelet-derived growth factor receptor (PDGF-R) phosphopeptide containing Tyr-857 does not bind appreciably to the Src SH2 domain, suggesting it is not the PDGF-R binding site for Src as previously reported. However, another PDGF-R-derived phosphopeptide containing Tyr-751 does bind the Src SH2 domain (with an affinity approximately 2 orders of magnitude lower than that of YEEI-P). All of the phosphopeptides which bind to the Src SH2 domain contain a glutamic acid at position -3 or -4 with respect to phosphotyrosine; changing this residue to alanine greatly diminishes binding. We have also tested Src SH2 mutants for their binding properties and have interpreted our results in light of the recent crystal structure solution for the Src SH2 domain. Mutations in various conserved and nonconserved residues (R155A, R155K, N198E, H201R, and H201L) cause slight reductions in binding, while two mutations cause severe reductions. The W148E mutant domain, which alters the invariant tryptophan that marks the N-terminal border of the SH2 domain, binds poorly to phosphopeptides. Inclusion of the SH3 domain in the fusion protein partially restores the binding by the W148E mutant. A change in the invariant arginine that coordinates twice with phosphotyrosine in the peptide (R175L) results in a nearly complete loss of binding. The R175L mutant does display high affinity for the PDGF-R peptide containing Tyr-751, via an interaction that is at least partly phosphotyrosine independent. We have used this interaction to show that the R175L mutation also disrupts the intramolecular interaction between the Src SH2 domain and the phosphorylated C terminus within the context of the entire Src protein; thus, the binding properties observed for mutant domains in an in vitro assay appear to mimic those that occur in vivo.