Solution NMR structure of S100B bound to the high-affinity target peptide TRTK-12

Computational Design of Macrocyclic Binders of S100B(ββ): Novel Peptide Theranostics

S100B(ββ) proteins are a family of multifunctional proteins that are present in several tissues and regulate a wide variety of cellular processes. Their altered expression levels have been associated with several human diseases, such as cancer, inflammatory disorders and neurodegenerative conditions, and hence are of interest as a therapeutic target and a biomarker. Small molecule inhibitors of S100B(ββ) have achieved limited success. Guided by the wealth of available experimental structures of S100B(ββ) in complex with diverse peptides from various protein interacting partners, we combine comparative structural analysis and molecular dynamics simulations to design a series of peptides and their analogues (stapled) as S100B(ββ) binders. The stapled peptides were subject to in silico mutagenesis experiments, resulting in optimized analogues that are predicted to bind to S100B(ββ) with high affinity, and were also modified with imaging agents to serve as diagnostic tools. These stapled peptides can serve as theranostics, which can be used to not only diagnose the levels of S100B(ββ) but also to disrupt the interactions of S100B(ββ) with partner proteins which drive disease progression, thus serving as novel therapeutics.

Specificity of Molecular Fragments Binding to S100B versus S100A1 as Identified by NMR and Site Identification by Ligand Competitive Saturation (SILCS)

S100B, a biomarker of malignant melanoma, interacts with the p53 protein and diminishes its tumor suppressor function, which makes this S100 family member a promising therapeutic target for treating malignant melanoma. However, it is a challenge to design inhibitors that are specific for S100B in melanoma versus other S100-family members that are important for normal cellular activities. For example, S100A1 is most similar in sequence and structure to S100B, and this S100 protein is important for normal skeletal and cardiac muscle function. Therefore, a combination of NMR and computer aided drug design (CADD) was used to initiate the design of specific S100B inhibitors. Fragment-based screening by NMR, also termed "SAR by NMR," is a well-established method, and was used to examine spectral perturbations in 2D [1H, 15N]-HSQC spectra of Ca2+-bound S100B and Ca2+-bound S100A1, side-by-side, and under identical conditions for comparison. Of the 1000 compounds screened, two were found to be specific for binding Ca2+-bound S100A1 and four were found to be specific for Ca2+-bound S100B, respectively. The NMR spectral perturbations observed in these six data sets were then used to model how each of these small molecule fragments showed specificity for one S100 versus the other using a CADD approach termed Site Identification by Ligand Competitive Saturation (SILCS). In summary, the combination of NMR and computational approaches provided insight into how S100A1 versus S100B bind small molecules specifically, which will enable improved drug design efforts to inhibit elevated S100B in melanoma. Such a fragment-based approach can be used generally to initiate the design of specific inhibitors for other highly homologous drug targets.

Interfering with S100B-effector protein interactions for cancer therapy

S100 calcium-binding protein B (S100B) is overexpressed in various malignant tumors, where it regulates cancer cell proliferation and metabolism by physical interactions with other molecules. Interfering with S100B-effector protein interactions is a potential strategy to treat malignant tumors. Although some S100B inhibitors have been discovered by virtual screening (VS), most target the S100B-p53 interaction. Hence, there is scope for the discovery of other S100B-effector protein interaction modulators for malignant tumors. In this review, we provide an overview of S100B-effector protein interaction inhibitor discovery using VS and discuss promising S100B-effector protein interaction targets that permit in silico analysis for drug discovery.

A Peptide-PNA Hybrid Beacon for Sensitive Detection of Protein Biomarkers in Biological Fluids

Specific and rapid detection of proteins in biological fluids poses a challenging problem. In biological fluids, many proteins are present at low concentrations, requiring high affinity and specificity of the beacon-protein interaction. We report the design of a peptide-PNA hybrid beacon that exploits the dimeric nature of a target protein, S100B, a biomarker for brain trauma, to enhance binding affinity and specificity. The complementary base-pairing of the PNA bases brings the two arms of the beacon, one carrying an Alexa tag and the other carrying a Dabcyl moiety, into proximity, thus quenching Alexa fluorescence. Each of the arms carries a sequence that binds to one of the subunits. Binding to the target separates the quencher from the probe lifting the quenching of fluorescence. Enhanced affinity and specificity resulting from simultaneously binding to two sites allowed specific detection of S100B at low-nanomolar concentrations in the presence of serum. The design can be easily adapted for the detection of proteins containing multiple binding sites and could prove useful for rapid and sensitive biomarker detection.

Sensitively detecting mTBI biomarker S100B by using peptide-modified ratiometric fluorescent C/AuNCs nanoprobe

Mild traumatic brain injury (mTBI) has become a tough nut in forensic science because of its minor damages but serious consequences. Utilizing biomarkers to diagnose mTBI has become a promising approach due to various shortcomings of traditional diagnostic methods. In this work, we developed a peptide-modified ratiometric fluorescent nanoprobe based on carbon dots (CDs) and gold nanoclusters (AuNCs) for the measurements of a pivotal biomarker S100B protein in the early diagnosis of mTBI. It has been found that florescence intensity of AuNCs at 580 nm was decreased as report signal while the florescence intensity of CDs was unchanged as reference signal in this sensing system when the surface modified peptide bind tightly with calcium-activated S100B. Under the optimized conditions, S100B concentration ranging from 0.03 to 1 μg/mL was successfully determined within 30 min, and the detection limit of 0.01 μg/mL was acquired through the standard rule (S/N = 3). Moreover, the detection of S100B in spiked blood samples were conducted with satisfactory recoveries. The as-prepared ratiometric fluorescent nanoprobe was proved to be a time-saving, convenient, and sensitive strategy, and it showed great prospects in the early diagnosis of mTBI in forensic practice.

High-throughput competitive fluorescence polarization assay reveals functional redundancy in the S100 protein family

The calcium-binding, vertebrate-specific S100 protein family consists of 20 paralogs in humans (referred as the S100ome), with several clinically important members. To explore their protein-protein interactions (PPIs) quantitatively, we have chosen an unbiased, high-throughput, competitive fluorescence polarization (FP) assay that revealed a partial functional redundancy when the complete S100ome (n = 20) was tested against numerous model partners (n = 13). Based on their specificity, the S100ome can be grouped into two distinct classes: promiscuous and orphan. In the first group, members bound to several ligands (> 4-5) with comparable high affinity, while in the second one, the paralogs bound only one partner weakly, or no ligand was identified. Our results demonstrate that FP assays are highly suitable for quantitative interaction profiling of selected protein families. Moreover, we provide evidence that PPI-based phenotypic characterization can complement or even exceed the information obtained from the sequence-based phylogenetic analysis of the S100ome, an evolutionary young protein family.

Monitoring Interactions Between S100B and the Dopamine D2 Receptor Using NMR Spectroscopy

S100B is a dimeric EF-hand protein that undergoes a calcium-induced conformational change and interacts with a wide range of proteins to modulate their functions. The dopamine D2 receptor is one potential S100B binding partner that may play a key role in neurological processing. In this chapter, we describe the use of NMR spectroscopy to examine the interaction between calcium-bound S100B and the third intracellular loop (IC3) from the dopamine D2 receptor. We provide details that allow the strength of the interaction (K _d) between the two proteins to be determined and the IC3 site of interaction on the structure of S100B to be identified. Both these characteristics can be identified from a single series of nondestructive experiments.

Structural Basis for S100B Interaction with its Target Proteins

The S100B protein is an intra- and extracellular signaling protein that plays a role in a multitude of cellular processes and abnormal S100B is associated with various neurological diseases and cancers. S100B recognizes and binds effector proteins in a calcium-dependent manner. S100B has been shown to interact with the actin capping protein CapZ, protein kinase C, Hdm2 and 4, RAGE receptor, and p⁵³, among others. These protein partners interact with a common area on the S100B protein surface, validating the method of using the consensus sequence for S100B target search. In addition, each S100B target protein distinguishes itself by additional contacts with S100B. This perspective suggests that the combination of sequence homology search and structural analysis promises to identify newer S100B-binding partners beyond the use of the consensus sequence alone as the given example in the XPB subunit of the TFIIH general transcription factor. XPB is a helicase required for both transcription and DNA repair. Inherited xpb mutations are associated with human disease Xeroderma Pigmentasum, Cockayne syndrome, and trichothiodystrophy. S100B protein is likely associated with much more biological pathways and processes. We believe that S100B will attract more and more attentions in the scientific community and S100B related studies will have important implications in human health and medicine.

S100A4 in cancer progression and metastasis: A systematic review

Metastasis is the leading cause of cancer-related death and directly associates with cancer progression, resistance to anticancer therapy, and poor patient survival. Current efforts focusing on the underlying molecular mechanisms of cancer metastasis attract a special attention to cancer researchers. The epithelial-mesenchymal transition is a complex of molecular program during embryogenesis, inflammation, tissue fibrosis, and cancer progression and metastasis. S100A4, an important member of S100 family proteins, functions to increase the tumor progression and metastasis. The molecular mechanisms of S100A4 involving in the progression and metastasis are diverse in various malignant tumors. Detection of S100A4 expression becomes a promising candidate biomarker in cancer early diagnosis and prediction of cancer metastasis and therefore, S100A4 may be a therapeutic target. This review summarized up to date advancement on the role of S100A4 in human cancer development, progression, and metastasis and the underlying molecular events and then strategies to target S100A4 expression experimentally.

The Activation of Protein Kinase A by the Calcium-Binding Protein S100A1 Is Independent of Cyclic AMP

Biochemical and structural studies demonstrate that S100A1 is involved in a Ca²⁺-dependent interaction with the type 2α and type 2β regulatory subunits of protein kinase A (PKA) (RIIα and RIIβ) to activate holo-PKA. The interaction was specific for S100A1 because other calcium-binding proteins (i.e., S100B and calmodulin) had no effect. Likewise, a role for S100A1 in PKA-dependent signaling was established because the PKA-dependent subcellular redistribution of HDAC4 was abolished in cells derived from S100A1 knockout mice. Thus, the Ca²⁺-dependent interaction between S100A1 and the type 2 regulatory subunits represents a novel mechanism that provides a link between Ca²⁺ and PKA signaling, which is important for the regulation of gene expression in skeletal muscle via HDAC4 cytosolic–nuclear trafficking.

Simultaneous inhibition of key growth pathways in melanoma cells and tumor regression by a designed bidentate constrained helical peptide

Protein-protein interactions are part of a large number of signaling networks and potential targets for drug development. However, discovering molecules that can specifically inhibit such interactions is a major challenge. S100B, a calcium-regulated protein, plays a crucial role in the proliferation of melanoma cells through protein-protein interactions. In this article, we report the design and development of a bidentate conformationally constrained peptide against dimeric S100B based on a natural tight-binding peptide, TRTK-12. The helical conformation of the peptide was constrained by the substitution of α-amino isobutyric acid--an amino acid having high helical propensity--in positions which do not interact with S100B. A branched bidentate version of the peptide was bound to S100B tightly with a dissociation constant of 8 nM. When conjugated to a cell-penetrating peptide, it caused growth inhibition and rapid apoptosis in melanoma cells. The molecule exerts antiproliferative action through simultaneous inhibition of key growth pathways, including reactivation of wild-type p53 and inhibition of Akt and STAT3 phosphorylation. The apoptosis induced by the bidentate constrained helix is caused by direct migration of p53 to mitochondria. At moderate intravenous dose, the peptide completely inhibits melanoma growth in a mouse model without any significant observable toxicity. The specificity was shown by lack of ability of a double mutant peptide to cause tumor regression at the same dose level. The methodology described here for direct protein-protein interaction inhibition may be effective for rapid development of inhibitors against relatively weak protein-protein interactions for de novo drug development.

Structural and thermodynamic characterization of the recognition of the S100-binding peptides TRTK12 and p53 by calmodulin

Calmodulin (CaM) is a multifunctional messenger protein that activates a wide variety of signaling pathways in eukaryotic cells in a calcium-dependent manner. CaM has been proposed to be functionally distinct from the S100 proteins, a related family of eukaryotic calcium-binding proteins. Previously, it was demonstrated that peptides derived from the actin-capping protein, TRTK12, and the tumor-suppressor protein, p53, interact with multiple members of the S100 proteins. To test the specificity of these peptides, they were screened using isothermal titration calorimetry against 16 members of the human S100 protein family, as well as CaM, which served as a negative control. Interestingly, both the TRTK12 and p53 peptides were found to interact with CaM. These interactions were further confirmed by both fluorescence and nuclear magnetic resonance spectroscopies. These peptides have distinct sequences from the known CaM target sequences. The TRTK12 peptide was found to independently interact with both CaM domains and bind with a stoichiometry of 2:1 and dissociations constants Kd,C-term  = 2 ± 1 µM and Kd,N-term  = 14 ± 1 µM. In contrast, the p53 peptide was found to interact only with the C-terminal domain of CaM, Kd,C-term = 2 ± 1 µM, 25°C. Using NMR spectroscopy, the locations of the peptide binding sites were mapped onto the structure of CaM. The binding sites for both peptides were found to overlap with the binding interface for previously identified targets on both domains of CaM. This study demonstrates the plasticity of CaM in target binding and may suggest a possible overlap in target specificity between CaM and the S100 proteins.

Novel interactions of the TRTK12 peptide with S100 protein family members: specificity and thermodynamic characterization

The S100 protein family consists of small, dimeric proteins that exert their biological functions in response to changing calcium concentrations. S100B is the best-studied member and has been shown to interact with more than 20 binding partners in a calcium-dependent manner. The TRTK12 peptide, derived from the consensus binding sequence for S100B, has previously been found to interact with S100A1 and has been proposed to be a general binding partner of the S100 family. To test this hypothesis and gain a better understanding of the specificity of binding for the S100 proteins, 16 members of the human S100 family were screened against this peptide and its alanine variants. Novel interactions were found with only two family members, S100P and S100A2, indicating that TRTK12 selectively interacts with a small subset of the S100 proteins. Substantial promiscuity was observed in the binding site of S100B thereby accommodating variations in the peptide sequence, while S100A1, S100A2, and S100P exhibited larger differences in the binding constants for the TRTK12 alanine variants. This suggests that single-point substitutions can be used to selectively modulate the affinity of TRTK12 peptides for individual S100 proteins. This study has important implications for the rational drug design of inhibitors for the S100 proteins, which are involved in a variety of cancers and neurodegenerative diseases.

Target binding to S100B reduces dynamic properties and increases Ca(2+)-binding affinity for wild type and EF-hand mutant proteins

Mutations in the second EF-hand (D61N, D63N, D65N, and E72A) of S100B were used to study its Ca(2+) binding and dynamic properties in the absence and presence of a bound target, TRTK-12. With (D63N)S100B as an exception ((D63N)K(D)=50±9 μM), Ca(2+) binding to EF2-hand mutants were reduced by more than 8-fold in the absence of TRTK-12 ((D61N)K(D)=412±67 μM, (D65N)K(D)=968±171 μM, and (E72A)K(D)=471±133 μM), when compared to wild-type protein ((WT)K(D)=56±9 μM). For the TRTK-12 complexes, the Ca(2+)-binding affinity to wild type ((WT+TRTK)K(D)=12±10 μM) and the EF2 mutants was increased by 5- to 14-fold versus in the absence of target ((D61N+TRTK)K(D)=29±1.2 μM, (D63N+TRTK)K(D)=10±2.2 μM, (D65N+TRTK)K(D)=73±4.4 μM, and (E72A+TRTK)K(D)=18±3.7 μM). In addition, R(ex), as measured using relaxation dispersion for side-chain (15)N resonances of Asn63 ((D63N)S100B), was reduced upon TRTK-12 binding when measured by NMR. Likewise, backbone motions on multiple timescales (picoseconds to milliseconds) throughout wild type, (D61N)S100B, (D63N)S100B, and (D65N)S100B were lowered upon binding TRTK-12. However, the X-ray structures of Ca(2+)-bound (2.0Å) and TRTK-bound (1.2Å) (D63N)S100B showed no change in Ca(2+) coordination; thus, these and analogous structural data for the wild-type protein could not be used to explain how target binding increased Ca(2+)-binding affinity in solution. Therefore, a model for how S100B-TRTK-12 complex formation increases Ca(2+) binding is discussed, which considers changes in protein dynamics upon binding the target TRTK-12.

Binding of two intrinsically disordered peptides to a multi-specific protein: a combined Monte Carlo and molecular dynamics study

The unique ability of intrinsically disordered proteins (IDPs) to fold upon binding to partner molecules makes them functionally well-suited for cellular communication networks. For example, the folding-binding of different IDP sequences onto the same surface of an ordered protein provides a mechanism for signaling in a many-to-one manner. Here, we study the molecular details of this signaling mechanism by applying both Molecular Dynamics and Monte Carlo methods to S100B, a calcium-modulated homodimeric protein, and two of its IDP targets, p53 and TRTK-12. Despite adopting somewhat different conformations in complex with S100B and showing no apparent sequence similarity, the two IDP targets associate in virtually the same manner. As free chains, both target sequences remain flexible and sample their respective bound, natively -helical states to a small extent. Association occurs through an intermediate state in the periphery of the S100B binding pocket, stabilized by nonnative interactions which are either hydrophobic or electrostatic in nature. Our results highlight the importance of overall physical properties of IDP segments, such as net charge or presence of strongly hydrophobic amino acids, for molecular recognition via coupled folding-binding.

A substantial fraction of our proteins are believed to be partly or completely disordered, meaning that they contain regions that lack a stable folded structure under typical physiological conditions. This is a feature which plays a key role in their functions. For example, it allows them to have many structurally different binding partners which in turn permits the construction of the intricate signaling and regulatory networks necessary to sustain complex biological organisms such as ourselves. Whereas measuring the binding strengths of associations involving disordered proteins is routine, the binding process itself is today still not fully understood. We use two different computational models to study the interactions of a folded protein, S100B, which can bind various disordered peptides. In particular, we compare two peptides whose structures are known when in complex with S100B. Our results suggest that, although the peptides assume different structures in the bound state, there are similarities in how they associate with S100B. The possibility to computationally model the interplay between proteins is an important complement to experiments, by identifying crucial steps in the binding process. This is essential to understand, e.g., how single mutations sometimes lead to serious diseases.

Thermodynamic and kinetic analysis of peptides derived from CapZ, NDR, p53, HDM2, and HDM4 binding to human S100B

S100B is a member of the S100 subfamily of EF-hand proteins that has been implicated in malignant melanoma and neurodegenerative conditions such as Alzheimer's disease and Parkinson's disease. Calcium-induced conformational changes expose a hydrophobic binding cleft, facilitating interactions with a wide variety of nuclear, cytoplasmic, and extracellular target proteins. Previously, peptides derived from CapZ, p53, NDR, HDM2, and HDM4 have been shown to interact with S100B in a calcium-dependent manner. However, the thermodynamic and kinetic basis of these interactions remains largely unknown. To gain further insight, we screened these peptides against the S100B protein using isothermal titration calorimetry and nuclear magnetic resonance. All peptides were found to have binding affinities in the low micromolar to nanomolar range. Binding-induced changes in the line shapes of S100B backbone (1)H and (15)N resonances were monitored to obtain the dissociation constants and the kinetic binding parameters. The large microscopic K(on) rate constants observed in this study (≥1 × 10(7) M(-1) s(-1)) suggest that S100B utilizes a "fly casting mechanism" in the recognition of these peptide targets.

Crystal structure of the S100A4-nonmuscle myosin IIA tail fragment complex reveals an asymmetric target binding mechanism

S100A4 is a member of the S100 family of calcium-binding proteins that is directly involved in tumor metastasis. It binds to the nonmuscle myosin IIA (NMIIA) tail near the assembly competence domain (ACD) promoting filament disassembly, which could be associated with increasing metastatic potential of tumor cells. Here, we investigate the mechanism of S100A4-NMIIA interaction based on binding studies and the crystal structure of S100A4 in complex with a 45-residue-long myosin heavy chain fragment. Interestingly, we also find that S100A4 binds as strongly to a homologous heavy chain fragment of nonmuscle myosin IIC as to NMIIA. The structure of the S100A4-NMIIA complex reveals a unique mode of interaction in the S100 family: A single, predominantly α-helical myosin chain is wrapped around the Ca(2+)-bound S100A4 dimer occupying both hydrophobic binding pockets. Thermal denaturation experiments of coiled-coil forming NMIIA fragments indicate that the coiled-coil partially unwinds upon S100A4 binding. Based on these results, we propose a model for NMIIA filament disassembly: Part of the random coil tailpiece and the C-terminal residues of the coiled-coil are wrapped around an S100A4 dimer disrupting the ACD and resulting in filament dissociation. The description of the complex will facilitate the design of specific drugs that interfere with the S100A4-NMIIA interaction.

Identification of calcium-independent and calcium-enhanced binding between S100B and the dopamine D2 receptor

S100B is a dimeric EF-hand protein that undergoes a calcium-induced conformational change and exposes a hydrophobic protein-binding surface. Recently S100B was identified as a binding partner of the dopamine D2 receptor in a bacterial two-hybrid screen involving the third intracellular loop (IC3). The low in vivo calcium concentration in bacteria (100–300 nM) suggests this interaction may occur in the absence of calcium. In this work the calcium-sensitive ability for S100B to recruit the IC3 of the dopamine D2 receptor was examined, and regions in both proteins required for complex formation were identified. Peptide array experiments identified the C-terminal 58 residues of the IC3 (IC3-C58) as the major interacting site for S100B. These experiments along with pull-down assays showed the IC3 interacts with S100B in the absence and presence of calcium. ¹H–¹⁵N HSQC experiments were used to identify residues, primarily in helices III and IV, utilized in the IC3-C58 interaction. NMR titration data indicated that although an interaction between apo-S100B and IC3-C58 occurs without calcium, the binding was enhanced more than 100-fold upon calcium binding. Further, it was established that shorter regions within IC3-C58 comprising its N- and C-terminal halves had diminished binding to Ca²⁺-S100B and did not display any observable affinity in the absence of calcium. This indicates that residue or structural components within both regions are required for optimal interaction with Ca²⁺-S100B. This work represents the first example of an S100B target that interacts with both the apo- and calcium-saturated forms of S100B.

The S100A10-annexin A2 complex provides a novel asymmetric platform for membrane repair

Membrane repair is mediated by multiprotein complexes, such as that formed between the dimeric EF-hand protein S100A10, the calcium- and phospholipid-binding protein annexin A2, the enlargeosome protein AHNAK, and members of the transmembrane ferlin family. Although interactions between these proteins have been shown, little is known about their structural arrangement and mechanisms of formation. In this work, we used a non-covalent complex between S100A10 and the N terminus of annexin A2 (residues 1-15) and a designed hybrid protein (A10A2), where S100A10 is linked in tandem to the N-terminal region of annexin A2, to explore the binding region, stoichiometry, and affinity with a synthetic peptide from the C terminus of AHNAK. Using multiple biophysical methods, we identified a novel asymmetric arrangement between a single AHNAK peptide and the A10A2 dimer. The AHNAK peptide was shown to require the annexin A2 N terminus, indicating that the AHNAK binding site comprises regions on both S100A10 and annexin proteins. NMR spectroscopy was used to show that the AHNAK binding surface comprised residues from helix IV in S100A10 and the C-terminal portion from the annexin A2 peptide. This novel surface maps to the exposed side of helices IV and IV' of the S100 dimeric structure, a region not identified in any previous S100 target protein structures. The results provide the first structural details of the ternary S100A10 protein complex required for membrane repair.

The design and delivery of a thermally responsive peptide to inhibit S100B-mediated neurodegeneration

S100B, a glial-secreted protein, is believed to play a major role in neurodegeneration in Alzheimer's disease, Down syndrome, traumatic brain injury, and spinocerebellar ataxia type 1 (SCA1). SCA1 is a trinucleotide repeat disorder in which the expanded polyglutamine mutation in the protein ataxin-1 primarily targets Purkinje cells of the cerebellum. Currently, the exact mechanism of S100B-mediated Purkinje cell damage in SCA1 is not clear. However, here we show that S100B may act via the activation of the receptor for advanced glycation end product (RAGE) signaling pathway, resulting in oxidative stress-mediated injury to mutant ataxin-1-expressing neurons. To combat S100B-mediated neurodegeneration, we have designed a selective thermally responsive S100B inhibitory peptide, Synb1-ELP-TRTK. Our therapeutic polypeptide was developed using three key elements: (1) the elastin-like polypeptide (ELP), a thermally responsive polypeptide, (2) the TRTK12 peptide, a known S100B inhibitory peptide, and (3) a cell-penetrating peptide, Synb1, to enhance intracellular delivery. Binding studies revealed that our peptide, Synb1-ELP-TRTK, interacts with its molecular target S100B and maintains a high S100B binding affinity as comparable with the TRTK12 peptide alone. In addition, in vitro studies revealed that Synb1-ELP-TRTK treatment reduces S100B uptake in SHSY5Y cells. Furthermore, the Synb1-ELP-TRTK peptide decreased S100B-induced oxidative damage to mutant ataxin-1-expressing neurons. To test the delivery capabilities of ELP-based therapeutic peptides to the cerebellum, we treated mice with fluorescently labeled Synb1-ELP and observed that thermal targeting enhanced peptide delivery to the cerebellum. Here, we have laid the framework for thermal-based therapeutic targeting to regions of the brain, particularly the cerebellum. Overall, our data suggest that thermal targeting of ELP-based therapeutic peptides to the cerebellum is a novel treatment strategy for cerebellar neurodegenerative disorders.

S100A1 and calmodulin regulation of ryanodine receptor in striated muscle

The release of Ca2+ ions from the sarcoplasmic reticulum through ryanodine receptor calcium release channels represents the critical step linking electrical excitation to muscular contraction in the heart and skeletal muscle (excitation-contraction coupling). Two small Ca2+ binding proteins, S100A1 and calmodulin, have been demonstrated to bind and regulate ryanodine receptor in vitro. This review focuses on recent work that has revealed new information about the endogenous roles of S100A1 and calmodulin in regulating skeletal muscle excitation-contraction coupling. S100A1 and calmodulin bind to an overlapping domain on the ryanodine receptor type 1 to tune the Ca2+ release process, and thereby regulate skeletal muscle function. We also discuss past, current and future work surrounding the regulation of ryanodine receptors by calmodulin and S100A1 in both cardiac and skeletal muscle, and the implications for excitation-contraction coupling.

Molecular determinants of S100B oligomer formation

Background

S100B is a dimeric protein that can form tetramers, hexamers and higher order oligomers. These forms have been suggested to play a role in RAGE activation.

Methodology/Principal Findings

Oligomerization was found to require a low molecular weight trigger/cofactor and could not be detected for highly pure dimer, irrespective of handling. Imidazol was identified as a substance that can serve this role. Oligomerization is dependent on both the imidazol concentration and pH, with optima around 90 mM imidazol and pH 7, respectively. No oligomerization was observed above pH 8, thus the protonated form of imidazol is the active species in promoting assembly of dimers to higher species. However, disulfide bonds are not involved and the process is independent of redox potential. The process was also found to be independent of whether Ca²⁺ is bound to the protein or not. Tetramers that are purified from dimers and imidazol by gel filtration are kinetically stable, but dissociate into dimers upon heating. Dimers do not revert to tetramer and higher oligomer unless imidazol is again added. Both tetramers and hexamers bind the target peptide from p53 with retained stoichiometry of one peptide per S100B monomer, and with high affinity (lgK = 7.3±0.2 and 7.2±0.2, respectively in 10 mM BisTris, 5 mM CaCl₂, pH 7.0), which is less than one order of magnitude reduced compared to dimer under the same buffer conditions.

Conclusion/Significance

S100B oligomerization requires protonated imidazol as a trigger/cofactor. Oligomers are kinetically stable after imidazol is removed but revert back to dimer if heated. The results underscore the importance of kinetic versus thermodynamic control of S100B protein aggregation.

The 1.5 Å crystal structure of human receptor for advanced glycation endproducts (RAGE) ectodomains reveals unique features determining ligand binding

Interaction of the pattern recognition receptor, RAGE with key ligands such as advanced glycation end products (AGE), S100 proteins, amyloid β, and HMGB1 has been linked to diabetic complications, inflammatory and neurodegenerative disorders, and cancer. To help answer the question of how a single receptor can recognize and respond to a diverse set of ligands we have investigated the structure and binding properties of the first two extracellular domains of human RAGE, which are implicated in various ligand binding and subsequent signaling events. The 1.5-Å crystal structure reveals an elongated molecule with a large basic patch and a large hydrophobic patch, both highly conserved. Isothermal titration calorimetry (ITC) and deletion experiments indicate S100B recognition by RAGE is an entropically driven process involving hydrophobic interaction that is dependent on Ca(2+) and on residues in the C'D loop (residues 54-67) of domain 1. In contrast, competition experiments using gel shift assays suggest that RAGE interaction with AGE is driven by the recognition of negative charges on AGE-proteins. We also demonstrate that RAGE can bind to dsDNA and dsRNA. These findings reveal versatile structural features of RAGE that help explain its ability to recognize of multiple ligands.

The crystal structures of human S100B in the zinc- and calcium-loaded state at three pH values reveal zinc ligand swapping

S100B is a homodimeric zinc-, copper-, and calcium-binding protein of the family of EF-hand S100 proteins. Zn(2+) binding to S100B increases its affinity towards Ca(2+) as well as towards target peptides and proteins. Cu(2+) and Zn(2+) bind presumably to the same site in S100B. We determined the structures of human Zn(2+)- and Ca(2+)-loaded S100B at pH 6.5, pH 9, and pH 10 by X-ray crystallography at 1.5, 1.4, and 1.65Å resolution, respectively. Two Zn(2+) ions are coordinated tetrahedrally at the dimer interface by His and Glu residues from both subunits. The crystal structures revealed that ligand swapping occurs for one of the four ligands in the Zn(2+)-binding sites. Whereas at pH 9, the Zn(2+) ions are coordinated by His15, His25, His 85', and His 90', at pH 6.5 and pH 10, His90' is replaced by Glu89'. The results document that the Zn(2+)-binding sites are flexible to accommodate other metal ions such as Cu(2+). Moreover, we characterized the structural changes upon Zn(2+) binding, which might lead to increased affinity towards Ca(2+) as well as towards target proteins. We observed that in Zn(2+)-Ca(2+)-loaded S100B the C-termini of helix IV adopt a distinct conformation. Zn(2+) binding induces a repositioning of residues Phe87 and Phe88, which are involved in target protein binding. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

The Calcium-Dependent Interaction of S100B with Its Protein Targets

S100B is a calcium signaling protein that is a member of the S100 protein family. An important feature of S100B and most other S100 proteins (S100s) is that they often bind Ca²⁺ ions relatively weakly in the absence of a protein target; upon binding their target proteins, Ca²⁺-binding then increases by as much as from 200- to 400-fold. This manuscript reviews the structural basis and physiological significance of increased Ca²⁺-binding affinity in the presence of protein targets. New information regarding redundancy among family members and the structural domains that mediate the interaction of S100B, and other S100s, with their targets is also presented. It is the diversity among individual S100s, the protein targets that they interact with, and the Ca²⁺ dependency of these protein-protein interactions that allow S100s to transduce changes in [Ca²⁺]_{intracellular} levels into spatially and temporally unique biological responses.

In vitro screening and structural characterization of inhibitors of the S100B-p53 interaction

S100B is highly over-expressed in many cancers, including malignant melanoma. In such cancers, S100B binds wild-type p53 in a calcium-dependent manner, sequestering it, and promoting its degradation, resulting in the loss of p53-dependent tumor suppression activities. Therefore, S100B inhibitors may be able to restore wild-type p53 levels in certain cancers and provide a useful therapeutic strategy. In this regard, an automated and sensitive fluorescence polarization competition assay (FPCA) was developed and optimized to screen rapidly for lead compounds that bind Ca(2+)-loaded S100B and inhibit S100B target complex formation. A screen of 2000 compounds led to the identification of 26 putative S100B low molecular weight inhibitors. The binding of these small molecules to S100B was confirmed by nuclear magnetic resonance spectroscopy, and additional structural information was provided by x-ray crystal structures of several compounds in complexes with S100B. Notably, many of the identified inhibitors function by chemically modifying Cys84 in protein. These results validate the use of high-throughput FPCA to facilitate the identification of compounds that inhibit S100B. These lead compounds will be the subject of future optimization studies with the ultimate goal of developing a drug with therapeutic activity for the treatment of malignant melanoma and/or other cancers with elevated S100B.

Fragmenting the S100B-p53 interaction: combined virtual/biophysical screening approaches to identify ligands

S100B contributes to cell proliferation by binding the C terminus of p53 and inhibiting its tumor suppressor function. The use of multiple computational approaches to screen fragment libraries targeting the human S100B-p53 interaction site is reported. This in silico screening led to the identification of 280 novel prospective ligands. NMR spectroscopic experiments revealed specific binding at the p53 interaction site for a set of these compounds and confirmed their potential for further rational optimization. The X-ray crystal structure determined for one of the binders revealed key intermolecular interactions, thus paving the way for structure-based ligand optimization.

The calcium-dependent interaction between S100B and the mitochondrial AAA ATPase ATAD3A and the role of this complex in the cytoplasmic processing of ATAD3A

S100 proteins comprise a multigene family of EF-hand calcium binding proteins that engage in multiple functions in response to cellular stress. In one case, the S100B protein has been implicated in oligodendrocyte progenitor cell (OPC) regeneration in response to demyelinating insult. In this example, we report that the mitochondrial ATAD3A protein is a major, high-affinity, and calcium-dependent S100B target protein in OPC. In OPC, ATAD3A is required for cell growth and differentiation. Molecular characterization of the S100B binding domain on ATAD3A by nuclear magnetic resonance (NMR) spectroscopy techniques defined a consensus calcium-dependent S100B binding motif. This S100B binding motif is conserved in several other S100B target proteins, including the p53 protein. Cellular studies using a truncated ATAD3A mutant that is deficient for mitochondrial import revealed that S100B prevents cytoplasmic ATAD3A mutant aggregation and restored its mitochondrial localization. With these results in mind, we propose that S100B could assist the newly synthesized ATAD3A protein, which harbors the consensus S100B binding domain for proper folding and subcellular localization. Such a function for S100B might also help to explain the rescue of nuclear translocation and activation of the temperature-sensitive p53val135 mutant by S100B at nonpermissive temperatures.

Design of high-affinity S100-target hybrid proteins

S100B and S100A10 are dimeric, EF-hand proteins. S100B undergoes a calcium-dependent conformational change allowing it to interact with a short contiguous sequence from the actin-capping protein CapZ (TRTK12). S100A10 does not bind calcium but is able to recruit the N-terminus of annexin A2 important for membrane fusion events, and to form larger multiprotein complexes such as that with the cation channel proteins TRPV5/6. In this work, we have designed, expressed, purified, and characterized two S100-target peptide hybrid proteins comprised of S100A10 and S100B linked in tandem to annexin A2 (residues 1-15) and CapZ (TRTK12), respectively. Different protease cleavage sites (tobacco etch virus, PreScission) were incorporated into the linkers of the hybrid proteins. In situ proteolytic cleavage monitored by (1)H-(15)N HSQC spectra showed the linker did not perturb the structures of the S100A10-annexin A2 or S100B-TRTK12 complexes. Furthermore, the analysis of the chemical shift assignments ((1)H, (15)N, and (13)C) showed that residues T102-S108 of annexin A2 formed a well-defined alpha-helix in the S100A10 hybrid while the TRTK12 region was unstructured at the N-terminus with a single turn of alpha-helix from D108-K111 in the S100B hybrid protein. The two S100 hybrid proteins provide a simple yet extremely efficient method for obtaining high yields of intact S100 target peptides. Since cleavage of the S100 hybrid protein is not necessary for structural characterization, this approach may be useful as a scaffold for larger S100 complexes.

The effects of CapZ peptide (TRTK-12) binding to S100B-Ca2+ as examined by NMR and X-ray crystallography

Structure-based drug design is underway to inhibit the S100B-p53 interaction as a strategy for treating malignant melanoma. X-ray crystallography was used here to characterize an interaction between Ca(2)(+)-S100B and TRTK-12, a target that binds to the p53-binding site on S100B. The structures of Ca(2+)-S100B (1.5-A resolution) and S100B-Ca(2)(+)-TRTK-12 (2.0-A resolution) determined here indicate that the S100B-Ca(2+)-TRTK-12 complex is dominated by an interaction between Trp7 of TRTK-12 and a hydrophobic binding pocket exposed on Ca(2+)-S100B involving residues in helices 2 and 3 and loop 2. As with an S100B-Ca(2)(+)-p53 peptide complex, TRTK-12 binding to Ca(2+)-S100B was found to increase the protein's Ca(2)(+)-binding affinity. One explanation for this effect was that peptide binding introduced a structural change that increased the number of Ca(2+) ligands and/or improved the Ca(2+) coordination geometry of S100B. This possibility was ruled out when the structures of S100B-Ca(2+)-TRTK-12 and S100B-Ca(2+) were compared and calcium ion coordination by the protein was found to be nearly identical in both EF-hand calcium-binding domains (RMSD=0.19). On the other hand, B-factors for residues in EF2 of Ca(2+)-S100B were found to be significantly lowered with TRTK-12 bound. This result is consistent with NMR (15)N relaxation studies that showed that TRTK-12 binding eliminated dynamic properties observed in Ca(2+)-S100B. Such a loss of protein motion may also provide an explanation for how calcium-ion-binding affinity is increased upon binding a target. Lastly, it follows that any small-molecule inhibitor bound to Ca(2+)-S100B would also have to cause an increase in calcium-ion-binding affinity to be effective therapeutically inside a cell, so these data need to be considered in future drug design studies involving S100B.

Small molecules bound to unique sites in the target protein binding cleft of calcium-bound S100B as characterized by nuclear magnetic resonance and X-ray crystallography

Structural studies are part of a rational drug design program aimed at inhibiting the S100B-p53 interaction and restoring wild-type p53 function in malignant melanoma. To this end, structures of three compounds (SBi132, SBi1279, and SBi523) bound to Ca(2+)-S100B were determined by X-ray crystallography at 2.10 A (R(free) = 0.257), 1.98 A (R(free) = 0.281), and 1.90 A (R(free) = 0.228) resolution, respectively. Upon comparison, SBi132, SBi279, and SBi523 were found to bind in distinct locations and orientations within the hydrophobic target binding pocket of Ca(2+)-S100B with minimal structural changes observed for the protein upon complex formation with each compound. Specifically, SBi132 binds nearby residues in loop 2 (His-42, Phe-43, and Leu-44) and helix 4 (Phe-76, Met-79, Ile-80, Ala-83, Cys-84, Phe-87, and Phe-88), whereas SBi523 interacts with a separate site defined by residues within loop 2 (Ser-41, His-42, Phe-43, Leu-44, Glu-45, and Glu-46) and one residue on helix 4 (Phe-87). The SBi279 binding site on Ca(2+)-S100B overlaps the SBi132 and SBi523 sites and contacts residues in both loop 2 (Ser-41, His-42, Phe-43, Leu-44, and Glu-45) and helix 4 (Ile-80, Ala-83, Cys-84, Phe-87, and Phe-88). NMR data, including saturation transfer difference (STD) and (15)N backbone and (13)C side chain chemical shift perturbations, were consistent with the X-ray crystal structures and demonstrated the relevance of all three small molecule-S100B complexes in solution. The discovery that SBi132, SBi279, and SBi523 bind to proximal sites on Ca(2+)-S100B could be useful for the development of a new class of molecule(s) that interacts with one or more of these binding sites simultaneously, thereby yielding novel tight binding inhibitors specific for blocking protein-protein interactions involving S100B.

Solution structure of S100A1 bound to the CapZ peptide (TRTK12)

As is typical for S100-target protein interactions, a Ca 2+-dependent conformational change in S100A1 is required to bind to a 12-residue peptide (TRTK12) derived from the actin-capping protein CapZ. In addition, the Ca 2+-binding affinity of S100A1 is found to be tightened (greater than threefold) when TRTK12 is bound. To examine the biophysical basis for these observations, we determined the solution NMR structure of TRTK12 in a complex with Ca 2+-loaded S100A1. When bound to S100A1, TRTK12 forms an amphipathic helix (residues N6 to S12) with several favorable hydrophobic interactions observed between W7, I10, and L11 of the peptide and a well-defined hydrophobic binding pocket in S100A1 that is only present in the Ca 2+-bound state. Next, the structure of S100A1-TRTK12 was compared to that of another S100A1-target complex (i.e., S100A1-RyRP12), which illustrated how the binding pocket in Ca 2+-S100A1 can accommodate peptide targets with varying amino acid sequences. Similarities and differences were observed when the structures of S100A1-TRTK12 and S100B-TRTK12 were compared, providing insights regarding how more than one S100 protein can interact with the same peptide target. Such comparisons, including those with other S100-target and S100-drug complexes, provide the basis for designing novel small-molecule inhibitors that could be specific for blocking one or more S100-target protein interactions.

Binding of S100 proteins to RAGE: an update

The Receptor for Advanced Glycation Endproducts (RAGE) is a multi-ligand receptor of the immunoglobulin family. RAGE interacts with structurally different ligands probably through the oligomerization of the receptor on the cell surface. However, the exact mechanism is unknown. Among RAGE ligands are members of the S100 protein family. S100 proteins are small calcium binding proteins with high structural homology. Several members of the family have been shown to interact with RAGE in vitro or in cell-based assays. Interestingly, many RAGE ligands appear to interact with distinct domains of the extracellular portion of RAGE and to trigger various cellular effects. In this review, we summarize the modes of S100 protein-RAGE interaction with regard to their cellular functions.

S100A1 and calmodulin compete for the same binding site on ryanodine receptor

In heart and skeletal muscle an S100 protein family member, S100A1, binds to the ryanodine receptor (RyR) and promotes Ca(2+) release. Using competition binding assays, we further characterized this system in skeletal muscle and showed that Ca(2+)-S100A1 competes with Ca(2+)-calmodulin (CaM) for the same binding site on RyR1. In addition, the NMR structure was determined for Ca(2+)-S100A1 bound to a peptide derived from this CaM/S100A1 binding domain, a region conserved in RyR1 and RyR2 and termed RyRP12 (residues 3616-3627 in human RyR1). Examination of the S100A1-RyRP12 complex revealed residues of the helical RyRP12 peptide (Lys-3616, Trp-3620, Lys-3622, Leu-3623, Leu-3624, and Lys-3626) that are involved in favorable hydrophobic and electrostatic interactions with Ca(2+)-S100A1. These same residues were shown previously to be important for RyR1 binding to Ca(2+)-CaM. A model for regulating muscle contraction is presented in which Ca(2+)-S100A1 and Ca(2+)-CaM compete directly for the same binding site on the ryanodine receptor.

Structure of the S100A6 complex with a fragment from the C-terminal domain of Siah-1 interacting protein: a novel mode for S100 protein target recognition

S100A6 is a member of the S100 subfamily of EF-hand Ca (2+) binding proteins that has been shown to interact with calcyclin binding protein/Siah-1 interacting protein (CacyBP/SIP or SIP), a subunit of an SCF-like E3 ubiquitin ligase complex (SCF-TBL1) formed under genotoxic stress. SIP serves as a scaffold in this complex, linking the E2-recruiting module Siah-1 to the substrate-recruiting module Skp1-TBL1. A cell-based functional assay suggests that S100A6 modulates the activity of SCF-TBL1. The results from the cell-based experiments could be enhanced if it were possible to selectively inhibit S100A6-SIP interactions without perturbing any other functions of the two proteins. To this end, the structure of the S100A6-SIP complex was determined in solution by NMR and the strength of the interaction was characterized by isothermal titration calorimetry. In an initial step, the minimal S100A6 binding region in SIP was mapped to a 31-residue fragment (Ser189-Arg219) in the C-terminal domain. The structure of the S100A6-SIP(189-219) complex revealed that SIP(189-219) forms two helices, the first of which (Met193-Tyr200) interacts with S100A6 in a canonical binding mode. The second helix (Met207-Val216) lies over the S100A6 dimer interface, a mode of binding to S100A6 that has not previously been observed for any target bound to an S100 protein. A series of structure-based SIP mutations showed reduced S100A6 binding affinity, setting the stage for direct functional analysis of S100A6-SIP interactions.

IQ motif selectivity in human IQGAP1: binding of myosin essential light chain and S100B

IQGAPs are cytoskeletal scaffolding proteins which link signalling pathways to the reorganisation of actin and microtubules. Human IQGAP1 has four IQ motifs each of which binds to calmodulin. The same region has been implicated in binding to two calmodulin-like proteins, the myosin essential light chain Mlc1sa and the calcium and zinc ion binding protein S100B. Using synthetic peptides corresponding to the four IQ motifs of human IQGAP1, we showed by native gel electrophoresis that only the first IQ motif interacts with Mlc1sa. This IQ motif, and also the fourth, interacts with the budding yeast myosin essential light chain Mlc1p. The first and second IQ motifs interact with S100B in the presence of calcium ions. This clearly establishes that S100B can interact with its targets through IQ motifs in addition to interacting via previously reported sequences. These results are discussed in terms of the function of IQGAP1 and IQ motif recognition.

Divalent metal ion complexes of S100B in the absence and presence of pentamidine

As part of an effort to inhibit S100B, structures of pentamidine (Pnt) bound to Ca(2+)-loaded and Zn(2+),Ca(2+)-loaded S100B were determined by X-ray crystallography at 2.15 A (R(free)=0.266) and 1.85 A (R(free)=0.243) resolution, respectively. These data were compared to X-ray structures solved in the absence of Pnt, including Ca(2+)-loaded S100B and Zn(2+),Ca(2+)-loaded S100B determined here (1.88 A; R(free)=0.267). In the presence and absence of Zn(2+), electron density corresponding to two Pnt molecules per S100B subunit was mapped for both drug-bound structures. One Pnt binding site (site 1) was adjacent to a p53 peptide binding site on S100B (+/-Zn(2+)), and the second Pnt molecule was mapped to the dimer interface (site 2; +/-Zn(2+)) and in a pocket near residues that define the Zn(2+) binding site on S100B. In addition, a conformational change in S100B was observed upon the addition of Zn(2+) to Ca(2+)-S100B, which changed the conformation and orientation of Pnt bound to sites 1 and 2 of Pnt-Zn(2+),Ca(2+)-S100B when compared to Pnt-Ca(2+)-S100B. That Pnt can adapt to this Zn(2+)-dependent conformational change was unexpected and provides a new mode for S100B inhibition by this drug. These data will be useful for developing novel inhibitors of both Ca(2+)- and Ca(2+),Zn(2+)-bound S100B.

Structure of Ca2+-bound S100A4 and its interaction with peptides derived from nonmuscle myosin-IIA

S100A4, also known as mts1, is a member of the S100 family of Ca2+-binding proteins that is directly involved in tumor invasion and metastasis via interactions with specific protein targets, including nonmuscle myosin-IIA (MIIA). Human S100A4 binds two Ca2+ ions with the typical EF-hand exhibiting an affinity that is nearly 1 order of magnitude tighter than that of the pseudo-EF-hand. To examine how Ca2+ modifies the overall organization and structure of the protein, we determined the 1.7 A crystal structure of the human Ca2+-S100A4. Ca2+ binding induces a large reorientation of helix 3 in the typical EF-hand. This reorganization exposes a hydrophobic cleft that is comprised of residues from the hinge region,helix 3, and helix 4, which afford specific target recognition and binding. The Ca2+-dependent conformational change is required for S100A4 to bind peptide sequences derived from the C-terminal portion of the MIIA rod with submicromolar affinity. In addition, the level of binding of Ca2+ to both EF-hands increases by 1 order of magnitude in the presence of MIIA. NMR spectroscopy studies demonstrate that following titration with a MIIA peptide, the largest chemical shift perturbations and exchange broadening effects occur for residues in the hydrophobic pocket of Ca2+-S100A4. Most of these residues are not exposed in apo-S100A4 and explain the Ca2+ dependence of formation of theS100A4-MIIA complex. These studies provide the foundation for understanding S100A4 target recognition and may support the development of reagents that interfere with S100A4 function.

S100A1 binds to the calmodulin-binding site of ryanodine receptor and modulates skeletal muscle excitation-contraction coupling

S100A1, a 21-kDa dimeric Ca2+-binding protein, is an enhancer of cardiac Ca2+ release and contractility and a potential therapeutic agent for the treatment of cardiomyopathy. The role of S100A1 in skeletal muscle has been less well defined. Additionally, the precise molecular mechanism underlying S100A1 modulation of sarcoplasmic reticulum Ca2+ release in striated muscle has not been fully elucidated. Here, utilizing a genetic approach to knock out S100A1, we demonstrate a direct physiological role of S100A1 in excitation-contraction coupling in skeletal muscle. We show that the absence of S100A1 leads to decreased global myoplasmic Ca2+ transients following electrical excitation. Using high speed confocal microscopy, we demonstrate with high temporal resolution depressed activation of sarcoplasmic reticulum Ca2+ release in S100A1-/- muscle fibers. Through competition assays with sarcoplasmic reticulum vesicles and through tryptophan fluorescence experiments, we also identify a novel S100A1-binding site on the cytoplasmic face of the intact ryanodine receptor that is conserved throughout striated muscle and corresponds to a previously identified calmodulin-binding site. Using a 12-mer peptide of this putative binding domain, we demonstrate low micromolar binding affinity to S100A1. NMR spectroscopy reveals this peptide binds within the Ca2+-dependent hydrophobic pocket of S100A1. Taken together, these data suggest that S100A1 plays a significant role in skeletal muscle excitation-contraction coupling, primarily through specific interactions with a conserved binding domain of the ryanodine receptor. This warrants further investigation into the use of S100A1 as a therapeutic target for the treatment of both cardiac and skeletal myopathies.

New insights into mechanism and regulation of actin capping protein

The heterodimeric actin capping protein, referred to here as "CP," is an essential element of the actin cytoskeleton, binding to the barbed ends of actin filaments and regulating their polymerization. In vitro, CP has a critical role in the dendritic nucleation process of actin assembly mediated by Arp2/3 complex, and in vivo, CP is important for actin assembly and actin-based process of morphogenesis and differentiation. Recent studies have provided new insight into the mechanism of CP binding the barbed end, which raises new possibilities for the dynamics of CP and actin in cells. In addition, a number of molecules that bind and regulate CP have been discovered, suggesting new ideas for how CP may integrate into diverse processes of cell physiology.

A biosensor of S100A4 metastasis factor activation: inhibitor screening and cellular activation dynamics

S100A4, a member of the S100 family of Ca2+-binding proteins, displays elevated expression in malignant human tumors compared with benign tumors, and increased expression correlates strongly with poor patient survival. S100A4 has a direct role in metastatic progression, likely due to the modulation of actomyosin cytoskeletal dynamics, which results in increased cellular motility. We developed a fluorescent biosensor (Mero-S100A4) that reports on the Ca2+-bound, activated form of S100A4. Direct attachment of a novel solvatochromatic reporter dye to S100A4 results in a sensor that, upon activation, undergoes a 3-fold enhancement in fluorescence, thus providing a sensitive assay for use in vitro and in vivo. In cells, localized activation of S100A4 at the cell periphery is observed during random migration and following stimulation with lysophosphatidic acid, a known activator of cell motility and proliferation. Additionally, a screen against a library of FDA-approved drugs with the biosensor identified an array of phenothiazines as inhibitors of myosin-II associated S100A4 function. These data demonstrate the utility of the new biosensor both for drug discovery and for probing the cellular dynamics controlled by the S100A4 metastasis factor.

Theoretical study on binding of S100B protein

S100B protein is one of the factors involved in the down-regulation of tumor suppressor protein p53, a transcription activator that signals for cycle arrest and apoptosis. As the inactivation of normal p53 functions is found in over half of human cancers, restoration of normal p53 functions through the destruction or prevention of S100B--p53 complexes represents a possible approach for the development of anti-cancer drugs. The aim of this work was to propose the S100B binding interface through an examination of the literature and use of molecular modeling (MM) techniques with AutoDock program and the AMBER force field. We propose two residues in the S100B binding pocket (Val56, Phe76) and two residues on the protein surface (Val52, Ala83) are essential for ligand binding. The data presented here indicate that interactions with these four residues are necessary for a reduction in the incidence of the S100B--p53 complex. Additionally, we have tried to explain a mechanism for the action of pentamidine, the best-known S100B ligand, and have proposed two S100B--pentamidine structures. The results presented here may be useful for the efficient design of new S100B ligands.

Structural and functional insights into RAGE activation by multimeric S100B

Nervous system development and plasticity require regulation of cell proliferation, survival, neurite outgrowth and synapse formation by specific extracellular factors. The EF-hand protein S100B is highly expressed in human brain. In the extracellular space, it promotes neurite extension and neuron survival via the receptor RAGE (receptor for advanced glycation end products). The X-ray structure of human Ca(2+)-loaded S100B was determined at 1.9 A resolution. The structure revealed an octameric architecture of four homodimeric units arranged as two tetramers in a tight array. The presence of multimeric forms in human brain extracts was confirmed by size-exclusion experiments. Recombinant tetrameric, hexameric and octameric S100B were purified from Escherichia coli and characterised. Binding studies show that tetrameric S100B binds RAGE with higher affinity than dimeric S100B. Analytical ultracentrifugation studies imply that S100B tetramer binds two RAGE molecules via the V-domain. In line with these experiments, S100B tetramer caused stronger activation of cell growth than S100B dimer and promoted cell survival. The structural and the binding data suggest that tetrameric S100B triggers RAGE activation by receptor dimerisation.

Hexameric Calgranulin C (S100A12) Binds to the Receptor for Advanced Glycated End Products (RAGE) Using Symmetric Hydrophobic Target-binding Patches

Calgranulin C (S100A12) is a member of the S100 family of proteins that undergoes a conformational change upon calcium binding allowing them to interact with target molecules and initiate biological responses; one such target is the receptor for advanced glycation products (RAGE). The RAGE-calgranulin C interaction mediates a pro-inflammatory response to cellular stress and can contribute to the pathogenesis of inflammatory lesions. The soluble extracellular part of RAGE (sRAGE) was shown to decrease the inflammation response possibly by scavenging RAGE-activating ligands. Here, by using high resolution NMR spectroscopy, we identified the sRAGE-calgranulin C interaction surface. Ca2+ binding creates two symmetric hydrophobic surfaces on Ca2+-calgranulin C that allow calgranulin C to bind to the C-type immunoglobulin domain of RAGE. Apo-calgranulin C also binds to sRAGE using a completely different surface and with substantially lower affinity, thus underscoring the role of Ca2+ binding to S100 proteins as a molecular switch. By using native gel electrophoresis, chromatography, and fluorescence spectroscopy, we established that sRAGE forms tetramers that bind to hexamers of Ca2+-calgranulin C. This arrangement creates a large platform for effectively transmitting RAGE-dependent signals from extracellular S100 proteins to the cytoplasmic signaling complexes.