Interaction between Btk TH and SH3 domain

Several mechanisms are involved in the regulation of cellular signaling. Bruton tyrosine kinase (Btk) of the Tec family contains in the Tec homology (TH) domain a proline-rich region (PRR) capable of interacting with several SH3 domains. The Btk has the SH3 domain adjacent to the TH domain. CD and fluorescence spectroscopy were used to study the binding of two peptides corresponding to segments in the PRR to the Btk SH3 domain. The peptide for the N-terminal half of the PRR binds specifically, whereas the other peptide had hardly any affinity. The TH domain has about four times lower affinity to the SH3 domain than the peptide, 17.0 vs 3.9 microM. The interaction was further tested with an SH3 domain construct that contained the PRR. The two peptides cannot compete for the binding to the extended protein and the TH domain has two times lower affinity to the extended SH3 domain. The intra- or intermolecular interaction between the TH and SH3 domain might have regulatory function also in the other Tec family members.     

Tec homology (TH) adjacent to the PH domain

The pleckstrin homology (PH) domain is extended in the Btk kinase family by a region designated the TH (Tec homology) domain, which consists of about 80 residues preceding the SH3 domain. The TH domain contains a conserved 27 amino acid stretch designated the Btk motif and a proline-rich region. Sequence similarity was found to a putative Ras GTPase activating protein and a human interferon-γ binding protein both in the PH domain and the Btk motif region. SLK1/SSP31 protein kinase and a non-catalytic p85 subunit of PI-3 kinase had similarity only with the proline rich region. The identification of a PH domain extension in some signal transduction proteins in different species suggests that this region is involved in protein—protein interactions.     

The solution structure and intramolecular associations of the Tec kinase SRC homology 3 domain.

Tec is the prototypic member of a family of intracellular tyrosine kinases that includes Txk, Bmx, Itk, and Btk. Tec family kinases share similarities in domain structure with Src family kinases, but one of the features that differentiates them is a proline-rich region (PRR) preceding their Src homology (SH) 3 domain. Evidence that the PRR of Itk can bind in an intramolecular fashion to its SH3 domain and the lack of a regulatory tyrosine in the C terminus indicates that Tec kinases must be regulated by a different set of intramolecular interactions to the Src kinases. We have determined the solution structure of the Tec SH3 domain and have investigated interactions with its PRR, which contains two SH3-binding sites. We demonstrate that in vitro, the Tec PRR can bind in an intramolecular fashion to the SH3. However, the affinity is lower than that for dimerization via reciprocal PRR-SH3 association. Using site-directed mutagenesis we show that both sites can bind the Tec SH3 domain; site 1 (155KTLPPAP161) binds intramolecularly, while site 2 (165KRRPPPPIPP174) cannot and binds in an intermolecular fashion. These distinct roles for the SH3 binding sites in Tec family kinases could be important for protein targeting and enzyme activation.     

Determinants of intra versus intermolecular self-association within the regulatory domains of Rlk and Itk

A protein fragment from the Tec family member Rlk (also known as Txk) containing a single proline-rich ligand adjacent to a Src homology 3 (SH3) domain has been investigated by nuclear magnetic resonance (NMR) spectroscopy. Analysis of the concentration dependence of the chemical shifts, NMR linewidths and self-diffusion coefficients reveal that the Rlk fragment dimerizes in solution. Mutation of two critical prolines in the proline-rich ligand abolishes dimerization. Furthermore, analysis of the extrapolated chemical shifts at infinite dilution reveal that intramolecular binding of the proline-rich ligand to the SH3 domain is disfavored. This is in contrast to the corresponding fragment of Itk, for which the proline-rich ligand/SH3 interaction occurs exclusively in an intramolecular fashion and no intermolecular binding is observed. Comparison of the Itk and Rlk sequences reveals that Rlk contains five fewer residues than Itk in the linker region between the proline-rich ligand and the SH3 domain. To assess whether linker length is a molecular determinant of intra- versus intermolecular self-association, we varied the length of the linker in both Rlk and Itk and analyzed the resulting variants by NMR. Intramolecular binding in Itk is reduced by shortening the linker and conversely a longer linker between the proline-rich ligand and the SH3 domain in Rlk enhances intramolecular self-association. Association constants for the binding of peptides corresponding to the proline-rich ligand with their respective SH3 domains were also measured by NMR. The protein/peptide data combined with the association constants for binding of each proline-rich peptide to the corresponding SH3 domain provide an explanation for the opposing modes of self-association within the otherwise closely related Rlk and Itk proteins.     

Competing modes of self-association in the regulatory domains of Bruton's tyrosine kinase: Intramolecular contact versus asymmetric homodimerization

A nuclear magnetic resonance (NMR) investigation of a fragment of the nonreceptor Tec family tyrosine kinase Btk has revealed an intricate set of coupled monomer-dimer equilibria. The Btk fragment studied contains two consecutive proline-rich motifs followed by a single Src homology 3 (SH3) domain. We provide evidence for an asymmetric homodimer in which the amino-terminal proline sequence of one monomer contacts the opposite SH3 binding pocket, whereas the carboxy-terminal proline sequence of the other monomer is engaged by the second SH3 domain across the dimer interface. We show that the asymmetric homodimer structure is mimicked by a heterodimer formed in an equimolar mixture of complimentary mutants: one carrying mutations in the amino-terminal proline stretch; the other, in the carboxy-terminal proline motif. Moreover, a monomeric species characterized by an intramolecular complex between the amino-terminal proline motif and the SH3 domain predominates at low concentration. Association constants were determined for each of the competing equilibria by NMR titration. The similarity of the determined K(a) values reveals a delicate balance between the alternative conformational states available to Btk. Thus, changes in the local concentration of Btk itself, or co-localization with exogenous signaling molecules that have high affinity for either proline sequence or the SH3 domain, can significantly alter species composition and regulate Btk kinase activity.     

Solution structure of Grb2 reveals extensive flexibility necessary for target recognition

Grb2 is an adaptor protein composed of a single SH2 domain flanked by two SH3 domains. Grb2 functions as an important evolutionary conserved link between a variety of cell membrane receptors and the Ras/MAP kinase-signaling cascade. Here, we describe the solution structure of Grb2 as revealed by NMR and small angle X-ray scattering measurements. We demonstrate that Grb2 is a flexible protein in which the C-terminal SH3 domain is connected to the SH2 domain via a flexible linker. This is in contrast to the previously described Grb2 crystal structure, which showed a compact structure with intramolecular contact between two SH3 domains. Binding experiments on Grb2 and peptides containing two different proline-rich sequences indicate that Grb2 adapts the relative position and orientation of the two SH3 domains to bind bivalently to the target peptide sequences.     

The SH3 domain of a M7 interacts with its C-terminal proline-rich region

Myosins play essential roles in migration, cytokinesis, endocytosis, and adhesion. They are composed of a large N-terminal motor domain with ATPase and actin binding sites and C-terminal neck and tail regions, whose functional roles and structural context in the protein are less well characterized. The tail regions of myosins I, IV, VII, XII, and XV each contain a putative SH3 domain that may be involved in protein-protein interactions. SH3 domains are reported to bind proline-rich motifs, especially "PxxP" sequences, and such interactions serve regulatory functions. The activity of Src, PI3, and Itk kinases, for example, is regulated by intramolecular interactions between their SH3 domain and internal proline-rich sequences. Here, we use NMR spectroscopy to reveal the structure of a protein construct from Dictyostelium myosin VII (DdM7) spanning A1620-T1706, which contains its SH3 domain and adjacent proline-rich region. The SH3 domain forms the signature beta-barrel architecture found in other SH3 domains, with conserved tryptophan and tyrosine residues forming a hydrophobic pocket known to bind "PxxP" motifs. In addition, acidic residues in the RT or n-Src loops are available to interact with the basic anchoring residues that are typically found in ligands or proteins that bind SH3 domains. The DdM7 SH3 differs in the hydrophobicity of the second pocket formed by the 3(10) helix and following beta-strand, which contains polar rather than hydrophobic side chains. Most unusual, however, is that this domain binds its adjacent proline-rich region at a surface remote from the region previously identified to bind "PxxP" motifs. The interaction may affect the orientation of the tail without sacrificing the availability of the canonical "PxxP"-binding surface.     

Directed discovery of bivalent peptide ligands to an SH3 domain

The Caenorhabditis elegans SEM-5 SH3 domains recognize proline-rich peptide segments with modest affinity. We developed a bivalent peptide ligand that contains a naturally occurring proline-rich binding sequence, tethered by a glycine linker to a disulfide-closed loop segment containing six variable residues. The glycine linker allows the loop segment to explore regions of greatest diversity in sequence and structure of the SH3 domain: the RT and n-Src loops. The bivalent ligand was optimized using phage display, leading to a peptide (PP-G(4)-L) with 1000-fold increased affinity for the SEM-5 C-terminal SH3 domain over that of a natural ligand. NMR analysis of the complex confirms that the peptide loop segment is targeted to the RT and n-Src loops and parts of the beta-sheet scaffold of this SH3 domain. This binding region is comparable to that targeted by a natural non-PXXP peptide to the p67(phox) SH3 domain, a region not known to be targeted in the Grb2 SH3 domain family. PP-G(4)-L may aid in the discovery of additional binding partners of Grb2 family SH3 domains.     

A specific intermolecular association between the regulatory domains of a Tec family kinase

Interleukin-2 tyrosine kinase (Itk), is a T-cell specific tyrosine kinase of the Tec family. We have examined a novel intermolecular interaction between the SH3 and SH2 domains of Itk. In addition to the interaction between the isolated domains, we have found that the dual SH3/SH2 domain-containing fragment of Itk self-associates in a specific manner in solution. Tec family members contain the SH3, SH2 and catalytic domains common to many kinase families but are distinguished by a unique amino-terminal sequence, which contains a proline-rich stretch. Previous work has identified an intramolecular regulatory association between the proline-rich region and the adjacent SH3 domain of Itk. The intermolecular interaction between the SH3 and SH2 domains of Itk that we describe provides a possible mechanism for displacement of this intramolecular regulatory sequence, a step that may be required for full Tec kinase activation. Additionally, localization of the interacting surfaces on both the SH3 and SH2 domains by chemical shift mapping has provided information about the molecular details of this recognition event. The interaction involves the conserved aromatic binding pocket of the SH3 domain and a newly defined binding surface on the SH2 domain. The interacting residues on the SH2 domain do not conform to the consensus motif for an SH3 proline-rich ligand. Interestingly, we note a striking correlation between the SH2 residues that mediate this interaction and those residues that, when mutated in the Tec family member Btk, cause the hereditary immune disorder, X-linked agamaglobulinemia.     

Rational design and purification of human Bruton's tyrosine kinase SH3-SH2 protein for structure-function studies

Bruton's tyrosine kinase (Btk) is a cytoplasmic protein tyrosine kinase consisting of N-terminal pleckstrin homology (PH) domain followed by Tec homology (TH) domain, Src homology 3 and 2 (SH3 and SH2) domains, and a C-terminal kinase domain. Mutations in the human BTK gene cause the severe immunodeficiency disease X-linked agammaglobulinemia (XLA). The structural and functional basis of several XLA-causing mutations remains unknown, since only the structures of the PH and SH3 domains of human Btk are currently available. In this study, we overexpressed and purified a protein consisting of the SH3 and SH2 domains of human Btk for biochemical and structural analysis. The purified protein was only partially soluble and had a tendency to dimerize, which made it unsuitable for further studies. To overcome the problems of low solubility and dimerization, subdomain interactions were engineered without altering the function of the protein.     

SNX9 as an adaptor for linking synaptojanin-1 to the Cdc42 effector ACK1

Sorting nexin 9 (SNX9, also referred to as SH3PX1) is a binding partner for the non-receptor and Cdc42-associated kinase (ACK) in Drosophila and mammals. ACK1 is known to bind clathrin and influence EGF receptor endocytosis. SNX9 comprises an N-terminal Src homology domain 3 (SH3), a central PHOX homology (PX) domain, and a carboxyl-terminal coiled-coil region. In order to investigate SNX9 further we have made use of a novel in vivo biotinylation system to label various GST-SH3 domains and perform blot overlays, thereby identifying synaptojanin-1 as a partner for SNX9. Biotinylated SH3 domains were also used for specific identification of target proline-rich sequences in synaptojanin and ACK1 on synthetic peptides arrays. Direct assessment of SH3 binding efficiencies at different positions within the extensive proline-rich regions of these proteins were thus determined. While SNX9 targets a number of sequences within the proline-rich regions of synaptojanin, a single site was identified in human ACK1. By testing the association of various truncations of ACK1 with SNX9 we confirmed the dominant SNX9 binding domain in human ACK1 (residues 920-955). In the presence of SNX9 we find that synaptojanin is able to colocalize with distinct ACK1 containing vesicles, indicating that this tyrosine kinase is linked to many components involved in vesicle dynamics including clathrin, AP2 and synaptojanin-1.     

The 1.1 A resolution crystal structure of the p130cas SH3 domain and ramifications for ligand selectivity

The Crk-associated tyrosine kinase substrate p130cas (CAS) is a docking protein containing an SH3 domain near its N terminus, followed by a short proline-rich segment, a large central substrate domain composed of 15 repeats of the four amino acid sequence YxxP, a serine-rich region and a carboxy-terminal domain, which possesses consensus binding sites for the SH2 and SH3 domains of Src (YDYV and RPLPSPP, respectively). The SH3 domain of CAS mediates its interaction with several proteins involved in signaling pathways such as focal adhesion kinase (FAK), tyrosine phosphatases PTP1B and PTP-PEST, and the guanine nucleotide exchange factor C3G. As a homolog of the corresponding Src docking domain, the CAS SH3 domain binds to proline-rich sequences (PxxP) of its interacting partners that can adopt a polyproline type II helix. We have determined a high-resolution X-ray structure of the recombinant human CAS SH3 domain. The domain, residues 1-69, crystallized in two related space groups, P2(1) and C222(1), that provided diffraction data to 1.1 A and 2.1 A, respectively. The crystal structure shows, in addition to the conserved SH3 domain architecture, the way in which the CAS characteristic amino acids form an atypically charged ligand-binding surface. This arrangement provides a rationale for the unusual ligand recognition motif exhibited by the CAS SH3 domain. The structure enables modelling of the docking interactions to its ligands, for example from focal adhesion kinase, and supports structure-based drug design of inhibitors of the CAS-FAK interaction.     

Molecular and structural characterization of five novel mutations in the Bruton's tyrosine kinase gene from patients with X-linked agammaglobulinemia.

BACKGROUND: The Btk (Bruton's tyrosine kinase) gene has been shown to be mutated in the human immunodeficiency disease, XLA (X-linked agammaglobulinemia). Btk is a member of the Tec family of cytosolic protein tyrosine kinases with distinct functional domains PH, TH, SH3, SH2, and kinase. Mutations have been observed in each of the Btk subdomains in XLA. We have analyzed the Btk gene in six XLA patients from five unrelated families. MATERIALS AND METHODS: DNA was prepared from the patients peripheral blood. The Btk exons including the junctional sequences were analyzed by single-strand conformation polymorphism (SSCP) followed by direct nucleotide sequencing after PCR-amplification. For structural analysis, the missense mutations were introduced into three-dimensional models of the PH and kinase domains of Btk and the outcome was predicted based on the knowledge of the protein function. RESULTS: Five novel mutations and two novel polymorphisms, all of which resulted from single-base alterations, were identified. Three of the five mutations were in the PH domain and two were in the kinase domain of Btk. Three of these mutations were of the missense type, two of which altered the same codon in the PH domain; the third one was located in the kinase domain. The fourth mutation was a point deletion in the PH domain causing a frameshift followed by premature termination. The fifth mutation was a splice donor-site mutation within the kinase domain which could result in an exon skipping. In four of the five instances, mothers of the patients were shown to be obligate carriers. In one instance, a sibling sister was identified as a heterozygote establishing her as a carrier. CONCLUSIONS: Functional consequences of the mutations causing frameshifts and altered splicing can be inferred directly. Functional consequences of the missense mutations were interpreted by 3-dimensional structural modeling of Btk domains. It is proposed that the two PH domain mutations will interfere with membrane localization while the kinase domain mutation will interfere with the enzymatic function of Btk. This study provides further insight into the role of Btk in XLA.     

Formin binding proteins bear WWP/WW domains that bind proline-rich peptides and functionally resemble SH3 domains.

The formins, proteins involved in murine limb and kidney development, contain a proline-rich region that matches consensus sequences for Src homology 3 (SH3) ligands. To identify proteins that interact with formins, we used this proline-rich region to screen mouse limb bud expression libraries for formin binding proteins (FBPs). As expected, we found one class of FBPs that contains SH3 domains, including two novel members of this class. In addition, however, we also found a novel class of FBPs that contains one or two copies of a 26 amino acid homology region that has been recently termed the WWP or WW motif. We demonstrate that WWP/WW domains as short as 26 amino acids can act as modular protein-binding interfaces that bind with high affinity to proline-rich sequences that are similar and, in some cases, identical to SH3 ligands. Furthermore, we find that the WWP/WW domain can compete with the Abl SH3 domain in binding a proline-rich peptide present in formin. Our results suggest that these novel protein interaction domains can perform functions similar to those of SH3 domains and, thus, might regulate SH3 interactions with target proteins through competitive binding.     

Structural basis of the auto-inhibition mechanism of nonreceptor tyrosine kinase PTK6

Protein tyrosine kinase 6 (PTK6) is composed of SH3, SH2, and Kinase domains, with a linker region (Linker) between the SH2 and Kinase domains. Here, we report the structural basis of the SH3-Linker interaction that results in auto-inhibition of PTK6. The solution structures of the SH3 domain and SH3/Linker complex were determined by NMR spectroscopy. The structure of the SH3 domain forms a conventional β-barrel with two β-sheets comprised of five β-strands. However, the molecular topology and charge distribution of PTK6-SH3 slightly differs from that of the other SH3 domains. The structure of the N-terminal Linker within the complex showed that the proline-rich region (P175-P187) of the Linker forms a compact hairpin structure through hydrophobic interactions. The structure of the SH3/Linker complex revealed intra-molecular interaction between the amino acid pairs R22/E190, W44/W184, N65/P177, and Y66/P179. Mutations in PTK6 at R22, W44, N65, and Y66 residues in the SH3 domain increased catalytic activity compared with wild-type protein, implying that specific interactions between hydrophobic residues in the proline-rich linker region and hydrophobic residues in the SH3 domain are mainly responsible for down-regulating the catalytic activity of PTK6.     

Six X-linked agammaglobulinemia-causing missense mutations in the Src homology 2 domain of Bruton's tyrosine kinase: phosphotyrosine-binding and circular dichroism analysis

Src homology 2 (SH2) domains recognize phosphotyrosine (pY)-containing sequences and thereby mediate their association to ligands. Bruton's tyrosine kinase (Btk) is a cytoplasmic protein tyrosine kinase, in which mutations cause a hereditary immunodeficiency disease, X-linked agammaglobulinemia (XLA). Mutations have been found in all Btk domains, including SH2. We have analyzed the structural and functional effects of six disease-related amino acid substitutions in the SH2 domain: G302E, R307G, Y334S, L358F, Y361C, and H362Q. Also, we present a novel Btk SH2 missense mutation, H362R, leading to classical XLA. Based on circular dichroism analysis, the conformation of five of the XLA mutants studied differs from the native Btk SH2 domain, while mutant R307G is structurally identical. The binding of XLA mutation-containing SH2 domains to pY-Sepharose was reduced, varying between 1 and 13% of that for the native SH2 domain. The solubility of all the mutated proteins was remarkably reduced. SH2 domain mutations were divided into three categories: 1) Functional mutations, which affect residues presumably participating directly in pY binding (R307G); 2) structural mutations that, via conformational change, not only impair pY binding, but severely derange the structure of the SH2 domain and possibly interfere with the overall conformation of the Btk molecule (G302E, Y334S, L358F, and H362Q); and 3) structural-functional mutations, which contain features from both categories above (Y361C).     

Ponsin/SH3P12: an l-afadin- and vinculin-binding protein localized at cell-cell and cell-matrix adherens junctions

We recently isolated a novel actin filament (F-actin)-binding protein, afadin, that has two isoforms, l- and s-afadins. l-Afadin is ubiquitously expressed and specifically localized at zonula adherens (ZA) in epithelial cells and at cell-cell adherens junction (AJ) in nonepithelial cells, whereas s-afadin is abundantly expressed in neural tissue. l-Afadin has one PDZ domain, three proline-rich regions, and one F-actin-binding domain, whereas s-afadin lacks the third proline-rich region and the F-actin-binding domain. To understand the molecular mechanism of the specific localization of l-afadin at ZA in epithelial cells and at cell-cell AJ in nonepithelial cells, we attempted here to identify an l-afadin-binding protein(s) and isolated a protein, named ponsin. Ponsin had many splicing variants and the primary structures of two of them were determined. Both the two variants had three Src homology 3 (SH3) domains and turned out to be splicing variants of SH3P12. The third proline-rich region of l-afadin bound to the region of ponsin containing the second and third SH3 domains. Ponsin was ubiquitously expressed and localized at ZA in epithelial cells, at cell-cell AJ in nonepithelial cells, and at cell-matrix AJ in both types of cells. Ponsin furthermore directly bound vinculin, an F-actin-binding protein localized at ZA in epithelial cells, at cell-cell AJ in nonepithelial cells, and at cell-matrix AJ in both types of cells. Vinculin has one proline-rich region where two proline-rich sequences are located. The proline-rich region bound to the region of ponsin containing the first and second SH3 domains. l-Afadin and vinculin bound to ponsin in a competitive manner and these three proteins hardly formed a ternary complex. These results indicate that ponsin is an l-afadin- and vinculin-binding protein localized at ZA in epithelial cells, at cell-cell AJ in nonepithelial cells, and at cell-matrix AJ in both types of cells.     

The CAP-Gly domain of CYLD associates with the proline-rich sequence in NEMO/IKKgamma

CYLD was originally identified as the human familial cylindromatosis tumor suppressor. Recently, it was reported that CYLD directly interacts with NEMO/IKKgamma and TRAF2 in the NF-kappaB signaling pathway. The two proteins bind to a region of CYLD that contains a Cys-box motif and the third cytoskeleton-associated protein-glycine conserved (CAP-Gly) domain. Here we report that the third CAP-Gly domain of CYLD specifically interacts with one of the two proline-rich sequences of NEMO/IKKgamma. The tertiary structure of the CAP-Gly domain shares the five-stranded beta sheet topology with the SH3 domain, which is well known as a proline-rich sequence-recognition domain. However, chemical shift mapping revealed that the peptide binding site of the CAP-Gly domain is formed without the long peptide binding loop characteristic of the SH3 domain. Therefore, CAP-Gly is likely to be a novel proline-rich sequence binding domain with a mechanism different from that of the SH3 domain.     

Dynamics of the Tec‐family tyrosine kinase SH3 domains

The Src Homology 3 (SH3) domain is an important regulatory domain found in many signaling proteins. X‐ray crystallography and NMR structures of SH3 domains are generally conserved but other studies indicate that protein flexibility and dynamics are not. We previously reported that based on hydrogen exchange mass spectrometry (HX MS) studies, there is variable flexibility and dynamics among the SH3 domains of the Src‐family tyrosine kinases and related proteins. Here we have extended our studies to the SH3 domains of the Tec family tyrosine kinases (Itk, Btk, Tec, Txk, Bmx). The SH3 domains of members of this family augment the variety in dynamics observed in previous SH3 domains. Txk and Bmx SH3 were found to be highly dynamic in solution by HX MS and Bmx was unstructured by NMR. Itk and Btk SH3 underwent a clear EX1 cooperative unfolding event, which was localized using pepsin digestion and mass spectrometry after hydrogen exchange labeling. The unfolding was localized to peptide regions that had been previously identified in the Src‐family and related protein SH3 domains, yet the kinetics of unfolding were not. Sequence alignment does not provide an easy explanation for the observed dynamics behavior, yet the similarity of location of EX1 unfolding suggests that higher‐order structural properties may play a role. While the exact reason for such dynamics is not clear, such motions can be exploited in intra‐ and intermolecular binding assays of proteins containing the domains.     

Specificity determinants of a novel Nck interaction with the juxtamembrane domain of the epidermal growth factor receptor

Nck is a ubiquitously expressed adaptor protein containing Src homology 2 (SH2) and Src homology 3 (SH3) domains. It integrates downstream effector proteins with cell membrane receptors, such as the epidermal growth factor receptor (EGFR). EGFR plays a critical role in cellular proliferation and differentiation. The 45-residue juxtamembrane domain of EGFR (JM), located between the transmembrane and kinase domains, regulates receptor activation and trafficking to the basolateral membrane of polarized epithelia through a proline-rich motif that resembles a consensus SH3 domain binding site. We demonstrate here that the JM region can bind to Nck, showing a notable binding preference for the second SH3 domain. To elucidate the structural determinants for this interaction, we have determined the NMR solution structures of both the first and second Nck SH3 domains (Nck1-1 and Nck1-2). These domains adopt a canonical SH3 beta-barrel-like fold, containing five antiparallel strands separated by three loop regions and one 3 10-helical turn. Chemical shift perturbation studies have identified the residues that form the binding cleft of Nck1-2, which are primarily located in the RT and n-Src loops. JM binds to Nck1-2 with an affinity of approximately 80 microM through a positively charged sequence near the N-terminus, as opposed to the polyproline sequence. The two Nck SH3 domains exhibit both steric and electrostatic differences in their RT-Src and n-Src loops, and a model of the Nck1-2 domain complexed with the JM highlights the factors that define the putative binding mode for this ligand.