Molecular characterization and tissue distribution of a novel member of the S100 family of EF-hand proteins

From bench to bedside: Calprotectin (S100A8/S100A9) as a biomarker in rheumatoid arthritis

S100A9/S100A8 (calprotectin), a member of the S100 protein family, has been shown to play a pivotal role in innate immunity activation. Calprotectin plays a critical role in the pathogenesis of rheumatoid arthritis (RA), as it triggers chemotaxis, phagocyte migration and modulation of neutrophils and macrophages. Higher calprotectin levels have been found in synovial fluid, plasma, and serum from RA patients. Recent studies have demonstrated better correlations between serum or plasma calprotectin and composite inflammatory disease activity indexes than c-reactive protein (CRP) or the erythrocyte sedimentation rate (ESR). Calprotectin serum levels decreased after treatment, independently of the DMARD type or strategy. Calprotectin has shown the strongest correlations with other sensitive techniques to detect inflammation, such as ultrasound. Calprotectin independently predicts radiographic progression. However, its value as a biomarker of treatment response and flare after tapering is unclear. This update reviews the current understanding of calprotectin in RA and discusses possible applications as a biomarker in clinical practice.

Identification of Novel Genes Involved in Hyperglycemia in Mice

Current attempts to prevent and manage type 2 diabetes have been moderately effective, and a better understanding of the molecular roots of this complex disease is important to develop more successful and precise treatment options. Recently, we initiated the collective diabetes cross, where four mouse inbred strains differing in their diabetes susceptibility were crossed with the obese and diabetes-prone NZO strain and identified the quantitative trait loci (QTL) Nidd13/NZO, a genomic region on chromosome 13 that correlates with hyperglycemia in NZO allele carriers compared to B6 controls. Subsequent analysis of the critical region, harboring 644 genes, included expression studies in pancreatic islets of congenic Nidd13/NZO mice, integration of single-cell data from parental NZO and B6 islets as well as haplotype analysis. Finally, of the five genes (Acot12, S100z, Ankrd55, Rnf180, and Iqgap2) within the polymorphic haplotype block that are differently expressed in islets of B6 compared to NZO mice, we identified the calcium-binding protein S100z gene to affect islet cell proliferation as well as apoptosis when overexpressed in MIN6 cells. In summary, we define S100z as the most striking gene to be causal for the diabetes QTL Nidd13/NZO by affecting β-cell proliferation and apoptosis. Thus, S100z is an entirely novel diabetes gene regulating islet cell function.

Frequent lineage-specific substitution rate changes support an episodic model for protein evolution

Since the inception of the molecular clock model for sequence evolution, the investigation of protein divergence has revolved around the question of a more or less constant change of amino acid sequences, with specific overall rates for each family. Although anomalies in clock-like divergence are well known, the assumption of a constant decay rate for a given protein family is usually taken as the null model for protein evolution. However, systematic tests of this null model at a genome-wide scale have lagged behind, despite the databases’ enormous growth. We focus here on divergence rate comparisons between very closely related lineages since this allows clear orthology assignments by synteny and reliable alignments, which are crucial for determining substitution rate changes. We generated a high-confidence dataset of syntenic orthologs from four ape species, including humans. We find that despite the appearance of an overall clock-like substitution pattern, several hundred protein families show lineage-specific acceleration and deceleration in divergence rates, or combinations of both in different lineages. Hence, our analysis uncovers a rather dynamic history of substitution rate changes, even between these closely related lineages, implying that one should expect that a large fraction of proteins will have had a history of episodic rate changes in deeper phylogenies. Furthermore, each of the lineages has a separate set of particularly fast diverging proteins. The genes with the highest percentage of branch-specific substitutions are ADCYAP1 in the human lineage (9.7%), CALU in chimpanzees (7.1%), SLC39A14 in the internal branch leading to humans and chimpanzees (4.1%), RNF128 in gorillas (9%), and S100Z in gibbons (15.2%). The mutational pattern in ADCYAP1 suggests a biased mutation process, possibly through asymmetric gene conversion effects. We conclude that a null model of constant change can be problematic for predicting the evolutionary trajectories of individual proteins.

S100 proteins: An emerging cynosure in pregnancy & adverse reproductive outcome

S100 proteins are calcium (Ca²⁺)-binding proteins and these have an important function in progression, manifestation and therapeutic aspects of various inflammatory, metabolic and neurodegenerative disorders. Based on their involvement in intracellular or extracellular regulatory effects, S100 proteins are classified into three subgroups: one subgroup is specialized in exerting only intracellular effects, other performs both intracellular and extracellular functions and the third subgroup members only display extracellular regulatory effects. S100 proteins are expressed particularly in vertebrates and have cell-specific expression. Functionally, S100 proteins act through their surface receptors and regulate cell functions in autocrine or paracrine mode. Receptor for advanced glycation end products (RAGEs) and toll-like receptor 4 are the main surface receptors. S100 proteins participate in the regulation of cellular differentiation, proliferation, apoptosis and inflammation along with Ca²⁺ homeostasis, energy metabolism and cellular migration, and perform the respective functions through their interaction with transcription factors, nucleic acids, enzymes, receptors, cytoskeleton system, etc. Currently, their role in adverse pregnancy outcomes and compromised reproductive health is being explored. These proteins are present in amniotic fluid, endometrium tissue and foetal brain; therefore, it is quite likely that alterations in the expression levels of S100 family members will be affecting the particular function they are involved in and ultimately affecting the pregnancy in adverse manner. The current review discusses about an association of S100 proteins in pregnancy disorders such as endometriosis, intrauterine growth retardation and miscarriage.

Evidence for the principle of minimal frustration in the evolution of protein folding landscapes

Theoretical and experimental studies have firmly established that protein folding can be described by a funneled energy landscape. This funneled energy landscape is the result of foldable protein sequences evolving following the principle of minimal frustration, which allows proteins to rapidly fold to their native biologically functional conformations. For a protein family with a given functional fold, the principle of minimal frustration suggests that, independent of sequence, all proteins within this family should fold with similar rates. However, depending on the optimal living temperature of the organism, proteins also need to modulate their thermodynamic stability. Consequently, the difference in thermodynamic stability should be primarily caused by differences in the unfolding rates. To test this hypothesis experimentally, we performed comprehensive thermodynamic and kinetic analyses of 15 different proteins from the thioredoxin family. Eight of these thioredoxins were extant proteins from psychrophilic, mesophilic, or thermophilic organisms. The other seven protein sequences were obtained using ancestral sequence reconstruction and can be dated back over 4 billion years. We found that all studied proteins fold with very similar rates but unfold with rates that differ up to three orders of magnitude. The unfolding rates correlate well with the thermodynamic stability of the proteins. Moreover, proteins that unfold slower are more resistant to proteolysis. These results provide direct experimental support to the principle of minimal frustration hypothesis.

Molecular inhibition mechanisms of cell migration and invasion by coix polysaccharides in A549 NSCLC cells via targeting S100A4

S100 calcium binding protein A4 (S100A4) promotes extracellular signal transduction, intercellular adhesion, motility and mobility. Different extracts from Coix lachryma-jobi have been used for the treatment of various types of cancer in Asia. In our previous study, the polysaccharide fraction extact, CP1, induced cell apoptosis of non‑small cell lung cancer cells. In the current study, CP1 inhibited migration and invasion of A549 cells in a scratch wound healing assay and matrigel invasion assay, respectively. Furthermore, reverse transcription‑polymerase chain reaction and western blotting demonstrated that CP1 downregulated the gene and protein expression levels of S100A4. In silico docking analysis demonstrated that polysaccharides may not interfere with dimerization, whereas, the affinity of polysaccharides for an S100A4‑NMIIA pocket was margnially greater than at the dimerization sites. Thus, CP1 inhibited A549 cell migration and invasion potentially via downregulation of S100A4, and may also interact with the binding site of S100A4‑NMIIA, which indicated that CP1 has potential as an alternative cancer chemotherapeutic by targeting S100A4.

Multiple Evolutionary Origins of Ubiquitous Cu2+ and Zn2+ Binding in the S100 Protein Family

The S100 proteins are a large family of signaling proteins that play critical roles in biology and disease. Many S100 proteins bind Zn2+, Cu2+, and/or Mn2+ as part of their biological functions; however, the evolutionary origins of binding remain obscure. One key question is whether divalent transition metal binding is ancestral, or instead arose independently on multiple lineages. To tackle this question, we combined phylogenetics with biophysical characterization of modern S100 proteins. We demonstrate an earlier origin for established S100 subfamilies than previously believed, and reveal that transition metal binding is widely distributed across the tree. Using isothermal titration calorimetry, we found that Cu2+ and Zn2+ binding are common features of the family: the full breadth of human S100 paralogs-as well as two early-branching S100 proteins found in the tunicate Oikopleura dioica-bind these metals with μM affinity and stoichiometries ranging from 1:1 to 3:1 (metal:protein). While binding is consistent across the tree, structural responses to binding are quite variable. Further, mutational analysis and structural modeling revealed that transition metal binding occurs at different sites in different S100 proteins. This is consistent with multiple origins of transition metal binding over the evolution of this protein family. Our work reveals an evolutionary pattern in which the overall phenotype of binding is a constant feature of S100 proteins, even while the site and mechanism of binding is evolutionarily labile.

Oncogenic RAS directs silencing of tumor suppressor genes through ordered recruitment of transcriptional repressors

Green and colleagues previously identified 28 cofactors through which oncogenic RAS directs transcriptional silencing of Fas and other tumor suppressor genes. The authors now uncover a highly ordered RAS-directed silencing pathway; silencing is initiated by binding of ZFP354B, a sequence-specific DNA-binding protein, and culminates in DNA methyltransferase DNMT1 recruitment. Importantly, RAS-directed silencing requires continuous function of RAS and its cofactors and can be rapidly reversed, which may have therapeutic implications for the treatment of RAS-positive cancers.

We previously identified 28 cofactors through which a RAS oncoprotein directs transcriptional silencing of Fas and other tumor suppressor genes (TSGs). Here we performed RNAi-based epistasis experiments and found that RAS-directed silencing occurs through a highly ordered pathway that is initiated by binding of ZFP354B, a sequence-specific DNA-binding protein, and culminates in recruitment of the DNA methyltransferase DNMT1. RNAi and pharmacological inhibition experiments reveal that silencing requires continuous function of RAS and its cofactors and can be rapidly reversed, which may have therapeutic implications for reactivation of silenced TSGs in RAS-positive cancers.

Functions of S100 proteins

The S100 protein family consists of 24 members functionally distributed into three main subgroups: those that only exert intracellular regulatory effects, those with intracellular and extracellular functions and those which mainly exert extracellular regulatory effects. S100 proteins are only expressed in vertebrates and show cell-specific expression patterns. In some instances, a particular S100 protein can be induced in pathological circumstances in a cell type that does not express it in normal physiological conditions. Within cells, S100 proteins are involved in aspects of regulation of proliferation, differentiation, apoptosis, Ca2+ homeostasis, energy metabolism, inflammation and migration/invasion through interactions with a variety of target proteins including enzymes, cytoskeletal subunits, receptors, transcription factors and nucleic acids. Some S100 proteins are secreted or released and regulate cell functions in an autocrine and paracrine manner via activation of surface receptors (e.g. the receptor for advanced glycation end-products and toll-like receptor 4), G-protein-coupled receptors, scavenger receptors, or heparan sulfate proteoglycans and N-glycans. Extracellular S100A4 and S100B also interact with epidermal growth factor and basic fibroblast growth factor, respectively, thereby enhancing the activity of the corresponding receptors. Thus, extracellular S100 proteins exert regulatory activities on monocytes/macrophages/microglia, neutrophils, lymphocytes, mast cells, articular chondrocytes, endothelial and vascular smooth muscle cells, neurons, astrocytes, Schwann cells, epithelial cells, myoblasts and cardiomyocytes, thereby participating in innate and adaptive immune responses, cell migration and chemotaxis, tissue development and repair, and leukocyte and tumor cell invasion.

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.

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.

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.

Conformational dynamics and structural plasticity play critical roles in the ubiquitin recognition of a UIM domain

Ubiquitin-interacting motifs (UIMs) are an important class of protein domains that interact with ubiquitin or ubiquitin-like proteins. These approximately 20-residue-long domains are found in a variety of ubiquitin receptor proteins and serve as recognition modules towards intracellular targets, which may be individual ubiquitin subunits or polyubiquitin chains attached to a variety of proteins. Previous structural studies of interactions between UIMs and ubiquitin have shown that UIMs adopt an extended structure of a single alpha-helix, containing a hydrophobic surface with a conserved sequence pattern that interacts with key hydrophobic residues on ubiquitin. In light of this large body of structural studies, details regarding the presence and the roles of structural dynamics and plasticity are surprisingly lacking. In order to better understand the structural basis of ubiquitin-UIM recognition, we have characterized changes in the structure and dynamics of ubiquitin upon binding of a UIM domain from the yeast Vps27 protein. The solution structure of a ubiquitin-UIM fusion protein designed to study these interactions is reported here and found to consist of a well-defined ubiquitin core and a bipartite UIM helix. Moreover, we have studied the plasticity of the docking interface, as well as global changes in ubiquitin due to UIM binding at the picoseconds-to-nanoseconds and microseconds-to-milliseconds protein motions by nuclear magnetic resonance relaxation. Changes in generalized-order parameters of amide groups show a distinct trend towards increased structural rigidity at the UIM-ubiquitin interface relative to values determined in unbound ubiquitin. Analysis of (15)N Carr-Purcell-Meiboom-Gill relaxation dispersion measurements suggests the presence of two types of motions: one directly related to the UIM-binding interface and the other induced to distal parts of the protein. This study demonstrates a case where localized interactions among protein domains have global effects on protein motions at timescales ranging from picoseconds to milliseconds.

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.

S100A8/A9: a Janus-faced molecule in cancer therapy and tumorgenesis

Correlations exist between the abundance of S100 proteins and disease pathologies. Indeed, this is evidenced by the heterodimeric S100 protein complex S100A8/A9 which has been shown to be involved in inflammatory and neoplastic disorders. However, S100A8/A9 appears as a Janus-faced molecule in this context. On the one hand, it is a powerful apoptotic agent produced by immune cells, making it a very fascinating tool in the battle against cancer. It spears the risk to induce auto-immune response and may serve as a lead compound for cancer-selective therapeutics. In contrast, S100A8/A9 expression in cancer cells has also been associated with tumor development, cancer invasion or metastasis. Clearly, there is a dichotomy and future investigations into the role of S100A8/A9 in cancer biology need to consider both sides of the same coin.

Advances in calcium-binding protein S100A4 and digestive neoplasms

S100A4 protein, encoded by S100A4 gene, is a kind of calcium-binding regulatory proteins. Combining with the calcium ions, this protein plays an important role in the progress of tumor growth and metastasis. Recent investigations concluded that there is a relationship between S100A4 protein and tumor invasion or metastasis. This article reviews the biochemical property of this protein and summarizes its action and potential mechanism in digestive tumor progression and metastasis.

RAGE (Receptor for Advanced Glycation Endproducts), RAGE ligands, and their role in cancer and inflammation

The Receptor for Advanced Glycation Endproducts [RAGE] is an evolutionarily recent member of the immunoglobulin super-family, encoded in the Class III region of the major histocompatability complex. RAGE is highly expressed only in the lung at readily measurable levels but increases quickly at sites of inflammation, largely on inflammatory and epithelial cells. It is found either as a membrane-bound or soluble protein that is markedly upregulated by stress in epithelial cells, thereby regulating their metabolism and enhancing their central barrier functionality. Activation and upregulation of RAGE by its ligands leads to enhanced survival. Perpetual signaling through RAGE-induced survival pathways in the setting of limited nutrients or oxygenation results in enhanced autophagy, diminished apoptosis, and (with ATP depletion) necrosis. This results in chronic inflammation and in many instances is the setting in which epithelial malignancies arise. RAGE and its isoforms sit in a pivotal role, regulating metabolism, inflammation, and epithelial survival in the setting of stress. Understanding the molecular structure and function of it and its ligands in the setting of inflammation is critically important in understanding the role of this receptor in tumor biology.

Structural and functional diversification in the teleost S100 family of calcium-binding proteins

BACKGROUND

Among the EF-Hand calcium-binding proteins the subgroup of S100 proteins constitute a large family with numerous and diverse functions in calcium-mediated signaling. The evolutionary origin of this family is still uncertain and most studies have examined mammalian family members.

RESULTS

We have performed an extensive search in several teleost genomes to establish the s100 gene family in fish. We report that the teleost S100 repertoire comprises fourteen different subfamilies which show remarkable similarity across six divergent teleost species. Individual species feature distinctive subsets of thirteen to fourteen genes that result from local gene duplications and gene losses. Eight of the fourteen S100 subfamilies are unique for teleosts, while six are shared with mammalian species and three of those even with cartilaginous fish. Several S100 family members are found in jawless fish already, but none of them are clear orthologs of cartilaginous or bony fish s100 genes. All teleost s100 genes show the expected structural features and are subject to strong negative selection. Many aspects of the genomic arrangement and location of mammalian s100 genes are retained in the teleost s100 gene family, including a completely conserved intron/exon border between the two EF hands. Zebrafish s100 genes exhibit highly specific and characteristic expression patterns, showing both redundancy and divergence in their cellular expression. In larval tissue expression is often restricted to specific cell types like keratinocytes, hair cells, ionocytes and olfactory receptor neurons as demonstrated by in situ hybridization.

CONCLUSION

The origin of the S100 family predates at least the segregation of jawed from jawless fish and some extant family members predate the divergence of bony from cartilaginous fish. Despite a complex pattern of gene gains and losses the total repertoire size is remarkably constant between species. On the expression level the teleost S100 proteins can serve as precise markers for several different cell types. At least some of their functions may be related to those of their counterparts in mammals. Accordingly, our findings provide an excellent basis for future studies of the functions and interaction partners of s100 genes and finally their role in diseases, using the zebrafish as a model organism.

Placental S100 (S100P) and GATA3: markers for transitional epithelium and urothelial carcinoma discovered by complementary DNA microarray

The morphologic distinction between prostate and urothelial carcinoma can be difficult. To identify novel diagnostic markers that may aid in the differential diagnosis of prostate versus urothelial carcinoma, we analyzed expression patterns in prostate and bladder cancer tissues using complementary DNA microarrays. Together with our prior studies on renal neoplasms and normal kidney, these studies suggested that the gene for placental S100 (S100P) is specifically expressed in benign and malignant urothelial cells. Using tissue microarrays, a polyclonal antiserum against S100P protein stained 86% of 295 urothelial carcinomas while only 3% of 260 prostatic adenocarcinomas and 1% of 133 renal cell carcinomas stained. A commercially available monoclonal antibody against S100P stained 78% of 300 urothelial carcinomas while only 2% of 256 prostatic adenocarcinomas and none of 137 renal cell carcinomas stained. A second gene, GATA3, also showed high level expression in urothelial tumors by cDNA array. A commercially available monoclonal antibody against GATA3 stained 67% of 308 urothelial carcinomas, but none of the prostate or renal carcinomas. For comparison, staining was also performed for p63 and cytokeratin 5/6. p63 stained 87% of urothelial carcinomas whereas CK5/6 stained 54%. Importantly, when S100P and p63 were combined 95% of urothelial carcinomas were labeled by one or both markers. We conclude that the detection of S100P and GATA3 protein expression may help distinguish urothelial carcinomas from other genitourinary neoplasms that enter into the differential diagnosis.

Pathologies involving the S100 proteins and RAGE

The S100 proteins are exclusively expressed in vertebrates and are the largest subgroup within the superfamily of EF-hand Ca2(+)-binding proteins Generally, S100 proteins are organized as tight homodimers (some as heterodimers). Each subunit is composed of a C-terminal, 'canonical' EF-hand, common to all EF-hand proteins, and a N-terminal, 'pseudo' EF-hand, characteristic of S100 proteins. Upon Ca2(+)-binding, the C-terminal EF-hand undergoes a large conformational change resulting in the exposure of a hydrophobic surface responsible for target binding A unique feature of this protein family is that some members are secreted from cells upon stimulation, exerting cytokine- and chemokine-like extracellular activities via the Receptor for Advanced Glycation Endproducts, RAGE. Recently, larger assemblies of some S100 proteins (hexamers, tetramers, octamers) have been also observed and are suggested to be the active extracellular species required for receptor binding and activation through receptor multimerization Most S100 genes are located in a gene cluster on human chromosome 1q21, a region frequently rearranged in human cancer The functional diversification of S100 proteins is achieved by their specific cell- and tissue-expression patterns, structural variations, different metal ion binding properties (Ca2+, Zn2+ and Cu2+) as well as their ability to form homo-, hetero- and oligomeric assemblies Here, we review the most recent developments focussing on the biological functions of the S100 proteins and we discuss the presently available S100-specific mouse models and their possible use as human disease models In addition, the S100-RAGE interaction and the activation of various cellular pathways will be discussed. Finally, the close association of S100 proteins with cardiomyopathy, cancer, inflammation and brain diseases is summarized as well as their use in diagnosis and their potential as drug targets to improve therapies in the future.

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.

Amino acid substitutions affecting protein dynamics in eglin C do not affect heat capacity change upon unfolding

The heat capacity change upon unfolding (deltaC(p)) is a thermodynamic parameter that defines the temperature dependence of the thermodynamic stability of proteins; however, physical basis of the heat capacity change is not completely understood. Although empirical surface area-based calculations can predict heat capacity changes reasonably well, accumulating evidence suggests that changes in hydration of those surfaces is not the only parameter contributing to the observed heat capacity changes upon unfolding. Because packing density in the protein interior is similar to that observed in organic crystals, we hypothesized that changes in protein dynamics resulting in increased rigidity of the protein structure might contribute to the observed heat capacity change upon unfolding. Using differential scanning calorimetry we characterized the thermodynamic behavior of a serine protease inhibitor eglin C and two eglin C variants with altered native state dynamics, as determined by NMR. We found no evidence of changes in deltaC(p) in either of the variants, suggesting that changes in rigidity do not contribute to the heat capacity change upon unfolding in this model system.

Regulation of calcium homeostasis by S100RVP, an androgen-regulated S100 protein in the rat ventral prostate

BACKGROUND

S100RVP was previously identified as an androgen-response gene in the rat ventral prostate (RVP). Characterization of S100RVP is important for elucidating the function of S100 proteins in androgen action.

METHODS

The expression and subcellular localization of S100RVP were determined by Northern blot, in situ hybridization, and fluorescent microscopy. Calcium overlay and calcium ionophore sensitivity assays were performed to investigate the calcium binding and function of S100RVP.

RESULTS

S100RVP is abundantly expressed in the RVP epithelial cells. A green fluorescent protein(GFP)-S100RVP fusion protein is present in both the cytoplasm and nucleus of transfected cells. A GST-S100RVP fusion protein bound calcium in vitro at levels similar to known S100 proteins. Furthermore, GFP-S100RVP transfected LNCaP and PC3 cells exhibited reduced sensitivity to calcium ionophore-induced cell death, but not to UV-induced cell death.

CONCLUSION

The results of this study argue for a role of S100RVP in calcium homeostasis in the prostate.

Calcium-dependent tetramer formation of S100A8 and S100A9 is essential for biological activity

S100 proteins comprise the largest family of calcium-binding proteins. Members of this family usually form homo- or heterodimers, which may associate to higher-order oligomers in a calcium-dependent manner. The heterodimers of S100A8 and S100A9 represent the major calcium-binding proteins in phagocytes. Both proteins regulate migration of these cells via modulation of tubulin polymerization. Calcium binding induces formation of (S100A8/S100A9)2 tetramers. The functional relevance of these higher-order oligomers of S100 proteins, however, is not yet clear. To investigate the importance of higher-order oligomerization for S100 proteins, we created a set of mutations within S100A9 (N69A, E78A, N69A+E78A) destroying the high-affinity C-terminal calcium-binding site (EF-hand II). Mutations in EF-hand II did not interfere with formation of the S100A8/S100A9 heterodimer as demonstrated by yeast two-hybrid experiments and pull-down assays. In contrast, mass spectrometric analysis and density gradient centrifugation revealed that calcium-induced association of (S100A8/S100A9)2 tetramers was strictly dependent on a functional EF-hand II in S100A9. Failure of tetramer formation was associated with a lack of functional activity of S100A8/S100A9 complexes in promoting the formation of microtubules. Thus, our data demonstrate that calcium-dependent formation of (S100A8/S100A9)2 tetramers is an essential prerequisite for biological function. This is the first report showing a functional relevance of calcium-induced higher-order oligomerization in the S100 family.

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.

Heterodimeric interaction and interfaces of S100A1 and S100P

With the widespread use of yeast two-hybrid systems, many heterodimeric forms of S100 proteins have been found, although their biological significance is unknown. In the present study, S100A1 was found to interact with another S100 protein, S100P, by using the yeast two-hybrid system. The binding parameters of the interaction were obtained using an optical biosensor and show that S100P has a slightly higher affinity for S100A1 (K(d)=10-20 nM) when compared with that for self-association (K(d)=40-120 nM). The physical interaction of S100A1 and S100P was also demonstrated in living mammalian cells using a fluorescence resonance energy transfer technique. Preincubation of recombinant S100P with S100A1, before the biosensor assay, reduced by up to 50% the binding of S100P to a recombinant C-terminal fragment of non-muscle myosin A, one of its target molecules. Site-specific mutations of S100P and S100A1, combined with homology modelling of an S100P/S100A1 heterodimer using known S100P and S100A1 structures, allowed the hydrophobic interactions at the dimeric interface of the heterodimer to be defined and provide an explanation for the heterodimerization of S100P and S100A1 at the molecular level. These results have revealed the similarities and the differences between the S100P homodimer and the S100A1/S100P heterodimer.

Mechanism of thermostabilization in a designed cold shock protein with optimized surface electrostatic interactions

Using computational and sequence analysis of bacterial cold shock proteins, we designed a protein (CspB-TB) that has the core residues of mesophilic protein from Bacillus subtilis(CspB-Bs) and altered distribution of surface charged residues. This designed protein was characterized by circular dichroism spectroscopy, and found to have secondary and tertiary structure similar to that of CspB-Bs. The activity of the CspB-TB protein as measured by the affinity to a single-stranded DNA (ssDNA) template at 25 degrees C is somewhat higher than that of CspB-Bs. Furthermore, the decrease in the apparent binding constant to ssDNA upon increase in temperature is much more pronounced for CspB-Bs than for CspB-TB. Temperature-induced unfolding (as monitored by differential scanning calorimetry and circular dichroism spectroscopy) and urea-induced unfolding experiments were used to compare the stabilities of CspB-Bs and CspB-TB. It was found that CspB-TB is approximately 20 degrees C more thermostable than CspB-Bs. The thermostabilization of CspB-TB relative to CspB-Bs is achieved by decrease in the enthalpy and entropy of unfolding without affecting their temperature dependencies, i.e. these proteins have similar heat capacity changes upon unfolding. These changes in the thermodynamic parameters result in the global stability function, i.e. Gibbs energy, deltaG(T), that is shifted to higher temperatures with only small changes in the maximum stability. Such a mechanism of thermostabilization, although predicted from the basic thermodynamic considerations, has never been identified experimentally.

S100P stimulates cell proliferation and survival via receptor for activated glycation end products (RAGE)

S100P is a member of the S100 protein family that is expressed in several malignant neoplasms. Currently the effects of this molecule on cell function are unknown. In the present study we investigated the biological effects and mechanisms of action of S100P using NIH3T3 cells. Expression of S100P in NIH3T3 cells led to the presence of S100P in the culture medium, increased cellular proliferation, and enhanced survival after detachment from the culture substrate or after exposure to the chemotherapeutic agent 5-flurouracil. The proliferation and survival effects of S100P expression were duplicated in a time- and concentration-dependent manner by the extracellular addition of purified S100P to wild-type NIH3T3 cells and correlated with the activation of extracellular-regulated kinases (Erks) and NF-kappaB. To determine the mechanisms involved in these effects, we tested the hypothesis that S100P activated RAGE (receptor for activated glycation end products). We found that S100P co-immunoprecipitated with RAGE. Furthermore, the effects of S100P on cell signaling, proliferation, and survival were blocked by agents that interfere with RAGE including administration of an amphoterin-derived peptide known to antagonize RAGE activation, anti-RAGE antibodies, and by expression of a dominant negative RAGE. These data suggest that S100P can act in an autocrine manner via RAGE to stimulate cell proliferation and survival.

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.

Molecular mechanisms of S100-target protein interactions

S100 proteins have no known enzymatic activity and exert their intracellular effects via interaction with and regulation of the activity of other proteins, termed target proteins, in both a Ca(2+)-dependent and Ca(2+)-independent manner. Structural studies have identified the linker region between the two EF-hand Ca(2+) binding domains and the C-terminus as Ca(2+)-dependent target protein binding sites in several S100 family members. In fact, C-terminal aromatic residues are obligatory for interaction of S100A1 with several of its Ca(2+)-dependent target proteins. Pharmacological studies suggest the presence of additional Ca(2+)-dependent binding motifs on some family members. A minimum of seven family members interact with and regulate the activity of aldolase A in a Ca(2+)-independent manner. In the case of S100A1, Ca(2+)-independent target protein interactions utilize a binding motif distinct from the C-terminal Ca(2+)-dependent target protein binding site. Several studies suggest that ionic interactions participate in the interaction of S100 family members with Ca(2+)-independent target proteins. While some target proteins are activated by multiple family members, other target proteins exhibit family member-specific activation, i.e., they are activated by a single family member. As predicted, family member specific interactions appear to be mediated by regions that exhibit the most divergence in amino acid sequence among family members, the linker or "hinge" region and the C terminus. Further specificity in S100-target protein interactions may arise from the different biochemical/biophysical properties of the individual family members, including affinity for metal ions (Ca(2+), Zn(2+), and Cu(2+)), oligomerization properties, heterodimerization, post-translational modifications, and lipid-binding. Delineation of the structural motifs that mediate S100-target protein interactions and determination of the in vivo relevance of these interactions are needed to fully understand the role of S100 proteins in normal and diseased cells.

Monitoring of S100 homodimerization and heterodimeric interactions by the yeast two-hybrid system

The S100 family consists of 19 members, which function as transducers of calcium signals in a tissue-specific manner. Upon calcium binding, the conformation of many S100 proteins changes dramatically. Several hydrophobic residues are exposed, allowing the S100 proteins to interact with their target proteins, and thereby to transduce calcium signals into specific biological responses. To further elucidate the exact contribution of the S100 calciproteins in the calcium signalling pathways, several groups have applied the yeast two-hybrid technology to identify putative target proteins for the various S100 calciproteins. Two-hybrid large screens using S100 proteins as baits have confirmed the biochemical and structural feature of S100, which enable them to form homodimers and the ability of some members to form specific heterodimers in vivo. Yeast two-hybrid investigations have allowed the identification of conserved hydrophobic residues and domains that are crucial for the stabilization of S100 homo- and heterodimers. Furthermore, this method clearly underlines that the homo- and heterodimerization mechanisms differ among the members of the S100 family. However, several lines of evidence strongly suggest that two-hybrid methodology is limited to the analysis of interactions that are calcium-independent, since no target proteins other than S100 family members themselves have been detected with this methodology.

Intracellular and extracellular roles of S100 proteins

S100, a multigenic family of non-ubiquitous Ca(2+)-modulated proteins of the EF-hand type expressed in vertebrates exclusively, has been implicated in intracellular and extracellular regulatory activities. Members of this protein family have been shown to interact with several effector proteins within cells thereby regulating enzyme activities, the dynamics of cytoskeleton constituents, cell growth and differentiation, and Ca(2+) homeostasis. Structural information indicates that most of S100 proteins exist in the form of antiparallelly packed homodimers (in some cases heterodimers), capable of functionally crossbridging two homologous or heterologous target proteins in a Ca(2+)-dependent (and, in some instances, Ca(2+)-independent) manner. In addition, extracellular roles have been described for several S100 members, although secretion (via an unknown mechanism) has been documented for a few of them. Extracellular S100 proteins have been shown to exert regulatory effects on inflammatory cells, neurons, astrocytes, microglia, and endothelial and epithelial cells, and a cell surface receptor, RAGE, has been identified as a potential S100A12 and S100B receptor transducing the effects of these two proteins on inflammatory cells and neurons. Other cell surface molecules with ability to interact with S100 members have been identified, suggesting that RAGE might not be a universal S100 protein receptor and/or that a single S100 protein might interact with more than one receptor. Collectively, these data indicate that members of the S100 protein family are multifunctional proteins implicated in the regulation of a variety of cellular activities.

Myeloid cell function in MRP-14 (S100A9) null mice

Myeloid-related protein 14 (MRP-14) and its heterodimeric partner, MRP-8, are cytosolic calcium-binding proteins, highly expressed in neutrophils and monocytes. To understand the function of MRP-14, we performed targeted disruption of the MRP-14 gene in mice. MRP-14(-/-) mice showed no obvious phenotype and were fertile. MRP-8 mRNA but not protein is present in the myeloid cells of these mice, suggesting that the stability of MRP-8 protein is dependent on MRP-14 expression. A compensatory increase in other proteins was not detected in cells lacking MRP-8 and MRP-14. Although the morphology of MRP-14(-/-) myeloid cells was not altered, they were significantly less dense. When Ca(2+) responses were investigated, there was no change in the maximal response to the chemokine MIP-2. At lower concentrations, however, there was reduced responsiveness in MRP-14(-/-) compared with MRP-14(+/+) neutrophils. This alteration in the ability to flux Ca(2+) did not impair the ability of the MRP-14(-/-) neutrophils to respond chemotactically to MIP-2. In addition, the myeloid cell functions of phagocytosis, superoxide burst, and apoptosis were unaffected in MRP-14(-/-) cells. In an in vivo model of peritonitis, MRP-14(-/-) mice showed no difference from wild-type mice in induced inflammatory response. The data indicate that MRP-14 and MRP-8 are dispensable for many myeloid cell functions.

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

S100P is a member of the S100 subfamily of calcium-binding proteins that are believed to be associated with various diseases, and in particular deregulation of S100P expression has been documented for prostate and breast cancer. Previously, we characterized the effects of metal binding on the conformational properties of S100P and proposed that S100P could function as a Ca2+ conformational switch. In this study we used fluorescence and CD spectroscopies and isothermal titration calorimetry to characterize the target-recognition properties of S100P using a model peptide, melittin. Based on these experimental data we show that S100P and melittin can interact in a Ca2+-dependent and -independent manner. Ca2+-independent binding occurs with low affinity (Kd approximately 0.2 mM), has a stoichiometry of four melittin molecules per S100P dimer and is presumably driven by favorable electrostatic interactions between the acidic protein and the basic peptide. In contrast, Ca2+-dependent binding of melittin to S100P occurs with high affinity (Kd approximately 5 microM) has a stoichiometry of two molecules of melittin per S100P dimer, appears to have positive cooperativity, and is driven by hydrophobic interactions. Furthermore, Ca2+-dependent S100P-melittin complex formation is accompanied by significant conformational changes: Melittin, otherwise unstructured in solution, adopts a helical conformation upon interaction with Ca2+-S100P. These results support a model for the Ca2+-dependent conformational switch in S100P for functional target recognition.

Molecular cloning and characterization of the human S100A14 gene encoding a novel member of the S100 family

S100 proteins form a growing subfamily of proteins related by Ca2+-binding motifs to the Efhand Ca2+-binding protein superfamily. By analyzing a human lung cancer cell line subtraction cDNA library, we have identified and characterized a new member of the human S100 family that we named S100A14 (GenBank acc. no. NM_020672). It encodes a mRNA present in several normal human tissues of epithelial origin, with the highest level of expression in colon. The full-length cDNA is 1067 nt in length, with a coding region predicting a protein of 104 amino acids that is 68% homologous to the S100A13 protein. The deduced amino acid sequence of the human S100A14 and its mouse homolog (identified as GenBank entry) contains two EF-hand Ca2+-binding domains, a myristoylation motif, a glycosylation site, and several potential protein kinase phosphorylation sites. We have mapped this gene to human chromosome 1q21, within a region where at least 15 other S100 genes are tightly clustered. A 3.2-kb genomic fragment containing the entire S100A14 was cloned and sequenced. The gene is split into four exons and three introns spanning a total of 2165 bp of genomic sequence. We examined the intracellular distribution of the epitope-tagged S100A14 protein in two human lung carcinoma cell lines and one immortalized monkey cell line. Pronounced staining was observed in the cytoplasm, suggesting an association with the plasma membrane and in the perinuclear area. We also provide evidence for heterogenic expression of S100A14 in tumors, demonstrating its overexpression in ovary, breast, and uterus tumors and underexpression in kidney, rectum, and colon tumors, a pattern suggesting distinct regulation with potentially important functions in malignant transformation.