Conformational and thermodynamic properties of peptide binding to the human S100P protein

Inhibition of the HIF-1 Survival Pathway as a Strategy to Augment Photodynamic Therapy Efficacy

Photodynamic therapy (PDT) is a non-to-minimally invasive treatment modality that utilizes photoactivatable drugs called photosensitizers to disrupt tumors with locally photoproduced reactive oxygen species (ROS). Photosensitizer activation by light results in hyperoxidative stress and subsequent tumor cell death, vascular shutdown and hypoxia, and an antitumor immune response. However, sublethally afflicted tumor cells initiate several survival mechanisms that account for decreased PDT efficacy. The hypoxia inducible factor 1 (HIF-1) pathway is one of the most effective cell survival pathways that contributes to cell recovery from PDT-induced damage. Several hundred target genes of the HIF-1 heterodimeric complex collectively mediate processes that are involved in tumor cell survival directly and indirectly (e.g., vascularization, glucose metabolism, proliferation, and metastasis). The broad spectrum of biological ramifications culminating from the activation of HIF-1 target genes reflects the importance of HIF-1 in the context of therapeutic recalcitrance. This chapter elaborates on the involvement of HIF-1 in cancer biology, the hypoxic response mechanisms, and the role of HIF-1 in PDT. An overview of inhibitors that either directly or indirectly impede HIF-1-mediated survival signaling is provided. The inhibitors may be used as pharmacological adjuvants in combination with PDT to augment therapeutic efficacy.

Bee Venom Components as Therapeutic Tools against Prostate Cancer

Prostate cancer is one of the most common cancers in men. Despite the development of a variety of therapeutic agents to treat either metastatic hormone-sensitive prostate cancer, advanced prostate cancer, or nonmetastatic/metastatic castration-resistant prostate cancer, the progression or spread of the disease often cannot be avoided. Additionally, the development of resistance of prostate cancer cells to available therapeutic agents is a well-known problem. Despite extensive and cost-intensive research over decades, curative therapy for metastatic prostate cancer is still not available. Therefore, additional therapeutic agents are still needed. The animal kingdom offers a valuable source of natural substances used for the treatment of a variety of diseases. Bee venom of the honeybee is a mixture of many components. It contains proteins acting as enzymes such as phospholipase A2, smaller proteins and peptides such as melittin and apamin, phospholipids, and physiologically active amines such as histamine, dopamine, and noradrenaline. Melittin has been shown to induce apoptosis in different cancer cell lines, including prostate cancer cell lines. It also influences cell proliferation, angiogenesis, and necrosis as well as motility, migration, metastasis, and invasion of tumour cells. Hence, it represents an interesting anticancer agent. In this review article, studies about the effect of bee venom components on prostate cancer cells are discussed. An electronic literature research was performed utilising PubMed in February 2021. All scientific publications, which examine this interesting subject, are discussed. Furthermore, the different types of application of these promising substances are outlined. The studies clearly indicate that bee venom or melittin exhibited anticancer effects in various prostate cancer cell lines and in xenografts. In most of the studies, a combination of bee venom or the modified melittin with another molecule was utilised in order to avoid side effects and, additionally, to target selectively the prostate cancer cells or the surrounding tissue. The studies showed that systemic side effects and unwanted damage to healthy tissue and organs could be minimised when the anticancer drug was not activated until binding to the cancer cells or the surrounding tissue. Different targets were used, such as the matrix metalloproteinase 2, hormone receptors expressed by prostate cancer cells, the extracellular domain of PSMA, and the fibroblast activation protein occurring in the stroma of prostate cancer cells. Another approach used loaded phosphate micelles, which were cleaved by the enzyme secretory phospholipase A2 produced by prostate cancer cells. In a totally different approach, targeted nanoparticles containing the melittin gene were used for prostate cancer gene therapy. By the targeted nonviral gene delivery, the gene encoding melittin was delivered to the prostate cancer cells without systemic side effects. This review of the scientific literature reveals totally different approaches using bee venom, melittin, modified melittin, or protoxin as anticancer agents. The toxic agents acted through several different mechanisms to produce their anti-prostate cancer effects. These mechanisms are not fully understood yet and more experimental studies are necessary to reveal the complete mode of action. Nevertheless, the researchers have conducted pioneering work. Based on these results, further experimental and clinical studies about melittin and modifications of this interesting agent deriving from nature are necessary and could possibly lead to a complementary treatment option for prostate cancer.

Effect of dimerized melittin on gastric cancer cells and antibacterial activity

Melittin is the peptide toxin found in bee venom and is effective against cancer cells. To enhance its activity, a branched dimeric form of melittin was designed. The monomeric form of the peptide was more cytotoxic against gastric cancer cells at low concentrations (1-5 μM) than the dimer form, while the cytotoxic effect was comparable at higher concentrations (10 μM). Confocal microscopy showed that both the monomer and dimer forms of melittin with fluorescent label at the C terminus penetrated the cytoplasm and localized at the cell nucleus and disrupted the cell membrane. The results indicated that both peptides localized in the nucleus and no significant difference in penetration was observed between monomer and dimer of melittin. Although the C and N termini are important for melittin activity, using C terminus for dimerization of the peptide resulted in similar activity for the monomer and dimer against bacteria and gastric cancer cells.

Structural insights into calcium-bound S100P and the V domain of the RAGE complex

The S100P protein is a member of the S100 family of calcium-binding proteins and possesses both intracellular and extracellular functions. Extracellular S100P binds to the cell surface receptor for advanced glycation end products (RAGE) and activates its downstream signaling cascade to meditate tumor growth, drug resistance and metastasis. Preventing the formation of this S100P-RAGE complex is an effective strategy to treat various disease conditions. Despite its importance, the detailed structural characterization of the S100P-RAGE complex has not yet been reported. In this study, we report that S100P preferentially binds to the V domain of RAGE. Furthermore, we characterized the interactions between the RAGE V domain and Ca²⁺-bound S100P using various biophysical techniques, including isothermal titration calorimetry (ITC), fluorescence spectroscopy, multidimensional NMR spectroscopy, functional assays and site-directed mutagenesis. The entropy-driven binding between the V domain of RAGE and Ca⁺²-bound S100P was found to lie in the micromolar range (K_d of ∼6 µM). NMR data-driven HADDOCK modeling revealed the putative sites that interact to yield a proposed heterotetrameric model of the S100P-RAGE V domain complex. Our study on the spatial structural information of the proposed protein-protein complex has pharmaceutical relevance and will significantly contribute toward drug development for the prevention of RAGE-related multifarious diseases.

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.

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.

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.

Modulation of quaternary structure of S100 proteins by calcium ions

It is well established that calcium binding leads to conformational changes in S100 proteins. These conformational changes are thought to activate the protein and render a protein conformation that is capable of binding other proteins. The basic quaternary structural motif of S100 proteins is a homodimer, however there is little information if higher order non-covalent oligomers are also formed and whether these oligomers are of functional relevance. To this end we performed equilibrium analytical ultracentrifugation experiments for 16 S100 proteins (S100A1, S100A2, S100A3, S100A4, S100A5, S100A6, S100A7, S100A8, S100A9, S100A10, S100A11, S100A12, S100A13, S100B, S100P, and S100Z) under reducing conditions in the absence and presence of calcium ions. We show that the addition of calcium promotes the formation of tetrameric structures which could be further enhanced under in vivo conditions where there is an additional effect of molecular crowding.

Annexin I and annexin II N-terminal peptides binding to S100 protein family members: specificity and thermodynamic characterization

The S100 proteins make up a family of dimeric calcium binding proteins that function in response to changing calcium levels. Several S100 binding proteins have been identified; however, the exact biological functions of the S100 proteins are largely unknown as there are several factors which modulate their functions. To address these issues, the specificity of binding of representative members of the human S100 proteins to short N-terminal peptides of annexin I (AnI) and annexin II (AnII) was investigated under controlled experimental conditions. AnI and AnII have been shown previously to interact with S100A11 and S100A10, respectively. This provided a unique opportunity to determine their binding specificity with the other members of the human S100 protein family. It was found that AnI binds S100A6 or S100A11 while AnII binds S100A10 or S100A11. This is the first report of the interaction between S100A6 and AnI. The fact that AnI and AnII bind to selected members of the S100 protein family shows that these interactions are specific and that the mode of binding is different from that of calmodulin, as it was found not to bind AnI or AnII. From the analysis of the thermodynamics of interactions, the binding seems to be entropically driven. It was found that both AnI and AnII undergo a coil-to-helix transition upon binding to their respective binding partners. The observation that there is an overlap in functionality is not surprising due to considerable sequence homology between S100 protein family members. In fact, the functional overlap can explain previous failures of S100 knockout constructs to show any detectable changes in phenotype despite numerous implications of these proteins in important cellular processes.

Hydrophobic interactions are the driving force for the binding of peptide mimotopes and Staphylococcal protein A to recombinant human IgG1

We studied the interaction of several nona-peptide mimotopes of different sequence and Staphylococcal protein A (SpA) with a recombinant human IgG1 antibody using isothermal titration calorimetry (ITC). The amino acid primary structure of the peptides was varied in order to identify the specific antibody-peptide binding sites. Additionally, the influence of temperature and salt concentration was investigated. An attempt was made to elucidate the structural changes upon complex formation using the determined thermodynamic parameters. The amino acid composition of the mimotopes determined their binding affinity. The binding constant K (a) of the mimotopes was in the range 1 x 10(4) to 1 x 10(6) M(-1). The binding constant of SpA was on the average about three orders of magnitude higher than that of the peptides. The binding constant of the peptides and of SpA decreased with temperature and the binding process was connected with negative changes in enthalpy, entropy, and heat capacity. The binding of the mimotopes to the Fab part of the IgG1 antibody and binding of SpA to the Fc part of the IgG1 antibody were mainly driven by hydrophobic effects and associated with a relatively large change in water-accessible surface area. Determinants for a strong/reduced antibody-peptide binding were identified.

Calcium-dependent and -independent interactions of the S100 protein family

The S100 proteins comprise at least 25 members, forming the largest group of EF-hand signalling proteins in humans. Although the proteins are expressed in many tissues, each S100 protein has generally been shown to have a preference for expression in one particular tissue or cell type. Three-dimensional structures of several S100 family members have shown that the proteins assume a dimeric structure consisting of two EF-hand motifs per monomer. Calcium binding to these S100 proteins, with the exception of S100A10, results in an approx. 40 degrees alteration in the position of helix III, exposing a broad hydrophobic surface that enables the S100 proteins to interact with a variety of target proteins. More than 90 potential target proteins have been documented for the S100 proteins, including the cytoskeletal proteins tubulin, glial fibrillary acidic protein and F-actin, which have been identified mostly from in vitro experiments. In the last 5 years, efforts have concentrated on quantifying the protein interactions of the S100 proteins, identifying in vivo protein partners and understanding the molecular specificity for target protein interactions. Furthermore, the S100 proteins are the only EF-hand proteins that are known to form both homo- and hetero-dimers, and efforts are underway to determine the stabilities of these complexes and structural rationales for their formation and potential differences in their biological roles. This review highlights both the calcium-dependent and -independent interactions of the S100 proteins, with a focus on the structures of the complexes, differences and similarities in the strengths of the interactions, and preferences for homo- compared with hetero-dimeric S100 protein assembly.

Elimination of the C-cap in ubiquitin - structure, dynamics and thermodynamic consequences

Single amino acid substitutions rarely produce substantial changes in protein structure. Here we show that substitution of the C-cap residue in the alpha-helix of ubiquitin with proline (34P variant) leads to dramatic structural changes. The resulting conformational perturbation extends over the last two turns of the alpha-helix and leads to enhanced flexibility for residues 27-37. Thermodynamic analysis of this ubiquitin variant using differential scanning calorimetry reveals that the thermal unfolding transition remains highly cooperative, exhibiting two-state behavior. Similarities with the wild type in the thermodynamic parameters (heat capacity change upon unfolding and m-value) of unfolding monitored by DSC and chemical denaturation suggests that the 34P variant has comparable buried surface area. The hydrophobic core of 34P variant is not packed as well as that of the wild type protein as manifested by a lower enthalpy of unfolding. The increased mobility of the polypeptide chain of this ubiquitin variant allows the transient opening of the hydrophobic core as evidenced by ANS binding. Taken together, these results suggest exceptional robustness of cooperativity in protein structures.

S100P is selectively upregulated in tumor cell lines challenged with DNA cross-linking agents

Bifunctional alkylating agents that cross-link DNA are implicated in the pathogenesis of therapy related myelodysplastic syndromes (MDS) and MDS related acute myeloid leukemia (MDR-AML). We exposed HL60 cells to the highest level of bifunctional alkylating nitrogen mustard mechlorethamine (HN2) that was consistent with recovery following suppressed growth. Microarray analyses showed minor changes in transcripts in HN2 treated cells. A moderate up-regulation of S100P mRNA was consistently observed after 1 day of exposure to bifunctional alkylating agents and expression was not induced with monofunctional agents. Elevated S100P protein/antigen was not detected until days later in a subset of non-mitotic G2 cells. Elevated S100P protein persisted over the course of a delayed recovery phase. The results confirm recent reports indicating that S100P is a survival factor. In addition, our results indicate that S100P has a specific role in G2 cell function associated with a prolonged phase of recovery after exposure to bifunctional alkylating agents.

Effects of Tween 20® and Tween 80® on the Stability of Albutropin During Agitation

The objectives of this work were to determine the effects of nonionic surfactants (Tween 20 and Tween 80) on agitation-induced aggregation of the recombinant fusion protein, Albutropintrade mark (human growth hormone genetically fused to human albumin), and to characterize the binding interactions between the surfactants and the protein. Knowing the binding stoichiometry would allow a rational choice of surfactant concentration to protect the protein from surface-induced aggregation. Fluorescence spectroscopy and isothermal titration calorimetry (ITC) were employed to study Albutropin surfactant binding. Albutropin was agitated at 25 +/- 2 degrees C to induce aggregation, and samples were taken during a 96-h incubation. Size-exclusion chromatography (SEC-HPLC) (HPLC, high-performance liquid chromatography) was used to detect and quantify the extent of protein aggregation. The effect of surfactants on the protein's free energy of unfolding was determined using guanidine HCl as a denaturant. Tween 20 and Tween 80 had saturable binding to Albutropin with a molar binding stoichiometry of 10:1 and 9:1 (surfactant:protein), respectively. Binding of the surfactants to Albutropin increased the free energy of unfolding by over 1 and 0.6 kcal/mol, respectively. In protein samples that were agitated in the absence of surfactant, soluble aggregates were detected within 24 h, and there was almost complete loss of monomer to soluble aggregates by the end of the 96-h experiment. At the molar binding stoichiometry, Tween 20 and Tween 80 prevented the formation of soluble aggregates, even though the concentrations of surfactants were well below their critical micelle concentrations (CMC). Tween 20 and Tween 80 protected Albutropin against agitation-induced aggregation, even at concentrations below the CMC. Equilibrium unfolding data indicate that Tween confer protection by increasing the free energy of unfolding of Albutropin.

Calcium- and magnesium-dependent interactions between calcium- and integrin-binding protein and the integrin alphaIIb cytoplasmic domain

Calcium- and integrin-binding protein (CIB) is a small EF-hand calcium-binding protein that is involved in hemostasis through its interaction with the alphaIIb cytoplasmic domain of integrinalphaIIbbeta(3). We have previously demonstrated that CIB lacks structural stability in the absence of divalent metal ions but that it acquires a well-folded conformation upon addition of Ca(2+) or Mg(2+). Here, we have used fluorescence spectroscopy, NMR spectroscopy, and isothermal titration calorimetry to demonstrate that both Ca(2+)-bound CIB (Ca(2+)-CIB) and the Mg(2+)-bound protein (Mg(2+)-CIB) bind with high affinity and through a similar mechanism to alphaIIb cytoplasmic domain peptides, but that metal-free CIB (apo-CIB) binds in a different manner. The interactions are thermodynamically distinct for Ca(2+)-CIB and Mg(2+)-CIB, but involve hydrophobic interactions in each case. Since the Mg(2+) concentration inside the cell is sufficient to saturate CIB at all times, our results imply that CIB would be capable of binding to the alphaIIb cytoplasmic domain independent of an intracellular Ca(2+) stimulus in vivo. This raises the question of whether CIB can act as a Ca(2+) sensor in alphaIIbbeta(3) signaling or if other regulatory mechanisms such as fibrinogen-induced conformational changes in alphaIIbbeta(3), post-translational modifications, or the binding of other accessory proteins mediate the interactions between CIB and alphaIIbbeta(3). Differences in NMR spectra do suggest, however, that Ca(2+)-binding to the Mg(2+)- CIB-alphaIIb complex induces subtle structural changes that could further modulate the activity of alphaIIbbeta(3).

Expression of S100P and its novel binding partner S100PBPR in early pancreatic cancer

S100P is a member of the S100 family of calcium-binding proteins and there have been several recent reports of its overexpression in pancreatic ductal adenocarcinoma (PDAC). We have used Far Western screening and in vitro interaction assays to identify and confirm a novel target protein for S100P. We have named this protein S100PBPR, and shown that its interaction with S100P is dependent on Ca(2+) or Mg(2+). S100PBPR was found to localize to cell nuclei where S100P is also present, and the two proteins co-immunoprecipitate. By in situ hybridization, S100PBPR transcript was found in islet cells but not duct cells of the healthy pancreas. Both S100P and S100PBPR were detected by quantitative real-time polymerase chain reaction in pancreatic intraepithelial neoplasia (PanIN) and PDAC samples, and in situ hybridization revealed the presence of S100PBPR transcript in malignant PDAC cells. These data suggest that an interaction between S100P and S100PBPR may be involved in early pancreatic cancer. S100P was further investigated in PanIN lesions and immunohistochemical analysis showed its expression to correlate significantly with increasing grade of PanINs, being found as early as PanIN-1 with more prevalent expression in PanIN-2 and -3. These data suggest that S100P can be added to the genetic progression model for PDAC.

Thermodynamic studies on the interaction of the third complement component and its inhibitor, compstatin

Compstatin is a 13-residue cyclic peptide that inhibits complement activation by binding to complement component, C3. Although the activity of compstatin has been improved severalfold using combinatorial and rational design approaches, the molecular basis for its interaction with C3 is not yet fully understood. In the present study, isothermal titration calorimetry was employed to dissect the molecular forces that govern the interaction of compstatin with C3 using four different compstatin analogs. Our studies indicate that the C3-compstatin interaction is an enthalpy-driven process. Substitution of the valine and histidine residues at positions 4 and 9 with tryptophan and alanine, respectively, resulted in the increase of enthalpy of the interaction, thereby increasing the binding affinity for C3. The data also suggest that the interaction is mediated by water molecules. These interfacial water molecules could be the source for unfavorable entropy and large negative heat capacity changes observed in the interaction. Although part of the negative heat capacity changes could be accounted for by the water molecules, the rest might be resulting from the conformational changes in C3 and/or compstatin up on binding. Finally, we propose based on the pK(a) values determined from the protonation studies that histidine on compstatin participates in protonation changes and contributes to the specificity of the interaction between compstatin and C3. These protonation changes vary significantly between the binding of different compstatin analogs to C3.

A survey of the year 2002 literature on applications of isothermal titration calorimetry

Isothermal titration calorimetry (ITC) is becoming widely accepted as a key instrument in any laboratory in which quantification of biomolecular interactions is a requisite. The method has matured with respect to general acceptance and application development over recent years. The number of publications on ITC has grown exponentially over the last 10 years, reflecting the general utility of the method. Here all the published works of the year 2002 in this area have been surveyed. We review the broad range of systems to which ITC is being directed and classify these into general areas highlighting key publications of interest. This provides an overview of what can be achieved using this method and what developments are likely to occur in the near future.

Ca2+-dependent binding and activation of dormant ezrin by dimeric S100P

S100 proteins are EF hand type Ca2+ binding proteins thought to function in stimulus-response coupling by binding to and thereby regulating cellular targets in a Ca2+-dependent manner. To isolate such target(s) of the S100P protein we devised an affinity chromatography approach that selects for S100 protein ligands requiring the biologically active S100 dimer for interaction. Hereby we identify ezrin, a membrane/F-actin cross-linking protein, as a dimer-specific S100P ligand. S100P-ezrin complex formation is Ca2+ dependent and most likely occurs within cells because both proteins colocalize at the plasma membrane after growth factor or Ca2+ ionophore stimulation. The S100P binding site is located in the N-terminal domain of ezrin and is accessible for interaction in dormant ezrin, in which binding sites for F-actin and transmembrane proteins are masked through an association between the N- and C-terminal domains. Interestingly, S100P binding unmasks the F-actin binding site, thereby at least partially activating the ezrin molecule. This identifies S100P as a novel activator of ezrin and indicates that activation of ezrin's cross-linking function can occur directly in response to Ca2+ transients.

S-100 proteins in the human peripheral nervous system

This article reviews the distribution of S100 proteins in the human peripheral nervous system. The expression of S100 by peripheral glial cells seems to be a distinctive fact of these cells, independently of their localization and their ability to myelinate or not. S100 proteins expressing cells include satellite cells of sensory, sympathetic and enteric ganglia, supporting cells of the adrenal medulla, myelinating and non-myelinating Schwann cells in the nerve trunks, and the Schwann-related cells of sensory corpuscles. In addition, S100 proteins are expressed in peripheral neurons. Most of them express S100alpha protein, and a subpopulation of sensory neurons in dorsal root ganglia contains S100beta protein or S100alpha plus S100beta proteins.

The crystal structure at 2A resolution of the Ca2+ -binding protein S100P

S100P is a small calcium-binding protein of the S100 EF-hand-containing family of proteins. Elevated levels of its mRNA are reported to be associated with the progression to hormone independence and metastasis of prostate cancer and to be associated with loss of senescence in human breast epithelial cells in vitro. The first structure of human recombinant S100P in calcium-bound form is now reported at 2.0A resolution by X-ray diffraction. A flexible linker connects the two EF-hand motifs. The protein exists as a homodimer formed by non-covalent interactions between large hydrophobic areas on monomeric S100P. Experiments with an optical biosensor to study binding parameters of the S100P monomer interaction showed that the association rate constant was faster in the presence of calcium than in their absence, whereas the dissociation rate constant was independent of calcium. The K(d) values were 64(+/-24)nM and 2.5(+/-0.8) microM in the presence and in the absence of calcium ions, respectively. Dimerization of S100P is demonstrated in vivo using the yeast two-hybrid system. The effect of mutation of specific amino acids suggests that dimerization in vivo can be affected by amino acids on the dimer interface and in the hydrophobic core.