Intertwined dimeric structure for the SH3 domain of the c-Src tyrosine kinase induced by polyethylene glycol binding

The impact of oncogenic mutations of the viral Src kinase on the structure and stability of the SH3 domain

Src kinase belongs to the family of Src-related nonreceptor tyrosine kinases. Because of its physiological role in cell growth and proliferation, its activity is strictly controlled by several mechanisms. Nevertheless, in viral Src kinase (v-Src) some of these mechanisms fail, and its uncontrolled activity is responsible for the occurrence of cancer. Here, the crystal structures of three SH3-domain mutants of v-Src were determined to unveil the effects of these oncogenic mutations in this regulatory domain. Mutations in the n-Src and distal loops have a low impact on the overall structure of the domain and its capacity to form intertwined dimers. However, mutations in the RT loop compromise the stability of the domain and make the protein very prone to aggregation. Additionally, these mutations prevent the formation of intertwined dimers. The results show a synergistic effect between mutations in the RT loop and those in the n-Src and distal loops. Analysis of the structures of the v-Src SH3-domain mutants and the closed inactive conformation of cellular Src kinase (c-Src) point to a loss of the interactions that are required to establish the compact inactive form of the kinase. Nevertheless, an analysis of structures of the c-Src SH3 domain complexed with class I and II peptides points to minor changes in the interactions between the v-Src SH3 domain and these peptides. In this way, the structures reported here indicate that mutations in the RT loop might impair the kinase regulation mechanism without affecting the recognition of short proline-rich motifs in the target proteins of the kinase, thus explaining the oncogenic behaviour of the protein.

Small molecule AX-024 reduces T cell proliferation independently of CD3ϵ/Nck1 interaction, which is governed by a domain swap in the Nck1-SH3.1 domain

Activation of the T cell receptor (TCR) results in binding of the adapter protein Nck (noncatalytic region of tyrosine kinase) to the CD3ϵ subunit of the TCR. The interaction was suggested to be important for the amplification of TCR signals and is governed by a proline-rich sequence (PRS) in CD3ϵ that binds to the first Src homology 3 (SH3) domain of Nck (Nck-SH3.1). Inhibition of this protein/protein interaction ameliorated inflammatory symptoms in mouse models of multiple sclerosis, psoriasis, and asthma. A small molecule, AX-024, was reported to inhibit the Nck/CD3ϵ interaction by physically binding to the Nck1-SH3.1 domain, suggesting a route to develop an inhibitor of the Nck1/CD3ϵ interaction for modulating TCR activity in autoimmune and inflammatory diseases. We show here that AX-024 reduces T cell proliferation upon weak TCR stimulation but does not significantly affect phosphorylation of Zap70 (ζ chain of T cell receptor–associated protein kinase 70). We also find that AX-024 is likely not involved in modulating the Nck/TCR interaction but probably has other targets in T cells. An array of biophysical techniques did not detect a direct interaction between AX-024 and Nck-SH3.1 in vitro. Crystal structures of the Nck-SH3.1 domain revealed its binding mode to the PRS in CD3ϵ. The SH3 domain tends to generate homodimers through a domain swap. Domain swaps observed previously in other SH3 domains indicate a general propensity of this protein fold to exchange structural elements. The swapped form of Nck-SH3.1 is unable to bind CD3ϵ, possibly representing an inactive form of Nck in cells.

The effect of an engineered ATCUN motif on the structure and biophysical properties of the SH3 domain of c-Src tyrosine kinase

Metal binding to sites engineered in proteins can provide an increase in their stability and facilitate new functions. Besides the sites introduced in purpose, sometimes they are present accidentally as a consequence of the expression system used to produce the protein. This happens with the copper- and nickel-binding (ATCUN) motif generated by the amino-terminal residues Gly-Ser-His. This ATCUN motif is fortuitously present in many proteins, but how it affects the structural and biophysical characterization of the proteins has not been studied. In this work, we have compared the structure and biophysical properties of a small modular domain, the SH3 domain of the c-Src tyrosine kinase, cloned with and without an ATCUN motif at the N terminus. At pH 7.0, the SH3 domain with the ATCUN motif binds nickel with a binding constant K_a = 28.0 ± 3.0 mM^-1. The formation of the nickel complex increases the thermal and chemical stability of the SH3 domain. A comparison of the crystal structures of the SH3 domain with and without the ATCUN motif shows that the binding of nickel does not affect the overall structure of the SH3 domain. In all crystal structures analyzed, residues Gly-Ser-His in complex with Ni²⁺ show a square planar geometry. The CD visible spectrum of the nickel complex shows that this geometry is also present in the solution. Therefore, our results not only show that the ATCUN motif might influence the biophysical properties of the protein, but also points to an advantageous stabilization of the protein with potential biotechnological applications.

Modified properties of alternan polymers arising from deletion of SH3-like motifs in Leuconostoc citreum ABK-1 alternansucrase

Alternansucrase (ALT, EC 2.4.1.140) catalyses the formation of an alternating 〈-1, 3/1, 6-linked glucan, with periodic branch points, from sucrose substrate. Beyond the catalytic domain, this enzyme harbours seven additional C-terminal SH3-like repeats. We herein generated two truncated alternansucrases, possessing deletions of three and seven adjacent SH3 motifs, giving Δ3SHALT and Δ7SHALT. Δ3SHALT and Δ7SHALT exhibited kcat/Km for transglycosylation activity 2.3- and 1.5-fold lower than wild-type ALT (WTALT), while hydrolysis was detected only in the truncated ALTs, oligosaccharide patterns and polymer glycosidic linkage were similar to that of WTALT. The viscosities of ALT polymers increase by ˜100-fold at 15% (w/v), with gel-like states formed at 12.5, 15.0, and 20.0% (w/v) produced by polymer from WTALT, Δ3SHALT, and Δ7SHALT, respectively. The average nanoparticle sizes of Δ3SHALT and Δ7SHALT polymers were 80 nm, compared to 90 nm from WTALT. In conclusion, even relatively subtle differences in the structure of ALT-produced alternan give rise to profound impact on the glucan polymer physicochemical properties.

A New Physiological Role for the DNA Molecule as a Protector against Drying Stress in Desiccation-Tolerant Microorganisms

The DNA molecule is associated with the role of encoding information required to produce RNA which is translated into proteins needed by the cell. This encoding involves information transmission to offspring or to other organisms by horizontal transfer. However, despite the abundance of this molecule in both the cell and the environment, its physiological role seems to be restricted mainly to that of a coding and inheritance molecule. In this paper, we report a new physiological role for the DNA molecule as involved in protection against desiccation, in addition to its well-established main information transfer and other recently reported functions such as bio-film formation in eDNA form. Desiccation-tolerant microorganisms such as Microbacterium sp. 3J1 significantly upregulate genes involved in DNA synthesis to produce DNA as part of their defensive mechanisms to protect protein structures and functions from drying according to RNA-seq analysis. We have observed the intracellular overproduction of DNA in two desiccation-tolerant microorganisms, Microbacterium sp. 3J1 and Arthrobacter siccitolerans 4J27, in response to desiccation signals. In addition, this conclusion can be made from our observations that synthetic DNA protects two proteins from drying and when part of a xeroprotectant preparation, DNA from various organisms including desiccation-sensitive species, does the same. Removal of DNA by nuclease treatment results in absence of this additive protective effect. We validated this role in biochemical and biophysical assays in proteins and occurs in trans even with short, single chains of synthetically produced DNA.

Creation of an apoptin-derived peptide that interacts with SH3 domains and inhibits glioma cell migration and invasion

Glioblastoma multiforme (GBM) is an aggressive tumor of the central nervous system characterized by high rates of recurrence, morbidity, and mortality. This study investigated the antitumor effects of an apoptin-derived peptide (ADP) on glioma cells and explored the underlying mechanisms. The U251, U87, and C6 glioma cell lines were used in the present study, and the expression of p-Akt, Akt, and MMP-9 was determined through Western blotting, quantitative real-time PCR, and hematoxylin and eosin (HE) staining. Tumor growth was evaluated by magnetic resonance imaging, and cell viability was assessed through an 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide MTT assay. Glioma cell metastasis was evaluated using transwell migration, invasion, and scratch-wound assays. An ADP was designed and synthesized based on the results of a domain-based analysis of the structure of apoptin. The ADP inhibited glioma cell viability, invasion and migration, and treatment with the synthesized ADP led to downregulation of p-Akt and MMP-9 and inhibited MMP-9 translation. The ADP also inhibited glioma invasion and migration in vivo, and HE staining showed decreases in the satellite-like invasion of cell masses and apoptotic cell populations after treatment with the ADP. Our findings demonstrate that treatment with an ADP can suppress glioma cell migration and invasion via the PI3K/Akt/MMP-9 signaling pathway and provide a new platform for the development of drugs for treating glioma.

From Binding-Induced Dynamic Effects in SH3 Structures to Evolutionary Conserved Sectors

Src Homology 3 domains are ubiquitous small interaction modules known to act as docking sites and regulatory elements in a wide range of proteins. Prior experimental NMR work on the SH3 domain of Src showed that ligand binding induces long-range dynamic changes consistent with an induced fit mechanism. The identification of the residues that participate in this mechanism produces a chart that allows for the exploration of the regulatory role of such domains in the activity of the encompassing protein. Here we show that a computational approach focusing on the changes in side chain dynamics through ligand binding identifies equivalent long-range effects in the Src SH3 domain. Mutation of a subset of the predicted residues elicits long-range effects on the binding energetics, emphasizing the relevance of these positions in the definition of intramolecular cooperative networks of signal transduction in this domain. We find further support for this mechanism through the analysis of seven other publically available SH3 domain structures of which the sequences represent diverse SH3 classes. By comparing the eight predictions, we find that, in addition to a dynamic pathway that is relatively conserved throughout all SH3 domains, there are dynamic aspects specific to each domain and homologous subgroups. Our work shows for the first time from a structural perspective, which transduction mechanisms are common between a subset of closely related and distal SH3 domains, while at the same time highlighting the differences in signal transduction that make each family member unique. These results resolve the missing link between structural predictions of dynamic changes and the domain sectors recently identified for SH3 domains through sequence analysis.

Small protein domains as Src Homology 3 often act as docking sites and serve as regulatory elements. To understand their role in the regulation of a protein’s activity, one needs to understand how their backbone and sidechain dynamics are affected when binding to peptides. We have therefore computationally analyzed eight different SH3 domain structures, predicting dynamical effects induced by binding through our MCIT approach that has been shown to correlate well with experimental data. We show first that binding the Src SH3 domain triggers a particular cascade of dynamic effects, which are compatible with an induced fit mechanism reported before. We then combined the predictions for the eight SH3 domains into different consensus models, with the aim of analyzing, for the first time from a structural perspective, commonalities and differences in the transduction mechanisms among these SH3 domains. These consensus results are, on one hand, in agreement with the domain sectors recently identified for the entire family of SH3 domains. On the other hand, they reveal also that differences exist between the different subgroups that were studied here, requiring extensive experimental investigations of the importance of these differences for the proteins wherein these SH3 domains can be found.

Crystallographic studies on protein misfolding: Domain swapping and amyloid formation in the SH3 domain

Oligomerization by 3D domain swapping is found in a variety of proteins of diverse size, fold and function. In the early 1960s this phenomenon was postulated for the oligomers of ribonuclease A, but it was not until the 1990s that X-ray diffraction provided the first experimental evidence of this special manner of oligomerization. Nowadays, structural information has allowed the identification of these swapped oligomers in over one hundred proteins. Although the functional relevance of this phenomenon is not clear, this alternative folding of protomers into intertwined oligomers has been related to amyloid formation. Studies on proteins that develop 3D domain swapping might provide some clues on the early stages of amyloid formation. The SH3 domain is a small modular domain that has been used as a model to study the basis of protein folding. Among SH3 domains, the c-Src-SH3 domain emerges as a helpful model to study 3D domain swapping and amyloid formation.

Electrostatic effects in the folding of the SH3 domain of the c-Src tyrosine kinase: pH-dependence in 3D-domain swapping and amyloid formation

The SH3 domain of the c-Src tyrosine kinase (c-Src-SH3) aggregates to form intertwined dimers and amyloid fibrils at mild acid pHs. In this work, we show that a single mutation of residue Gln128 of this SH3 domain has a significant effect on: (i) its thermal stability; and (ii) its propensity to form amyloid fibrils. The Gln128Glu mutant forms amyloid fibrils at neutral pH but not at mild acid pH, while Gln128Lys and Gln128Arg mutants do not form these aggregates under any of the conditions assayed. We have also solved the crystallographic structures of the wild-type (WT) and Gln128Glu, Gln128Lys and Gln128Arg mutants from crystals obtained at different pHs. At pH 5.0, crystals belong to the hexagonal space group P6₅22 and the asymmetric unit is formed by one chain of the protomer of the c-Src-SH3 domain in an open conformation. At pH 7.0, crystals belong to the orthorhombic space group P2₁2₁2₁, with two molecules at the asymmetric unit showing the characteristic fold of the SH3 domain. Analysis of these crystallographic structures shows that the residue at position 128 is connected to Glu106 at the diverging β-turn through a cluster of water molecules. Changes in this hydrogen-bond network lead to the displacement of the c-Src-SH3 distal loop, resulting also in conformational changes of Leu100 that might be related to the binding of proline rich motifs. Our findings show that electrostatic interactions and solvation of residues close to the folding nucleation site of the c-Src-SH3 domain might play an important role during the folding reaction and the amyloid fibril formation.

Structure of the c-Src-SH3 domain in complex with a proline-rich motif of NS5A protein from the hepatitis C virus

The non-structural hepatitis C virus proteins NS5A and NS5B form a complex through interaction with the SH2 and SH3 domains of the non-receptor Src tyrosine kinase, which seems essential for viral replication. We have crystallized the complex between the SH3 domain of the c-Src tyrosine kinase and the C-terminal proline rich motif of the NS5A protein (A349PPIPPPRRKR359). Crystals obtained at neutral pH belong to the space group I41, with a single molecule of the SH3/NS5A complex at the asymmetric unit. The NS5A peptide is bound in a reverse orientation (class II) and the comparison of this structure with those of the high affinity synthetic peptides APP12 and VSL12 shows some important differences at the salt bridge that drives the peptide orientation. Further conformational changes in residues placed apart from the binding site also seem to play an important role in the binding orientation of this peptide. Our results show the interaction of the SH3 domain of the c-Src tyrosine kinase with a proline rich motif in the NS5A protein and point to their potential interaction in vivo.

Atomic resolution structures of the c-Src SH3 domain in complex with two high-affinity peptides from classes I and II

The atomic resolution crystal structures of complexes between the SH3 domain of the c-Src tyrosine kinase and two high-affinity peptides belonging to class I and class II have been solved. The crystals of the Thr98Asp and Thr98Glu mutants in complex with the APP12 peptide (APPLPPRNRPRL) belonged to the trigonal space group P3121 and in both cases the asymmetric unit was composed of one molecule of the SH3-APP12 complex. The crystals of the Thr98Glu mutant in complex with the VSL12 peptide (VSLARRPLPLP) belonged to the trigonal space group P3221 and the asymmetric unit was also composed of a single molecule of the SH3-VSL12 complex. All crystals were obtained in the presence of PEG 300 under the same conditions as reported for the intertwined dimeric structure of the c-Src SH3 domain, but the presence of the peptide stabilizes the monomeric form of the domain. These structures allow a detailed analysis of the role of salt bridges, cation-π interactions and hydrogen bonds in the binding of proline-rich motifs to the c-Src SH3 domain. Moreover, these crystallographic structures allow the role of water molecules in the binding of these motifs to the c-Src SH3 domain to be studied for the first time.

The promiscuous binding of the Fyn SH3 domain to a peptide from the NS5A protein

The hepatitis C virus nonstructural 5A (NS5A) protein is a large zinc-binding phosphoprotein that plays an important role in viral RNA replication and is involved in altering signal transduction pathways in the host cell. This protein interacts with Fyn tyrosine kinase in vivo and regulates its kinase activity. The 1.5 Å resolution crystal structure of a complex between the SH3 domain of the Fyn tyrosine kinase and the C-terminal proline-rich motif of the NS5A-derived peptide APPIPPPRRKR has been solved. Crystals were obtained in the presence of ZnCl(2) and belonged to the tetragonal space group P4(1)2(1)2. The asymmetric unit is composed of four SH3 domains and two NS5A peptide molecules; only three of the domain molecules contain a bound peptide, while the fourth molecule seems to correspond to a free form of the domain. Additionally, two of the SH3 domains are bound to the same peptide chain and form a ternary complex. The proline-rich motif present in the NS5A protein seems to be important for RNA replication and virus assembly, and the promiscuous interaction of the Fyn SH3 domain with the NS5A C-terminal proline-rich peptide found in this crystallographic structure may be important in the virus infection cycle.

The crystal structure of the AF2331 protein from Archaeoglobus fulgidus DSM 4304 forms an unusual interdigitated dimer with a new type of alpha + beta fold

The structure of AF2331, a 11-kDa orphan protein of unknown function from Archaeoglobus fulgidus, was solved by Se-Met MAD to 2.4 A resolution. The structure consists of an alpha + beta fold formed by an unusual homodimer, where the two core beta-sheets are interdigitated, containing strands alternating from both subunits. The decrease in solvent-accessible surface area upon dimerization is unusually large (3960 A(2)) for a protein of its size. The percentage of the total surface area buried in the interface (41.1%) is one of the largest observed in a nonredundant set of homodimers in the PDB and is above the mean for nearly all other types of homo-oligomers. AF2331 has no sequence homologs, and no structure similar to AF2331 could be found in the PDB using the CE, TM-align, DALI, or SSM packages. The protein has been identified in Pfam 23.0 as the archetype of a new superfamily and is topologically dissimilar to all other proteins with the "3-Layer (BBA) Sandwich" fold in CATH. Therefore, we propose that AF2331 forms a novel alpha + beta fold. AF2331 contains multiple negatively charged surface clusters and is located on the same operon as the basic protein AF2330. We hypothesize that AF2331 and AF2330 may form a charge-stabilized complex in vivo, though the role of the negatively charged surface clusters is not clear.