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.     

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.     

Intermolecular interactions between the SH3 domain and the proline-rich TH region of Bruton's tyrosine kinase

The SH3 domain of Bruton's tyrosine kinase (Btk) is preceded by the Tec homology (TH) region containing proline-rich sequences. We have studied a protein fragment containing both the Btk SH3 domain and the proline-rich sequences of the TH region (PRR-SH3). Intermolecular NMR cross-relaxation measurements, gel permeation chromatography profiles, titrations with proline-rich peptides, and (15)N NMR relaxation measurements are all consistent with a monomer-dimer equilibrium with a dissociation constant on the order of 60 microM. The intermolecular interactions do, at least in part, involve proline-rich sequences in the TH region. This behavior of Btk PRR-SH3 may have implications for the functional action of Btk.     

SH2-Dependent Autophosphorylation within the Tec Family Kinase Itk

The Tec family kinase, Itk (interleukin-2 tyrosine kinase), undergoes an in cis autophosphorylation on Y180 within its Src homology 3 (SH3) domain. Autophosphorylation of the Itk SH3 domain by the Itk kinase domain is strictly dependent on the presence of the intervening Src homology 2 (SH2) domain. A direct docking interaction between the Itk kinase and SH2 domains brings the Itk SH3 domain into the active site where Y180 is then phosphorylated. We now identify the residues on the surface of the Itk SH2 domain responsible for substrate docking and show that this SH2 surface mediates autophosphorylation in the full-length Itk molecule. The canonical phospholigand binding site on the SH2 domain is not involved in substrate docking, instead the docking site consists of side chains from three loop regions (AB, EF and BG) and part of the βD strand. These results are extended into Btk (Bruton's tyrosine kinase), a Tec family kinase linked to the B-cell deficiency X-linked agammaglobulinemia (XLA). Our results suggest that some XLA-causing mutations might impair Btk phosphorylation.     

Stability and peptide binding specificity of Btk SH2 domain: molecular basis for X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is caused by mutations in the Bruton's tyrosine kinase (Btk). The absence of functional Btk leads to failure of B-cell development that incapacitates antibody production in XLA patients leading to recurrent bacterial infections. Btk SH2 domain is essential for phospholipase C-gamma phosphorylation, and mutations in this domain were shown to cause XLA. Recently, the B-cell linker protein (BLNK) was found to interact with the SH2 domain of Btk, and this association is required for the activation of phospholipase C-gamma. However, the molecular basis for the interaction between the Btk SH2 domain and BLNK and the cause of XLA remain unclear. To understand the role of Btk in B-cell development, we have determined the stability and peptide binding affinity of the Btk SH2 domain. Our results indicate that both the structure and stability of Btk SH2 domain closely resemble with other SH2 domains, and it binds with phosphopeptides in the order pYEEI > pYDEP > pYMEM > pYLDL > pYIIP. We expressed the R288Q, R288W, L295P, R307G, R307T, Y334S, Y361C, L369F, and 1370M mutants of the Btk SH2 domain identified from XLA patients and measured their binding affinity with the phosphopeptides. Our studies revealed that mutation of R288 and R307 located in the phosphotyrosine binding site resulted in a more than 200-fold decrease in the peptide binding compared to L295, Y334, Y361, L369, and 1370 mutations in the pY + 3 hydrophobic binding pocket (approximately 3- to 17-folds). Furthermore, mutation of the Tyr residue at the betaD5 position reverses the binding order of Btk SH2 domain to pYIIP > pYLDL > pYDEP > pYMEM > pYEEI. This altered binding behavior of mutant Btk SH2 domain likely leads to XLA.     

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.     

SH3 domain of Bruton's tyrosine kinase can bind to proline-rich peptides of TH domain of the kinase and p120cbl

X-linked agammaglobulinemia (XLA), an inherited disease, is caused by mutations in the Bruton's tyrosine kinase (BTK). The absence of functional BTK leads to failure of B-cell differentiation; this incapacitates antibody production in XLA patients, who suffer from recurrent, sometimes lethal, bacterial infections. BTK plays an important role in B-cell development; it interacts with several proteins in the context of signal transduction. Point mutation in the BTK gene that leads to deletion of C-terminal 14 aa residues of BTK SH3 domain was found in a patient family. To understand the role of BTK, we studied binding of BTK SH3 domain (aa 216–273, 58 residues) and truncated SH3 domain (216–259, 44 residues) with proline-rich peptides; the first peptide constitutes the SH3 domain of BTK, while the latter peptide lacks 14 amino acid residues of the C terminal. Proline-rich peptides selected from TH domain of BTK and p120^cbl were studied. It is known that BTK TH domain binds to SH3 domains of various proteins. We found that BTK SH3 domain binds to peptides of BTK TH domain. This suggests that BTK SH3 and TH domains may associate in inter- or intramolecular fashion, which raises the possibility that the kinase may be regulating its own activity by restricting the availability of both its ligand-binding modules. We also found that truncated SH3 domain binds to BTK TH domain peptide less avidly than does normal SH3 domain. Also, we show that the SH3 and truncated SH3 domains bind to peptide of p120^cbl, but the latter domain binds weakly. It is likely that the truncated SH3 domain fails to present to the ligand the crucial residues in the correct context, hence the weaker binding. These results delineate the importance of C terminal in binding of SH3 domains and indicate also that improper folding and the altered binding behavior of mutant BTK SH3 domain likely leads to XLA. Proteins 29:545–552, 1997. © 1997 Wiley-Liss, Inc.     

A remote substrate docking mechanism for the tec family tyrosine kinases

During T cell signaling, Itk selectively phosphorylates a tyrosine within its own SH3 domain and a tyrosine within PLCgamma1. We find that the remote SH2 domain in each of these substrates is required to achieve efficient tyrosine phosphorylation by Itk and extend this observation to two other Tec family kinases, Btk and Tec. Additionally, we detect a stable interaction between the substrate SH2 domains and the kinase domain of Itk and find that addition of specific, exogenous SH2 domains to the in vitro kinase assay competes directly with substrate phosphorylation. On the basis of these results, we show that the kinetic parameters of a generic peptide substrate of Itk are significantly improved via fusion of the peptide substrate to the SH2 domain of PLCgamma1. This work is the first characterization of a substrate docking mechanism for the Tec kinases and provides evidence of a novel, phosphotyrosine-independent regulatory role for the ubiquitous SH2 domain.     

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.     

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.     

Crystal structure of the SH3 domain of betaPIX in complex with a high affinity peptide from PAK2

The p21-activated kinases (PAKs) are important effector proteins of the small GTPases Cdc42 and Rac and control cytoskeletal rearrangements and cell proliferation. The direct interaction of PAKs with guanine nucleotide exchange factors from the PIX/Cool family, which is responsible for the localization of PAK kinases to focal complexes in the cell, is mediated by a 24-residue peptide segment in PAKs and an N-terminal src homology 3 (SH3) domain in PIX/Cool. The SH3-binding segment of PAK contains the atypical consensus-binding motif PxxxPR, which is required for unusually high affinity binding. In order to understand the structural basis for the high affinity and specificity of the PIX-PAK interaction, we solved crystal structures for the N-terminal SH3 domain of betaPIX and for the complex of the atypical binding segment of PAK2 with the N-terminal SH3 domain of betaPIX at 0.92 A and 1.3A resolution, respectively. The asymmetric unit of the crystal contains two SH3 domains and two peptide ligands. The bound peptide adopts a conformation that allows for intimate contacts with three grooves on the surface of the SH3 domain that lie between the n-Src and RT-loops. Most notably, the arginine residue of the PxxxPR motif forms a salt-bridge and is tightly coordinated by a number of residues in the SH3 domain. This arginine-specific interaction appears to be the key determinant for the high affinity binding of PAK peptides. Furthermore, C-terminal residues of the peptide engage in additional interactions with the surface of the RT-loop, which significantly increases binding specificity. Compared to a recent NMR structure of a similar complex, our crystal structure reveals an alternate binding mode. Finally, we compare our crystal structure with the recently published betaPIX/Cbl-b complex structure, and suggest the existence of a molecular switch.     

Protein-tyrosine phosphatase D1, a potential regulator and effector for Tec family kinases

Etk, also named Bmx, is a member of the Tec tyrosine kinase family, which is characterized by a multimodular structure including a pleckstrin homology (PH) domain, an SH3 domain, an SH2 domain, and a catalytic domain. The signaling mechanisms regulating Etk kinase activity remain largely unknown. To identify factor(s) regulating Etk activity, we used the PH domain and a linker region of Etk as a bait for a yeast two-hybrid screen. Three independent clones encoding protein-tyrosine phosphatase D1 (PTPD1) fragments were isolated. The binding of PTPD1 to Etk is specific since PTPD1 cannot associate with either the Akt PH domain or lamin. In vitro and in vivo binding studies demonstrated that PTPD1 can interact with Etk and that residues 726-848 of PTPD1 are essential for this interaction. Deletion analysis of Etk indicated that the PH domain is essential for PTPD1 interaction. Furthermore, the Etk-PTPD1 interaction stimulated the kinase activity of Etk, resulting in an increased phosphotyrosine content in both factors. The Etk-PTPD1 interaction also increased Stat3 activation. The effect of PTPD1 on Etk activation is specific since PTPD1 cannot potentiate Jak2 activity upon Stat3 activation. In addition, Tec (but not Btk) kinase can also be activated by PTPD1. Taken together, these findings indicate that PTPD1 can selectively associate with and stimulate Tec family kinases and modulate Stat3 activation.     

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.     

Mechanism and functional significance of Itk autophosphorylation

Tec family non-receptor tyrosine kinases (Itk, Btk, Tec, Rlk and Bmx) are characterized by the presence of an autophosphorylation site within the non-catalytic Src homology 3 (SH3) domain. The full-length Itk mutant containing phenylalanine in place of the autophosphorylated tyrosine has been studied in Itk-deficient primary T cells. These studies revealed that the non-phosphorylated enzyme restores Itk mediated signaling only partially. In spite of these insights, the precise role of the Tec kinase autophosphorylation site is unclear and the mechanism of the autophosphorylation reaction within the Tec kinases is not known. Here, we show both in vitro and in vivo that Itk autophosphorylation on Y180 within the SH3 domain occurs exclusively via an intramolecular, in cis mechanism. Using an in vitro kinase assay, we show that mutation of the Itk autophosphorylation site Y180 to Phe decreases kinase activity of the full-length enzyme by increasing Km for a peptide substrate. Moreover, mutation of Y180 to Glu, a residue chosen to mimic the phosphorylated tyrosine, alters the ligand-binding capability of the Itk SH3 domain in a ligand-dependent fashion. NMR chemical shift mapping gives residue-specific structural insight into the effect of the Y180E mutation on ligand binding. These data provide a molecular level context with which to interpret in vivo functional data and allow development of a structural model for Itk autophosphorylation.     

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.     

Evolution of High-Affinity Peptide Probes to Detect the SH3 Domain of Cancer Biomarker BCR–ABL

The BCR–ABL fusion protein is closely associated with the pathological progression of chronic myelogenous leukaemia and some other myeloproliferative diseases, which has long been recognized as one of the most important cancer biomarkers in the tumor diagnosis community. The SH3 domain of BCR–ABL is a small, conserved protein module that specifically recognizes and binds proline-rich peptide fragments. In the current study, we used a synthetic strategy to discover new peptide probes with high affinity binding to the BCR–ABL SH3 domain. In the procedure, a sequence-based machine learning predictor was developed based on a set of affinity-known SH3 binders, and the predictor was then used to guide the evolutional optimization of numerous virtual peptides to enrich high binding potency for the SH3 domain. Subsequently, a evolved peptide population was generated, from which ten peptides with the highest affinity scores were selected and their interaction free energies with SH3 domain were characterized systematically using a combination of molecular dynamics simulation and binding free energy analysis. Consequently, four peptides were suggested as promising candidates and their affinities toward SH3 domain were assayed; two peptides, APTYTPPPPP and APTYAPPPPP, were identified to have potent binding capability with dissociation constants K _d of 3 and 8 μM, respectively. Further, the structural basis and energetic property of SH3 domain in complex with APTYTPPPPP were examined in detail, revealing a non-specific interaction in SH3–peptide recognition that should render a broad ligand spectrum for the domain.     

Physical and functional interactions between SH2 and SH3 domains of the Src family protein tyrosine kinase p59fyn.

The Src family protein tyrosine kinases participate in signalling through cell surface receptors that lack intrinsic tyrosine kinase domains. All nine members of this family possess adjacent Src homology (SH2 and SH3) domains, both of which are essential for repression of the enzymatic activity. The repression is mediated by binding between the SH2 domain and a C-terminal phosphotyrosine, and the SH3 domain is required for this interaction. However, the biochemical basis of functional SH2-SH3 interaction is unclear. Here, we demonstrate that when the SH2 and SH3 domains of p59fyn (Fyn) were present as adjacent domains in a single protein, binding of phosphotyrosyl peptides and proteins to the SH2 domain was enhanced, whereas binding of a subset of cellular polypeptide ligands to the SH3 domain was decreased. An interdomain communication was further revealed by occupancy with domain-specific peptide ligands: occupancy of the SH3 domain with a proline-rich peptide enhanced phosphotyrosine binding to the linked SH2 domain, and occupancy of the SH2 domain with phosphotyrosyl peptides enhanced binding of certain SH3-specific cellular polypeptides. Second, we demonstrate a direct binding between purified SH2 and SH3 domains of Fyn and Lck Src family kinases. Heterologous binding between SH2 and SH3 domains of closely related members of the Src family, namely, Fyn, Lck, and Src, was also observed. In contrast, Grb2, Crk, Abl, p85 phosphatidylinositol 3-kinase, and GTPase-activating protein SH2 domains showed lower or no binding to Fyn or Lck SH3 domains. SH2-SH3 binding did not require an intact phosphotyrosine binding pocket on the SH2 domain; however, perturbations of the SH2 domain induced by specific high-affinity phosphotyrosyl peptide binding abrogated binding of the SH3 domain. SH3-SH2 binding was observed in the presence of proline-rich peptides or when a point mutation (W119K) was introduced in the putative ligand-binding pouch of the Fyn SH3 domain, although these treatments completely abolished the binding to p85 phosphatidylinositol 3-kinase and other SH3-specific polypeptides. These biochemical SH2-SH3 interactions suggest novel mechanisms of regulating the enzymatic activity of Src kinases and their interactions with other proteins.     

Identification of phosphorylation sites within the SH3 domains of Tec family tyrosine kinases

Tec family protein tyrosine kinases (TFKs) play a central role in hematopoietic cellular signaling. Initial activation takes place through specific tyrosine phosphorylation situated in the activation loop. Further activation occurs within the SH3 domain via a transphosphorylation mechanism, which for Bruton's tyrosine kinase (Btk) affects tyrosine 223. We found that TFKs phosphorylate preferentially their own SH3 domains, but differentially phosphorylate other member family SH3 domains, whereas non-related SH3 domains are not phosphorylated. We demonstrate that SH3 domains are good and reliable substrates. We observe that transphosphorylation is selective not only for SH3 domains, but also for dual SH3SH2 domains. However, the dual domain is phosphorylated more effectively. The major phosphorylation sites were identified as conserved tyrosines, for Itk Y180 and for Bmx Y215, both sites being homologous to the Y223 site in Btk. There is, however, one exception because the Tec-SH3 domain is phosphorylated at a non-homologous site, nevertheless a conserved tyrosine, Y206. Consistent with these findings, the 3D structures for SH3 domains point out that these phosphorylated tyrosines are located on the ligand-binding surface. Because a number of Tec family kinases are coexpressed in cells, it is possible that they could regulate the activity of each other through transphosphorylation.     

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.