The Grb2 adaptor

From adhesion complex to signaling hub: the dual role of dystroglycan

Dystroglycan (DG) is a transmembrane protein widely expressed in multiple cells and tissues. It is formed by two subunits, α- and β-DG, and represents a molecular bridge between the outside and the inside of the cell, which is essential for the mechanical and structural stability of the plasma membrane. The α-subunit is a cell-surface protein that binds to the extracellular matrix (ECM) and is tightly associated with the plasma membrane via a non-covalent interaction with the β-subunit, which, in turn, is a transmembrane protein that binds to the cytoskeletal actin. DG is a versatile molecule acting not only as a mechanical building block but also as a modulator of outside-inside signaling events. The cytoplasmic domain of β-DG interacts with different adaptor and cytoskeletal proteins that function as molecular switches for the transmission of ECM signals inside the cells. These interactions can modulate the involvement of DG in different biological processes, ranging from cell growth and survival to differentiation and proliferation/regeneration. Although the molecular events that characterize signaling through the ECM-DG-cytoskeleton axis are still largely unknown, in recent years, a growing list of evidence has started to fill the gaps in our understanding of the role of DG in signal transduction. This mini-review represents an update of recent developments, uncovering the dual role of DG as an adhesion and signaling molecule that might inspire new ideas for the design of novel therapeutic strategies for pathologies such as muscular dystrophy, cardiomyopathy, and cancer, where the DG signaling hub plays important roles.

Unveiling Metastable Ensembles of GRB2 and the Relevance of Interdomain Communication during Folding

The folding process of multidomain proteins is a highly intricate phenomenon involving the assembly of distinct domains into a functional three-dimensional structure. During this process, each domain may fold independently while interacting with others. The folding of multidomain proteins can be influenced by various factors, including their composition, the structure of each domain, or the presence of disordered regions, as well as the surrounding environment. Misfolding of multidomain proteins can lead to the formation of nonfunctional structures associated with a range of diseases, including cancers or neurodegenerative disorders. Understanding this process is an important step for many biophysical analyses such as stability, interaction, malfunctioning, and rational drug design. One such multidomain protein is growth factor receptor-bound protein 2 (GRB2), an adaptor protein that is essential in regulating cell survival. GRB2 consists of one central Src homology 2 (SH2) domain flanked by two Src homology 3 (SH3) domains. The SH2 domain interacts with phosphotyrosine regions in other proteins, while the SH3 domains recognize proline-rich regions on protein partners during cell signaling. Here, we combined computational and experimental techniques to investigate the folding process of GRB2. Through computational simulations, we sampled the conformational space and mapped the mechanisms involved by the free energy profiles, which may indicate possible intermediate states. From the molecular dynamics trajectories, we used the energy landscape visualization method (ELViM), which allowed us to visualize a three-dimensional (3D) representation of the overall energy surface. We identified two possible parallel folding routes that cannot be seen in a one-dimensional analysis, with one occurring more frequently during folding. Supporting these results, we used differential scanning calorimetry (DSC) and fluorescence spectroscopy techniques to confirm these intermediate states in vitro. Finally, we analyzed the deletion of domains to compare our model outputs to previously published results, supporting the presence of interdomain modulation. Overall, our study highlights the significance of interdomain communication within the GRB2 protein and its impact on the formation, stability, and structural plasticity of the protein, which are crucial for its interaction with other proteins in key signaling pathways.

Targeting the Grb2 cSH3 domain: Design, synthesis and biological evaluation of the first series of modulators

Growth factor receptor bound protein 2 (Grb2) is an adaptor protein featured by a nSH3-SH2-cSH3 domains. Grb2 finely regulates important cellular pathways such as growth, proliferation and metabolism and a minor lapse of this tight control may totally change the entire pathway to the oncogenic. Indeed, Grb2 is found overexpressed in many tumours type. Consequently, Grb2 is an attractive therapeutic target for the development of new anticancer drug. Herein, we reported the synthesis and the biological evaluation of a series of Grb2 inhibitors, developed starting from a hit-compound already reported by this research unit. The newly synthesized compounds were evaluated by kinetic binding experiments, and the most promising derivatives were assayed in a short panel of cancer cells. Five of the newly synthesized derivatives proved to be able to bind the targeted protein with valuable inhibitory concentration in one-digit micromolar concentration. The most active compound of this series, derivative 12, showed an inhibitory concentration of about 6 μM for glioblastoma and ovarian cancer cells, and an IC50 of 1.67 for lung cancer cell. For derivative 12, the metabolic stability and the ROS production was also evaluated. The biological data together with the docking studies led to rationalize an early structure activity relationship.

The influence of pH on the structure and stability of the Grb2 dimer reveals changes in the inter-domain and molecular interaction: Could it be a modulation mechanism?

Cancer cells present an increased replicative potential as a hallmark. The increased replication leads to a higher intracellular pH. Grb2, an adapter protein, is mainly involved in several types of cancers due to its role in signaling pathways responsible for cell growth and proliferation. At pH 7, we observed a more compact structure, as seen by DLS and ¹H NMR relaxation experiments, with high cooperativity within domains. On the other hand, we observed an increase in disordered structures at pH 8, with relative independence between domains characterized by higher melting temperatures and enthalpy of unfolding. CD and DLS corroborate with these observations at pH 8, conferring more flexibility among the domains, followed by lower unfolding cooperativity and increased hydrodynamic diameter at higher pH. In addition, ¹⁵N-HSQC chemical shift perturbations experiments showed significant differences in the positions of several amino acids spread on the Grb2 structure when pH was changed, which agrees with the previous results. Finally, the molecular dynamic analysis demonstrates that Grb2 presents a movement pattern where both SH3 domains move toward the center of the protein at pH 7. On the contrary, the pattern changes its direction at pH 8, where domains move outside the center of the protein, conferring a more elongated structure at higher pH. So, Grb2 presents significant structural and dynamic changes modulated by pH. If considering the role of Grb2 in cell signaling upstream, these conformational changes could be a critical mechanistic behavior of this protein, preventing/disrupting the stability of the cell signaling pathways related to cancer.

Specific phosphorylation of microtubule-associated protein 2c by extracellular signal-regulated kinase reduces interactions at its Pro-rich regions

Microtubule-associated protein 2 (MAP2) is an important neuronal target of extracellular signal-regulated kinase 2 (ERK2) involved in Raf signaling pathways, but mechanistic details of MAP2 phosphorylation are unclear. Here, we used NMR spectroscopy to quantitatively describe the kinetics of phosphorylation of individual serines and threonines in the embryonic MAP2 variant MAP2c. We carried out real-time monitoring of phosphorylation to discover major phosphorylation sites that were not identified in previous studies relying on specific antibodies. Our comparison with phosphorylation of MAP2c by a model cyclin-dependent kinase CDK2 and with phosphorylation of the MAP2c homolog Tau revealed differences in phosphorylation profiles that explain specificity of regulation of biological functions of MAP2c and Tau. To probe the molecular basis of the regulatory effect of ERK2, we investigated the interactions of phosphorylated and unphosphorylated MAP2c by NMR with single-residue resolution. As ERK2 phosphorylates mostly outside the regions binding microtubules, we studied the binding of proteins other than tubulin, namely regulatory subunit RIIα of cAMP-dependent protein kinase (PKA), adaptor protein Grb2, Src homology domain 3 of tyrosine kinases Fyn and Abl, and ERK2 itself. We found ERK2 phosphorylation interfered mostly with binding to proline-rich regions of MAP2c. Furthermore, our NMR experiments in SH-SY5Y neuroblastoma cell lysates showed that the kinetics of dephosphorylation are compatible with in-cell NMR studies and that residues targeted by ERK2 and PKA are efficiently phosphorylated in the cell lysates. Taken together, our results provide a deeper characterization of MAP2c phosphorylation and its effects on interactions with other proteins.

Negative regulation of receptor tyrosine kinases by ubiquitination: Key roles of the Cbl family of E3 ubiquitin ligases

Receptor tyrosine kinases (RTKs) serve as transmembrane receptors that participate in a broad spectrum of cellular processes including cellular growth, motility, differentiation, proliferation, and metabolism. Hence, elucidating the regulatory mechanisms of RTKs involved in an assortment of diseases such as cancers attracts increasing interest from researchers. Members of the Cbl family ubiquitin ligases (c-Cbl, Cbl-b and Cbl-c in mammals) have emerged as negative regulators of activated RTKs. Upon activation of RTKs by growth factors, Cbl binds to RTKs via its tyrosine kinase binding (TKB) domain and targets them for ubiquitination, thus facilitating their degradation and negative regulation of RTK signaling. RTKs such as epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGF), fibroblast growth factor receptor (FGFR) and hepatocyte growth factor receptor (HGFR) undergo ubiquitination upon interaction with Cbl family members. In this review, we summarize the current knowledge related to the negative regulation of RTKs by Cbl family proteins.

SOS1 interacts with Grb2 through regions that induce closed nSH3 conformations

Grb2 is an adaptor protein connecting the epidermal growth factor receptor and the downstream Son of sevenless 1 (SOS1), a Ras-specific guanine nucleotide exchange factor (RasGEF), which exchanges GDP by GTP. Grb2 contains three SH domains: N-terminal SH3 (nSH3), SH2, and C-terminal SH3 (cSH3). The C-terminal proline-rich (PR) domain of SOS1 regulates nSH3 open/closed conformations. Earlier, several nSH3 binding motifs were identified in the PR domain. More recently, we characterized by nuclear magnetic resonance and replica exchange simulations possible cSH3 binding regions. Among them, we discovered a cSH3-specific binding region. However, how PR binding at these sites regulates the nSH3/cSH3 conformation has been unclear. Here, we explore the nSH3/cSH3 interaction with linked and truncated PR segments using molecular dynamics simulations. Our 248 μs simulations include 620 distinct trajectories, each 400 ns. We construct the effective free energy landscape to validate the nSH3/cSH3 binding sites. The nSH3/cSH3-SOS1 peptide complex models indicate that strong peptide binders attract the flexible nSH3 n-Src loop, inducing a closed conformation of nSH3; by contrast, the cSH3 conformation remains unchanged. Inhibitors that disrupt the Ras-SOS1 interaction have been designed; the conformational details uncovered here may assist in the design of polypeptides inhibiting Grb2-SOS1 interaction, thus SOS1 recruitment to the membrane where Ras resides.

Crystal structure of the SH3 domain of growth factor receptor-bound protein 2

This study presents the crystal structure of the N-terminal SH3 (SH3N) domain of growth factor receptor-bound protein 2 (Grb2) at 2.5 Å resolution. Grb2 is a small (215-amino-acid) adaptor protein that is widely expressed and involved in signal transduction/cell communication. The crystal structure of full-length Grb2 has previously been reported (PDB entry 1gri). The structure of the isolated SH3N domain is consistent with the full-length structure. The structure of the isolated SH3N domain was solved at a higher resolution (2.5 Å compared with 3.1 Å for the previously deposited structure) and made it possible to resolve some of the loops that were missing in the full-length structure. In addition, interactions between the carboxy-terminal region of the SH3N domain and the Sos1-binding sites were observed in the structure of the isolated domain. Analysis of these interactions provided new information about the ligand-binding properties of the SH3N domain of Grb2.

High-Affinity Interactions of the nSH3/cSH3 Domains of Grb2 with the C-Terminal Proline-Rich Domain of SOS1

Grb2 is an adaptor protein that recruits Ras-specific guanine nucleotide exchange factor, Son of Sevenless 1 (SOS1), to the plasma membrane. SOS1 exchanges GDP by GTP, activating Ras. Grb2 consists of an SH2 domain flanked by N- and C-terminal SH3 domains (nSH3/cSH3). Grb2 nSH3/cSH3 domains have strong binding affinity for the SOS1 proline-rich (PR) domain that mediates the Grb2-SOS1 interaction. The nSH3/cSH3 domains have distinct preferred binding motifs: PxxPxR for nSH3 and PxxxRxxKP for cSH3 (x represents any natural amino acid). Several nSH3-binding motifs have been identified in the SOS1 PR domain but none specific for cSH3 binding. Even though both nSH3 and cSH3 exhibit the strongest binding to the SOS1 peptide PVPPPVPPRRRP, this mutually exclusive binding combined with other potential nSH3/cSH3 binding regions in SOS1 makes understanding the Grb2-SOS1 interaction challenging. To identify the SOS1-cSH3 binding sites, we selected seven potential binding segments in SOS1. The synthesized peptides were tested for their binding to nSH3/cSH3. Our NMR data reveal that the PKLPPKTYKREH peptide has strong binding affinity for cSH3, but very weak for nSH3. The binding specificity suggests that the most likely Grb2-SOS1 binding mode is through nSH3-PVPPPVPPRRRP and cSH3-PKLPPKTYKREH interactions, which is supported by replica-exchange simulations for the Grb2-SOS1 complex models. We propose that nSH3/cSH3 binding peptides, which effectively interrupt Grb2-SOS1 association, can serve as tumor suppressors. The Grb2-SOS1 mechanism outlined here offers new venues for future therapeutic strategies for upstream mutations in cancer, such as in EGFR.

Molecular characterization and expression analysis of protein enhancer of sevenless 2B from Artemia sinica at early embryonic development and during immune response to bacterial stimulation

Protein enhancer of sevenless 2B, E(sev)2B, is a key adapter protein in the Ras/MAPK signaling pathway which has been reported to be involved in innate immunity. In this study, the gene that encodes AsE(sev)2B was isolated from A. sinica. It was found to contain a 636 bp open reading frame encoding 211 amino acids with a calculated molecular mass of 24.357 kDa and a predicted isoelectric point of 5.39. The predicted protein contains a N-terminal Src homology 3 domain (SH3), a central Src homology 2 domain (SH2), and a C-terminal Src homology 3 domain (SH3). Homology analysis revealed that AsE(sev)2B shares 49%-95% identity with E(sev)2B homologs from other species. In this study, the expression pattern and location of AsE(sev)2B during different stages of embryonic development and bacterial challenge were investigated by means of real-time qPCR, Western blotting and immunohistochemistry. Results showed that the highest expression level of AsE(sev)2B was at 0 h. After challenged by Gram-positive bacteria and Gram-negative bacteria, AsE(sev)2B was remarkably upregulated at 10⁶ cellsL^-1 bacterial concentrations. These results suggested that AsE(sev)2B plays a vital role during early embryonic development and in immune responses against bacterial challenge.

The 11-Kilodalton Nonstructural Protein of Human Parvovirus B19 Facilitates Viral DNA Replication by Interacting with Grb2 through Its Proline-Rich Motifs

Lytic infection of human parvovirus B19 (B19V) takes place exclusively in human erythroid progenitor cells of bone marrow and fetal liver, which disrupts erythropoiesis. During infection, B19V expresses three nonstructural proteins (NS1, 11-kDa, and 7.5-kDa) and two structural proteins (VP1 and VP2). While NS1 is essential for B19V DNA replication, 11-kDa enhances viral DNA replication significantly. In this study, we confirmed the enhancement role of 11-kDa in viral DNA replication and elucidated the underlying mechanism. We found that 11-kDa specially interacts with cellular growth factor receptor-bound protein 2 (Grb2) during virus infection and in vitro We determined a high affinity interaction between 11-kDa and Grb2 that has an equilibrium dissociation constant (KD ) value of 18.13 nM. In vitro, one proline-rich motif was sufficient for 11-kDa to sustain a strong interaction with Grb2. In consistence, in vivo during infection, one proline-rich motif was enough for 11-kDa to significantly reduce phosphorylation of extracellular signal-regulated kinase (ERK). Mutations of all three proline-rich motifs of 11-kDa abolished its capability to reduce ERK activity and, accordingly, decreased viral DNA replication. Transduction of a lentiviral vector encoding a short hairpin RNA (shRNA) targeting Grb2 decreased the expression of Grb2 as well as the level of ERK phosphorylation, which resulted in an increase of B19V replication. These results, in concert, indicate that the B19V 11-kDa protein interacts with cellular Grb2 to downregulate ERK activity, which upregulates viral DNA replication.IMPORTANCE Human parvovirus B19 (B19V) infection causes hematological disorders and is the leading cause of nonimmunological fetal hydrops during pregnancy. During infection, B19V expresses two structural proteins, VP1 and VP2, and three nonstructural proteins, NS1, 11-kDa, and 7.5-kDa. While NS1 is essential, 11-kDa plays an enhancing role in viral DNA replication. Here, we elucidated a mechanism underlying 11-kDa protein-regulated B19V DNA replication. 11-kDa is tightly associated with cellular growth factor receptor-bound protein 2 (Grb2) during infection. In vitro, 11-kDa interacts with Grb2 with high affinity through three proline-rich motifs, of which at least one is indispensable for the regulation of viral DNA replication. 11-kDa and Grb2 interaction disrupts extracellular signal-regulated kinase (ERK) signaling, which mediates upregulation of B19V replication. Thus, our study reveals a novel mechanism of how a parvoviral small nonstructural protein regulates viral DNA replication by interacting with a host protein that is predominately expressed in the cytoplasm.

Overexpression of GRB2 Enhances Epithelial to Mesenchymal Transition of A549 Cells by Upregulating SNAIL Expression

GRB2 is an adaptor protein which interacts with phosphorylated TGF-β receptor and is critical for mammary tumour growth. We found that TGF-β1-induced EMT increased GRB2 expression in A549 cells (non-small cell lung cancer). Overexpression of GRB2 (A549^GRB2) enhanced cell invasion while knocking down GRB2 (A549^GRB2KD) reduced cell migration and invasion, probably due to increased vinculin and reduced Paxillin patches in A549^GRB2KD cell. TGF-β1-induced EMT was more pronounced in A549^GRB2 cells and attenuated in A549^GRB2KD cells. This could be due to the reduced expression of E-cadherin in A549^GRB2 and increased expression of E-cadherin in A549^GRB2KD cells, even before TGF-β1 stimulation. Expression of SNAIL was elevated in A549^GRB2 cells and was further enhanced by TGF-β1 stimulation, suggesting that GRB2 down-regulates E-cadherin by enhancing the expression of SNAIL. The N-SH3 domain of GRB2 was critical for suppressing E-cadherin expression, while the C-SH3 domain of GRB2 mediating interaction with proteins such as N-WASP was critical for promoting invasion, and the SH2 domain was critical for suppressing E-cadherin expression and invasion. Thus, our data suggests that GRB2 enhances EMT by suppressing E-cadherin expression and promoting invasion probably through N-WASP to promote metastasis.

Cell penetrating peptides to dissect host-pathogen protein-protein interactions in Theileria-transformed leukocytes

One powerful application of cell penetrating peptides is the delivery into cells of molecules that function as specific competitors or inhibitors of protein-protein interactions. Ablating defined protein-protein interactions is a refined way to explore their contribution to a particular cellular phenotype in a given disease context. Cell-penetrating peptides can be synthetically constrained through various chemical modifications that stabilize a given structural fold with the potential to improve competitive binding to specific targets. Theileria-transformed leukocytes display high PKA activity, but PKA is an enzyme that plays key roles in multiple cellular processes; consequently genetic ablation of kinase activity gives rise to a myriad of confounding phenotypes. By contrast, ablation of a specific kinase-substrate interaction has the potential to give more refined information and we illustrate this here by describing how surgically ablating PKA interactions with BAD gives precise information on the type of glycolysis performed by Theileria-transformed leukocytes. In addition, we provide two other examples of how ablating specific protein-protein interactions in Theileria-infected leukocytes leads to precise phenotypes and argue that constrained penetrating peptides have great therapeutic potential to combat infectious diseases in general.

Myogenic differentiation depends on the interplay of Grb2 and N-WASP

Myogenesis requires a well-coordinated withdrawal from cell cycle, morphological changes and cell fusion mediated by actin cytoskeleton. Grb2 is an adaptor protein whose central SH2 domain binds to phosphorylated tyrosine residues of activated receptors and activates intracellular signaling pathway, while its N-terminal and C-terminal SH3 domains bind to proline rich proteins such as N-WASP (Neural-Wiskott Aldrich Syndrome Protein). We found that the expression of Grb2 was increased at the beginning of differentiation and remained constant during differentiation in C2C12 myoblasts. Knocking down endogenous Grb2 expression caused a significant increase in the fusion index and expression of MyHC, a terminal differentiation marker when compared with the control. Over expression of Grb2 in C2C12 (C2C12^Grb2-Myc) reduced myotube formation and expression of MyHC. Similarly over expression of Grb2^P49L-Myc (N-terminal SH3 domain mutant) or Grb2^R86K-Myc (SH2 domain mutant) inhibited myogenic differentiation of C2C12 cells. However, the expression of Grb2^P206L-Myc (C-terminal SH3 domain mutant) did not inhibit myotube formation and expression of MyHC. This suggests that the C-terminal SH3 domain of Grb2 is critical for the inhibition of myogenic differentiation. The C2C12^Grb2-Myc cells have reduced phalloidin staining at late stages of differentiation. Expression of N-WASP in C2C12^Grb2-Myc cells rescued the myogenic defect and increased phalloidin staining (increased F-actin) in these cells. Thus our results suggest that Grb2 is a negative regulator of myogenesis and reduces myogenic differentiation by inhibiting actin polymerization/remodeling through its C-terminal SH3 domain.

TGF-β2 induces Grb2 to recruit PI3-K to TGF-RII that activates JNK/AP-1-signaling and augments invasiveness of Theileria-transformed macrophages

Theileria-infected macrophages display many features of cancer cells such as heightened invasive capacity; however, the tumor-like phenotype is reversible by killing the parasite. Moreover, virulent macrophages can be attenuated by multiple in vitro passages and so provide a powerful model to elucidate mechanisms related to transformed macrophage virulence. Here, we demonstrate that in two independent Theileria-transformed macrophage cell lines Grb2 expression is down-regulated concomitant with loss of tumor virulence. Using peptidimer-c to ablate SH2 and SH3 interactions of Grb2 we identify TGF-receptor II and the p85 subunit of PI3-K, as Grb2 partners in virulent macrophages. Ablation of Grb2 interactions reduces PI3-K recruitment to TGF-RII and decreases PIP3 production, and dampens JNK phosphorylation and AP-1-driven transcriptional activity down to levels characteristic of attenuated macrophages. Loss of TGF-R>PI3-K>JNK>AP-1 signaling negatively impacts on virulence traits such as reduced JAM-L/ITG4A and Fos-B/MMP9 expression that contribute to virulent macrophage adhesion and invasiveness.

Overexpression of GRB2 is correlated with lymph node metastasis and poor prognosis in esophageal squamous cell carcinoma

The adapter protein growth factor receptor-bound 2 (GRB2) is essential for various basic cellular functions by mediating the regulation of receptor tyrosine kinase (RTK) signaling, however, little is known about GRB2 expression in esophageal squamous cell carcinoma (ESCC). We sought to characterize GRB2 expression and its relationship with clinicopathological parameters and prognostic significance in ESCC patients. Here, it was presented that GRB2 was overexpressed in cytoplasm in 58.1% (100/172) of ESCC cases by immunohistochemistry. Survival analysis demonstrated overexpression of GRB2 protein was significantly related to poor prognosis of ESCC patients (P = 0.021). Furthermore, overexpression of GRB2 was significantly associated with the lymph node metastases. In addition, subgroup analysis according to lymph node metastasis revealed a shorter disease-free survival (DFS) in the ESCC patients with GRB2 overexpression than the patients with GRB2 low-expression (Means for DFS months: 33.8 versus 52.1). Finally, the significant difference between overexpression of GRB2 and poor survival rates exhibited in univariate analysis (P = 0.022) and multivariate Cox analysis (close to significance, P = 0.065), demonstrated that GRB2 was an independent factor in prognosis of ESCC patients. In conclusion, GRB2 expression status could be as a positive biomarker of ESCC progression and lymph node metastasis.

Identification of SH3 domain proteins interacting with the cytoplasmic tail of the a disintegrin and metalloprotease 10 (ADAM10)

The a disintegrin and metalloproteases (ADAMs) play a pivotal role in the control of development, adhesion, migration, inflammation and cancer. Although numerous substrates of ADAM10 have been identified, the regulation of its surface expression and proteolytic activity is still poorly defined. One current hypothesis is that both processes are in part modulated by protein-protein interactions mediated by the intracellular portion of the protease. For related proteases, especially proline-rich regions serving as docking sites for Src homology domain 3 (SH3) domain-containing proteins proved to be important for mediating regulatory interactions. In order to identify ADAM10-binding SH3 domain proteins, we screened the All SH3 Domain Phager library comprising 305 human SH3 domains using a GST fusion protein with the intracellular region of human ADAM10 as a bait for selection. Of a total of 291 analyzed phage clones, we found 38 SH3 domains that were precipitated with the ADAM10-derived fusion protein but not with GST. We verified the binding to the cytosolic portion of ADAM10 for several candidates by co-immunoprecipitation and/or pull down analyses. Intriguingly, several of the identified proteins have been implicated in regulating surface appearance and/or proteolytic activity of related ADAMs. Thus, it seems likely that they also play a role in ADAM10 biology.

Molecular cloning and characterization of growth factor receptor bound-protein in Clonorchis sinensis

Background

Clonorchis sinensis causes clonorchiasis, a potentially serious disease. Growth factor receptor-bound protein 2 (Grb2) is a cytosolic protein conserved among animals and plays roles in cellular functions such as meiosis, organogenesis and energy metabolism. In the present study, we report first molecular characters of growth factor receptor bound-protein (CsGrb2) from C. sinensis as counter part of Grb2 from animals and its possible functions in development and organogenesis of C. sinensis.

Methodology/Principal Findings

A CsGrb2 cDNA clone retrieved from the C. sinensis transcriptome encoded a polypeptide with a SH3-SH2-SH3 structure. Recombinant CsGrb2 was bacterially produced and purified to homogeneity. Native CsGrb2 with estimated molecular weight was identified from C. sinensis adult extract by western blotting using a mouse immune serum to recombinant CsGrb2. CsGrb2 transcripts was more abundant in the metacercariae than in the adults. Immunohistochemical staining showed that CsGrb2 was localized to the suckers, mesenchymal tissues, sperms in seminal receptacle and ovary in the adults, and abundantly expressed in most organs of the metacercariae. Recombinant CsGrb2 was evaluated to be little useful as a serodiagnostic reagent for C. sinesis human infections.

Conclusion

Grb2 protein found in C. sinensis was conserved among animals and suggested to play a role in the organogenesis, energy metabolism and mitotic spermatogenesis of C. sinensis. These findings from C. sinensis provide wider understanding on diverse function of Grb2 in lower animals such as platyhelminths.

Allostery mediates ligand binding to Grb2 adaptor in a mutually exclusive manner

Allostery plays a key role in dictating the stoichiometry and thermodynamics of multi-protein complexes driving a plethora of cellular processes central to health and disease. Herein, using various biophysical tools, we demonstrate that although Sos1 nucleotide exchange factor and Gab1 docking protein recognize two non-overlapping sites within the Grb2 adaptor, allostery promotes the formation of two distinct pools of Grb2-Sos1 and Grb2-Gab1 binary signaling complexes in concert in lieu of a composite Sos1-Grb2-Gab1 ternary complex. Of particular interest is the observation that the binding of Sos1 to the nSH3 domain within Grb2 sterically blocks the binding of Gab1 to the cSH3 domain and vice versa in a mutually exclusive manner. Importantly, the formation of both the Grb2-Sos1 and Grb2-Gab1 binary complexes is governed by a stoichiometry of 2:1, whereby the respective SH3 domains within Grb2 homodimer bind to Sos1 and Gab1 via multivalent interactions. Collectively, our study sheds new light on the role of allostery in mediating cellular signaling machinery.

Template-assisted and self-activating clicked peptide as a synthetic mimic of the SH2 domain

A new synthetic strategy for obtaining artificial receptors that selectively regulate and/or control specific protein/protein interactions was developed based on the template-assisted and the self-activating click reaction applied to a combinatorial library. Synthetic mimics of the Grb2-SH2 domain, examined as a model case, selectively bound to a target signaling protein to induce cytotoxicity and inhibit tumor growth in vivo.

Multivalent binding and facilitated diffusion account for the formation of the Grb2-Sos1 signaling complex in a cooperative manner

Despite its key role in driving cellular growth and proliferation through receptor tyrosine kinase (RTK) signaling, the Grb2-Sos1 macromolecular interaction remains poorly understood in mechanistic terms. Herein, using an array of biophysical methods, we provide evidence that although the Grb2 adaptor can potentially bind to all four PXψPXR motifs (designated herein S1-S4) located within the Sos1 guanine nucleotide exchange factor, the formation of the Grb2-Sos1 signaling complex occurs with a 2:1 stoichiometry. Strikingly, such bivalent binding appears to be driven by the association of the Grb2 homodimer to only two of four potential PXψPXR motifs within Sos1 at any one time. Of particular interest is the observation that of a possible six pairwise combinations in which S1-S4 motifs may act in concert for the docking of the Grb2 homodimer through bivalent binding, only S1 and S3, S1 and S4, S2 and S4, and S3 and S4 do so, while pairwise combinations of sites S1 and S2 and sites S2 and S3 appear to afford only monovalent binding. This salient observation implicates the role of local physical constraints in fine-tuning the conformational heterogeneity of the Grb2-Sos1 signaling complex. Importantly, the presence of multiple binding sites within Sos1 appears to provide a physical route for Grb2 to hop in a flip-flop manner from one site to the next through facilitated diffusion, and such rapid exchange forms the basis of positive cooperativity driving the bivalent binding of Grb2 to Sos1 with high affinity. Collectively, our study sheds new light on the assembly of a key macromolecular signaling complex central to cellular machinery in health and disease.

Mammalian son of sevenless Guanine nucleotide exchange factors: old concepts and new perspectives

The Son of Sevenless (Sos) factors were originally discovered 2 decades ago as specialized Ras activators in signaling pathways controlling the process of R7 cell development in the eye of Drosophila melanogaster. The 2 known members of the mammalian Sos family (Sos1 and Sos2) code for ubiquitously expressed, highly homologous (69% overall) proteins involved in coupling signals originated by cell surface receptor tyrosine kinases (RTKs) to downstream, Ras-dependent mitogenic signaling pathways. Mechanistically, the Sos proteins function as enzymatic factors interacting with Ras proteins in response to upstream stimuli to promote guanine nucleotide exchange (GDP/GTP) and subsequent formation of the active Ras-GTP complex. In this review, we summarize current knowledge on structural, regulatory, and functional aspects of the Sos family, focusing on specific aspects of Sos biology such as structure-function relationship, crosstalk with different signaling pathways, and in vivo functional significance as deduced from phenotypic characterization of Sos knockout mice and human genetic syndromes caused by germline hSos1 mutations.

Identification of a gene, Desiccate, contributing to desiccation resistance in Drosophila melanogaster

Suitable alterations in gene expression are believed to allow animals to survive drastic changes in environmental conditions. Drosophila melanogaster larvae cease eating and exit moist food to search for dry pupation sites after the foraging stage in what is known as the wandering stage. Although the behavioral change from foraging to wandering causes desiccation stress, the mechanism by which Drosophila larvae protect themselves from desiccation remains obscure. Here, we identified a gene, CG14686 (designated as Desiccate (Desi)), whose expression was elevated during the wandering stage. The Desi expression level was reversibly decreased by transferring wandering larvae to wet conditions and increased again by transferring them to dry conditions. Elevation of Desi expression was also observed in foraging larvae when they were placed in dry conditions. Desi encoded a 261-amino acid single-pass transmembrane protein with notable motifs, such as SH2 and PDZ domain-binding motifs and a cAMP-dependent protein kinase phosphorylation motif, in the cytoplasmic region, and its expression was observed mainly in the epidermal cells of the larval integuments. Overexpression of Desi slightly increased the larval resistance to desiccation stress during the second instar. Furthermore, Desi RNAi larvae lost more weight under dry conditions, and subsequently, their mortalities significantly increased compared with control larvae. Under dry conditions, consumption of carbohydrate was much higher in Desi RNAi larvae than control larvae. Based on these results, it is reasonable to conclude that Desi contributes to the resistance of Drosophila larvae to desiccation stress.

Recruitment of Pyk2 to SHPS-1 signaling complex is required for IGF-I-dependent mitogenic signaling in vascular smooth muscle cells

In vascular smooth muscle cells, IGF-I stimulates SHPS-1/SHP2/Src complex formation which is required for IGF-I-stimulated cell proliferation. Using SHP2/Src silencing and a Pyk2/Y402F mutant, we showed that Pyk2 was also recruited to the SHPS-1 complex. Pyk2 recruitment to SHPS-1 is mediated via the interaction of Pyk2 Tyr402 and the Src in response to IGF-I. Following Src/Pyk2 association, Src phosphorylates Pyk2 on Tyr881 providing a binding site for Grb2. Cells expressing Pyk2/Y881F showed decreased Grb2 recruitment to SHPS-1 and impaired Shc/Grb2 association. This change led to reduced Erk1/2 (MAP kinase) activation and cell proliferation in response to IGF-I. Our results show that, following its recruitment to the SHPS-1 signaling complex, Pyk2 localizes Grb2 in close proximity to Shc thereby facilitating Shc/Grb2 association which leads to Erk1/2 activation in response to IGF-I. Thus, Pyk2 recruitment to SHPS-1 plays an important role in regulating the IGF-I-stimulated mitogenic response.

Binding of flexible and constrained ligands to the Grb2 SH2 domain: structural effects of ligand preorganization

Structures of the Grb2 SH2 domain complexed with a series of pseudopeptides containing flexible (benzyl succinate) and constrained (aryl cyclopropanedicarboxylate) replacements of the phosphotyrosine (pY) residue in tripeptides derived from Ac-pYXN-NH(2) (where X = V, I, E and Q) were elucidated by X-ray crystallography. Complexes of flexible/constrained pairs having the same pY + 1 amino acid were analyzed in order to ascertain what structural differences might be attributed to constraining the phosphotyrosine replacement. In this context, a given structural dissimilarity between complexes was considered to be significant if it was greater than the corresponding difference in complexes coexisting within the same asymmetric unit. The backbone atoms of the domain generally adopt a similar conformation and orientation relative to the ligands in the complexes of each flexible/constrained pair, although there are some significant differences in the relative orientations of several loop regions, most notably in the BC loop that forms part of the binding pocket for the phosphate group in the tyrosine replacements. These variations are greater in the set of complexes of constrained ligands than in the set of complexes of flexible ligands. The constrained ligands make more direct polar contacts to the domain than their flexible counterparts, whereas the more flexible ligand of each pair makes more single-water-mediated contacts to the domain; there was no correlation between the total number of protein-ligand contacts and whether the phosphotyrosine replacement of the ligand was preorganized. The observed differences in hydrophobic interactions between the complexes of each flexible/constrained ligand pair were generally similar to those observed upon comparing such contacts in coexisting complexes. The average adjusted B factors of the backbone atoms of the domain and loop regions are significantly greater in the complexes of constrained ligands than in the complexes of the corresponding flexible ligands, suggesting greater thermal motion in the crystalline state in the former complexes. There was no apparent correlation between variations in crystal packing and observed structural differences or similarities in the complexes of flexible and constrained ligands, but the possibility that crystal packing might result in structural variations cannot be rigorously excluded. Overall, it appears that there are more variations in the three-dimensional structure of the protein and the ligand in complexes of the constrained ligands than in those of their more flexible counterparts.

Assembly of the Sos1-Grb2-Gab1 ternary signaling complex is under allosteric control

Allostery has evolved as a form of local communication between interacting protein partners allowing them to quickly sense changes in their immediate vicinity in response to external cues. Herein, using isothermal titration calorimetry (ITC) in conjunction with circular dichroism (CD) and macromolecular modeling (MM), we show that the binding of Grb2 adaptor--a key signaling molecule involved in the activation of Ras GTPase--to its downstream partners Sos1 guanine nucleotide exchange factor and Gab1 docker is under tight allosteric regulation. Specifically, our findings reveal that the binding of one molecule of Sos1 to the nSH3 domain allosterically induces a conformational change within Grb2 such that the loading of a second molecule of Sos1 onto the cSH3 domain is blocked and, in so doing, allows Gab1 access to the cSH3 domain in an exclusively non-competitive manner to generate the Sos1-Grb2-Gab1 ternary signaling complex.

KIF26A is an unconventional kinesin and regulates GDNF-Ret signaling in enteric neuronal development

The kinesin superfamily proteins (KIFs) are motor proteins that transport organelles and protein complexes in a microtubule- and ATP-dependent manner. We identified KIF26A as a new member of the murine KIFs. KIF26A is a rather atypical member as it lacks ATPase activity. Mice with a homozygous deletion of Kif26a developed a megacolon with enteric nerve hyperplasia. Kif26a-/- enteric neurons showed hypersensitivity for GDNF-Ret signaling, and we find that KIF26A suppressed GDNF-Ret signaling by direct binding and inhibition of Grb2, an essential component of GDNF/Akt/ERK signaling. We therefore propose that the unconventional kinesin KIF26A plays a key role in enteric nervous system development by repressing a cell growth signaling pathway.

Gasp, a Grb2-associating protein, is critical for positive selection of thymocytes

T cells develop in the thymus through positive and negative selection, which are responsible for shaping the T cell receptor (TCR) repertoire. To elucidate the molecular mechanisms involved in selection remains an area of intense interest. Here, we identified and characterized a gene product Gasp (Grb2-associating protein, also called Themis) that is critically required for positive selection. Gasp is a cytosolic protein with no known functional motifs that is expressed only in T cells, especially immature CD4/CD8 double positive (DP) thymocytes. In the absence of Gasp, differentiation of both CD4 and CD8 single positive cells in the thymus was severely inhibited, whereas all other TCR-induced events such as beta-selection, negative selection, peripheral activation, and homeostatic proliferation were unaffected. We found that Gasp constitutively associates with Grb2 via its N-terminal Src homology 3 domain, suggesting that Gasp acts as a thymocyte-specific adaptor for Grb2 or regulates Ras signaling in DP thymocytes. Collectively, we have described a gene called Gasp that is critical for positive selection.

SH3 domains of Grb2 adaptor bind to PXpsiPXR motifs within the Sos1 nucleotide exchange factor in a discriminate manner

Ubiquitously encountered in a wide variety of cellular processes, the Grb2-Sos1 interaction is mediated through the combinatorial binding of nSH3 and cSH3 domains of Grb2 to various sites containing PXpsiPXR motifs within Sos1. Here, using isothermal titration calorimetry, we demonstrate that while the nSH3 domain binds with affinities in the physiological range to all four sites containing PXpsiPXR motifs, designated S1, S2, S3, and S4, the cSH3 domain can only do so at the S1 site. Further scrutiny of these sites yields rationale for the recognition of various PXpsiPXR motifs by the SH3 domains in a discriminate manner. Unlike the PXpsiPXR motifs at S2, S3, and S4 sites, the PXpsiPXR motif at the S1 site is flanked at its C-terminus with two additional arginine residues that are absolutely required for high-affinity binding of the cSH3 domain. In striking contrast, these two additional arginine residues augment the binding of the nSH3 domain to the S1 site, but their role is not critical for the recognition of S2, S3, and S4 sites. Site-directed mutagenesis suggests that the two additional arginine residues flanking the PXpsiPXR motif at the S1 site contribute to free energy of binding via the formation of salt bridges with specific acidic residues in SH3 domains. Molecular modeling is employed to project these novel findings into the 3D structures of SH3 domains in complex with a peptide containing the PXpsiPXR motif and flanking arginine residues at the S1 site. Taken together, this study furthers our understanding of the assembly of a key signaling complex central to cellular machinery.

Prolectin, a glycan-binding receptor on dividing B cells in germinal centers

Prolectin, a previously undescribed glycan-binding receptor, has been identified by re-screening of the human genome for genes encoding proteins containing potential C-type carbohydrate-recognition domains. Glycan array analysis revealed that the carbohydrate-recognition domain in the extracellular domain of the receptor binds glycans with terminal alpha-linked mannose or fucose residues. Prolectin expressed in fibroblasts is found at the cell surface, but unlike many glycan-binding receptors it does not mediate endocytosis of a neoglycoprotein ligand. However, compared with other known glycan-binding receptors, the receptor contains an unusually large intracellular domain that consists of multiple sequence motifs, including phosphorylated tyrosine residues, that allow it to interact with signaling molecules such as Grb2. Immunohistochemistry has been used to demonstrate that prolectin is expressed on a specialized population of proliferating B cells in germinal centers. Thus, this novel receptor has the potential to function in carbohydrate-mediated communication between cells in the germinal center.

Overexpression of Grb2/HER2 signaling in Chinese gastric cancer: their relationship with clinicopathological parameters and prognostic significance

PURPOSE

Growth factor receptor-bound 2 (Grb2)-mediated HER2 signaling is thought to play a critical role in gastric cancer development, progression and metastasis. However, little is known about their expression in gastric cancer. In this study, we try to explore their relationship with clinicopathological parameters and prognostic significance in gastric cancer patients.

MATERIALS AND METHODS

We examined the expression of Grb2 and HER2 in normal gastric mucosa, primary gastric cancers, and lymph node metastases using immunohistochemical analysis of tissue microarrays containing specimens obtained from 1,143 patients with gastric cancer.

RESULTS

Grb2 was overexpressed in 48% (553/1,143) of primary tumors and 59% (155/262) of lymph node metastases. We observed significant differences in Grb2 expression between the primary tumors and the lymph node metastases (P < 0.01). Also, HER2 was overexpressed in 28% (321/1,143) of the primary tumors and 30% (79/262) of the lymph node metastases. Overexpression of Grb2 and Her2 was associated with age (>60 years), tumor location (cardia of stomach), adenocarcinoma, and high/moderate differentiation. A significant relationship was found between Grb2 and HER2 expression using Chi-Square Tests and Spearman Correlation. Overexpression of Grb2 correlated significantly with poor survival rates in both univariate and multivariate analysis.

CONCLUSIONS

Our data demonstrated a progressive amplification of Grb2 and HER2 expression in gastric carcinogenesis, suggesting the importance of Grb2 and HER2 as positive biomarkers for gastric cancer development and progression.

A microinjectable biological system, the Xenopus oocyte, as an approach to understanding signal transduction protein function

To study protein function in cellular signaling, manual microinjection is a direct technique, but limited by the small size of many cells. The giant vertebrate cell, the Xenopus laevis oocyte, is a perfect model system to perform these studies. Oocytes are numerous and synchronous cells, arrested in the G2 phase of the cell cycle and easily amenable to biochemical, electrophysiological, and cytological studies. We describe how to microinject proteins or peptides in this model and we study, as an example, the Grb2 transduction cascade.

Structural basis of the differential binding of the SH3 domains of Grb2 adaptor to the guanine nucleotide exchange factor Sos1

Grb2-Sos1 interaction, mediated by the canonical binding of N-terminal SH3 (nSH3) and C-terminal SH3 (cSH3) domains of Grb2 to a proline-rich sequence in Sos1, provides a key regulatory switch that relays signaling from activated receptor tyrosine kinases to downstream effector molecules such as Ras. Here, using isothermal titration calorimetry in combination with site-directed mutagenesis, we show that the nSH3 domain binds to a Sos1-derived peptide containing the proline-rich consensus motif PPVPPR with an affinity that is nearly threefold greater than that observed for the binding of cSH3 domain. We further demonstrate that such differential binding of nSH3 domain relative to the cSH3 domain is largely due to the requirement of a specific acidic residue in the RT loop of the beta-barrel fold to engage in the formation of a salt bridge with the arginine residue in the consensus motif PPVPPR. While this role is fulfilled by an optimally positioned D15 in the nSH3 domain, the chemically distinct and structurally non-equivalent E171 substitutes in the case of the cSH3 domain. Additionally, our data suggest that salt tightly modulates the binding of both SH3 domains to Sos1 in a thermodynamically distinct manner. Our data further reveal that, while binding of both SH3 domains to Sos1 is under enthalpic control, the nSH3 binding suffers from entropic penalty in contrast to entropic gain accompanying the binding of cSH3, implying that the two domains employ differential thermodynamic mechanisms for Sos1 recognition. Our new findings are rationalized in the context of 3D structural models of SH3 domains in complex with the Sos1 peptide. Taken together, our study provides structural basis of the differential binding of SH3 domains of Grb2 to Sos1 and a detailed thermodynamic profile of this key protein-protein interaction pertinent to cellular signaling and cancer.

The cytotoxicity of a Grb2-SH3 inhibitor in Bcr-Abl positive K562 cells

Chronic myelogenous leukemia (CML) is characterized by the presence of Bcr-Abl oncoprotein. Gleevec has been designed to treat many CML patients by specifically targeting Bcr-Abl, but resistance to it is already apparent in many cases. In CML cells, Bcr-Abl activates several signaling pathways, including the Ras-dependent pathway, in which growth factor receptor binding 2 (Grb2) acts as an adaptor protein. A specific Grb2-SH3 inhibitor (denoted as peptidimer-c) that disrupts Grb2-Sos complex was designed and synthesized in our laboratory. In this study, we investigated the effect and the molecular mechanism of this inhibitor. Peptidimer-c was shown to bind to Grb2 in K562 cells, a cell line over-expressing Bcr-Abl oncoprotein. It caused cytotoxicity in the cells, and inhibited their ability of colony formation in the semi-solid medium. It was shown to induce apoptosis of K562 cells in a dose-dependent mode, the apoptotic effect of peptidimer-c being associated with caspase-3 activation. The effect of peptidimer-c on growth inhibition was also shown to be accompanied by S-phase arrest of cell cycle mediated by down-regulation of cyclin A and Cdk2, as well as phospho-Cdk2. The above results indicated that peptidimer-c may be another potential therapeutic agent for CML, which can induce S-phase arrest in the Bcr-Abl positive K562.

A non-canonical Grb2-PLC-gamma1-Sos cascade triggered by lipovitellin 1, an apolipoprotein B homologue

The injection of the Grb2 adapter in Xenopus oocytes promotes G2/M transition without stimulation from a receptor only the first day after the oocytes removal from the ovaries. This cell cycle reinitiation is Ras-dependent and requires the SH2 and SH3 domains of Grb2. The SH2 domain of Grb2 binds the tyrosine phosphorylated lipovitellin1, a homologue of the human apolipoprotein B. The N-SH3 domain of Grb2 is linked to a proline-rich sequence of the C2 domain of PLC-gamma1, PLC-gamma1 itself is linked, through its SH3 domain, to the C-terminal proline-rich region of Sos. When Grb2-PLC-gamma1-Sos is associated, PLC-gamma1 is not phosphorylated on Y783 but shows a phospholipase activity. Inhibition of lipovitellin 1 or PLC-gamma1 avoids Grb2-induced cell cycle reinitiation. Therefore, the Grb2-lipovitellin 1 association is the starting point of a novel signaling pathway, where PLC-gamma1 binds Grb2 and recruits Sos.

Grb2-SH3 ligand inhibits the growth of HER2+ cancer cells and has antitumor effects in human cancer xenografts alone and in combination with docetaxel

HER2 represents an important signaling pathway in breast and other cancers. Herceptin has demonstrated clinical activity, but resistance is common. Thus, new therapeutic approaches to the HER2 pathway are needed. Grb2 is an adaptor protein involved in Ras-dependent signaling induced by HER2 receptors. A specific Grb2-SH3 ligand, designed and synthesized in our laboratory, called peptidimer-c, inhibited colony formation in HER2 overexpressing SKBr3 cancer cells. Combined treatment of peptidimer-c with docetaxel further inhibited both colony formation and tumor cell survival compared to docetaxel treatment alone. Efficacy of this combined treatment was correlated with a reduction in the phosphorylation of MAPK and AKT. Finally, peptidimer-c reduced the growth of a HER2(+) human breast cancer (BK111) xenograft in nude mice and potentiated the antitumor effect of docetaxel in a HER2+ hormone-independent human prostate adenocarcinoma (PAC120 HID28) xenograft. These results validate Grb2 as a new target for the HER2 pathway.

A system-wide analysis of differentially expressed genes in ectopic and eutopic endometrium

BACKGROUND

Decades of research suggest that endometriosis is a complex disorder, with varying severity, onset and progression. Many genes have been associated with endometriosis through a number of studies and now microarray analyses have added to the list of perturbed or differentially regulated genes. Thus, it is difficult to see 'the big picture' without first integrating these multiple, heterogeneous sources of high-quality information for analysis.

METHODS

The goal of this study was to infer correlative and/or causal trends by combining empirical microarray analysis with a historical knowledge base of genetic relationships in endometriosis via a program called IRIDESCENT.

RESULTS

Importantly, we found a number of genes, which may have a central role in endometriosis, despite the fact that few or no past studies have reported these associations.

CONCLUSIONS

Several genes listed as non-responders on the microarray were found to be regulated post-transcriptionally, illustrating the importance of integrating multiple data sources.

Structural and energetic aspects of Grb2-SH2 domain-swapping

The SH2 domain of growth factor receptor-bound protein 2 (Grb2) has been the focus of numerous studies, primarily because of the important roles it plays in signal transduction. More recently, it has emerged as a useful protein to study the consequences of ligand preorganization upon energetics and structure in protein-ligand interactions. The Grb2-SH2 domain is known to form a domain-swapped dimer, and as part of our investigations toward correlating structure and energetics in biological systems, we examined the effects that domain-swapping dimerization of the Grb2-SH2 domain had upon ligand binding affinities. Isothermal titration calorimetry was performed using Grb2-SH2 in both its monomeric and domain-swapped dimeric forms and a phosphorylated tripeptide AcNH-pTyr-Val-Asn-NH(2) that is similar to the Shc sequence recognized by Grb2-SH2 in vivo. The two binding sites of domain-swapped dimer exhibited a 4- and a 13-fold reduction in ligand affinity compared to monomer. Crystal structures of peptide-bound and uncomplexed forms of Grb2-SH2 domain-swapped dimer were obtained and reveal that the orientation of residues V122, V123, and R142 may influence the conformation of W121, an amino acid that is believed to play an important role in Grb2-SH2 ligand sequence specificity. These findings suggest that domain-swapping of Grb2-SH2 not only results in a lower affinity for a Shc-derived ligand, but it may also affect ligand specificity.

Dystroglycan expression is reduced during prostate tumorigenesis and is regulated by androgens in prostate cancer cells

Prostate cancer, the most frequently diagnosed cancer in Western men, can display a high variability in term of clinical aggressiveness and prognosis and none of the available markers is able to accurately predict its clinical course. Dystroglycan (DG), a non-integrin adhesion molecule, is a complex formed by two subunits, alpha- and beta-DG, which bind to extracellular matrix molecules and cytoskeleton, respectively. DG expression is frequently reduced in human cancers and has been related to tumor grade and aggressiveness. This study investigated the role of DG in human prostate tumorigenesis and its suitability as a prognostic marker. The expression level of extracellular alpha-DG subunit was frequently reduced in human prostate cancer cell lines and primary tumors and the percentage of positive tumor cells was significantly further decreased in vivo following androgen ablation therapy (median = 1%) compared to pre-treatment samples (median = 28%). A significant relationship was observed between alpha-DG staining on the post-treatment samples and tumor recurrence. A dose- and time-dependent decrease of DG expression also occurred in human prostate cancer cells following treatment with the anti-androgen flutamide. Stable expression of an exogenous DG cDNA in the LNCaP human prostate carcinoma cell line resulted in a marked inhibition of both anchorage-dependent and independent growth and of the in vivo tumorigenicity. These findings confirm and extend previous evidence that disturbances in the function of the DG complex might contribute to the definition of the malignant behavior of prostate cancer cells and suggest that androgens might regulate DG expression in these cells.

Design and Concise Synthesis of Fully Protected Analogues of l-γ-Carboxyglutamic Acid

The design and synthesis of four nonnaturally occurring amino acid analogues of l-gamma-carboxyglutamic acid (Gla), appropriately protected for Fmoc-based solid-phase peptide synthesis (SPPS), is described. These amino acids are Bu-Mal 2, BCAH 3, Pen-Mal 4, and Cm-Gla 5. These Gla analogues have been designed to replace the glutamic acid of position 1 in the cyclic decapeptide G1TE, which is a potent inhibitor of tyrosine kinase, to further enhance binding to the Grb2-SH2 domain of signal transduction receptors. In the new amino acids, the propionic acid side chain of Glu has been replaced by a malonyl or a carboxymethylmalonyl moiety located at different distances from the alpha-carbon to optimize interactions in the phosphotyrosine-binding cavity of the Grb2-SH2 domain. Additionally, a direct and efficient synthetic route for the preparation of Fmoc-protected l-gamma-carboxyglutamic acid, which is amenable to large-scale production, has been developed to provide this important and unique amino acid(1) in 55% overall yield.

Affinity chromatography for purification of the modular protein growth factor receptor-bound protein 2 and development of a screening test for growth factor receptor-bound protein 2 Src homology 3 domain inhibitor using peroxidase-linked ligand

Growth factor receptor-bound protein 2 (Grb2) is an adapter protein involved in the Ras-dependent signaling pathway that plays an important role in human cancers initiated by oncogenic receptors. Grb2 is constituted by one Src homology 2 domain surrounded by two SH3 domains, and the inhibition of the interactions produced by these domains could provide an antitumor approach. In evaluating chemical libraries, to search for potential Grb2 inhibitors, it was necessary to elaborate a rapid test for their screening. We have developed, first, a batch method based on the use of an affinity column bearing a Grb2-SH3 peptide ligand to isolate highly purified Grb2. We subsequently describe a very rapid 96-well screening of inhibitors based on a simple competition between purified Grb2 and a peroxidase-coupled proline-rich peptide.

Structural Comparison of the Unstable drkN SH3 Domain and a Stable Mutant†,‡

The N-terminal SH3 domain of the Drosophila adapter protein Drk (drkN SH3 domain) is marginally stable (DeltaG(U) = 1 kcal/mol) and exists in equilibrium between folded and highly populated unfolded states. The single substitution T22G, however, completely stabilizes the protein (DeltaG(U) = 4.0 kcal/mol). To probe the causes of instability of the wild-type (WT) protein and the dramatic stabilization of the mutant, we determined and compared nuclear magnetic resonance structures of the folded WT and mutant drkN SH3 domains. Residual dipolar coupling (RDC) and carbonyl chemical-shift anisotropy (C'-CSA) restraints measured for the WT and T22G domains were used for calculating the structures. The structures for the WT and mutant are highly similar. Thr22 of the WT and Gly22 of the mutant are at the i + 2 position of the diverging, type-II beta-turn. Interestingly, not only Gly22 but also Thr22 successfully adopt an alpha(L) conformation, required at this position of the turn, despite the fact that positive phi values are energetically unfavorable and normally disallowed for threonine residues. Forcing the Thr22 residue into this unnatural conformation increases the free energy of the folded state of the WT domain relative to its T22G mutant. Evidence for residual helix formation in the diverging turn region has been previously reported for the unfolded state of the WT drkN SH3 domain, and this, in addition to other residual structure, has been proposed to play a role in decreasing the free energy of the unfolded state of the protein. Together these data provide evidence that both increasing the free energy of the folded state and decreasing the free energy of the unfolded state of the protein contribute to instability of the WT drkN SH3 domain.

Binding affinity difference induced by the stereochemistry of the sulfoxide bridge of the cyclic peptide inhibitors of Grb2-SH2 domain: NMR studies for the structural origin

The SAR study on a phage library-derived non-phosphorylated cyclic peptide ligand of Grb2-SH2 domain indicates that the configuration of the cyclization linkage is crucial for assuming the active binding conformation. When the thioether linkage was oxidized to the two chiral sulfoxides, the R-configured sulfoxide-cyclized peptide displayed 10-30 times more potency than the corresponding S-configured one in binding affinity to the Grb2-SH2 domain. In this paper, the solution structures of such a pair of sulfoxide-bridged cyclic peptide diastereoisomers, i.e., cyclo[CH(2)CO-Gla(1)-L-Y-E-N-V-G-NPG-Y-(R/S)C(O)(10)]-amide, were determined by NMR and molecular dynamics simulation. Results indicate that the consensus sequence of Y(3)-E(4)-N(5)-V(6) in both diastereoisomers adopt a beta-turn conformation; however, the R-configured peptide forms an extended structure with a circular backbone conformation, while the S-configured isomer forms a compact structure with key residues buried inside the molecule. The average root-mean-square deviations were found to be 0.756 and 0.804 A, respectively. It is apparent that the chiral S-->O group played a key role in the solution structures of the sulfoxide-bridged cyclic peptides. The R-sulfoxide group forms an intramolecular hydrogen bond with the C-terminal amide, conferring a more rigid conformation with all residues protruding outside except for Leu2, in which the Gla1 and Tyr3 share an overlapping function as previous SAR studies proposed. Additionally, the extended structure endows a more hydrophilic binding surface of the R-configured peptide to facilitate its capture by its targeted protein. In comparison, the S-configured sulfoxide was embedded inside the ligand peptide leading to a compact structure, in which the essential residues of Gla1, Tyr3, and Asn5 form multiple intramolecular hydrogen bonds resulting in an unfavorable conformational change and a substantial loss of the interaction with the protein. The solution structures disclosed by our NMR and molecular dynamics simulation studies provide a molecular basis for understanding how the chirality of the cyclization linkage remarkably discriminates in terms of the binding affinity, thus advancing the rational design of potent non-phosphorylated inhibitors of Grb2-SH2 domain as antitumor agents.

Structure-activity relationships of small phosphopeptides, inhibitors of Grb2 SH2 domain, and their prodrugs

To develop potential antitumor agents directed toward HER2/ErbB2 overexpression in cancer, we have designed inhibitors of the recognition between the phosphotyrosine of the receptor and the SH2 domain of the adaptor protein Grb2. In the first part of the paper, we report the synthesis of mimetics of the constrained (alpha-Me)phosphotyrosine residue such as (alpha-Me)-4-phosphonomethylphenylalanine (-CH2PO3H2), (alpha-Me) 4-phosphonodifluoromethylphenylalanine (-CF2PO3H2), and (alpha-Me)-4-phosphonophenylalanine (-PO3H2). The incorporation of these residues in the mAZ-pTyr-Xaa-Asn-NH2 series provided compounds with very high affinity for the Grb2 SH2 domain, in the 10(-8)-10(-9) range of Kd values. These compounds behave as potent antagonists of the Grb2-Shc interaction. Our results highlight the importance of the doubly negative charge borne by the pY + 1 amino acid in accordance with the interactions observed in the complex crystallized between mAZ-pTyr-(alphaMe)pTyr-Asn-NH2 and the Grb2 SH2 domain. mAZ-pTyr-(alphaMe)pTyr-Asn-NH2 was derivatized as the S-acetyl thioester (SATE) of the phosphotyrosine residues, and its surrogates provided prodrugs with very potent antiproliferative activity on cells overexpressing HER2/ErbB2, with ED50 values amounting to 0.1 microM. Finally a new prodrug is put forth under the form of a monobenzyl ester of phosphate group that is as active as and much easier to synthesize than SATE prodrugs. These compounds show promising activity for further testing on in vivo models.

Tyrosine kinase receptors as attractive targets of cancer therapy

Receptor tyrosine kinases (RTKs) are the main mediators of the signaling network that transmit extracellular signals into the cell, and control cellular differentiation and proliferation. Recent and rapid advances in our understanding of cellular signaling by receptor tyrosine kinases, in normal and malignant cells, have brought to light the potential of RTKs as selective anti-cancer targets. Their activity is normally tightly controlled and regulated. Overexpression of RTK proteins or functional alterations caused by mutations in the corresponding genes or abnormal stimulation by autocrine growth factor loops contribute to constitutive RTK signaling, resulting in dysregulated cell growth and cancer. The mechanisms of uncontrolled RTK signaling that leads to cancer has provided the rationale for anti-RTK drug development. Herceptin, Gleevec, and Iressa are the first examples of drugs which have successfully translated basic research on oncogenes into cancer therapeutics. RTKs can be viewed as multifunctional targets, and strategies towards the prevention and inhibition of RTK signaling include antibodies, antagonist ligands, small molecule inhibitors of protein kinase activity, and inhibitors of protein-protein interactions. Progresses in the field of rational drug design and computational chemistry will vastly benefit from the availability of increasing structural knowledge of both the kinase domains and the ligand-binding sites of these receptors.