Capping protein binding to S100B: implications for the tentacle model for capping the actin filament barbed end

S100B binds tightly to a 12-amino acid peptide derived from heterodimeric capping protein. In native intact capping protein, this sequence is in the C terminus of the alpha-subunit, which is important for capping the actin filament. This C-terminal region is proposed to act as a flexible "tentacle," extending away from the body of capping protein in order to bind actin. To this hypothesis, we analyzed the interaction between S100B and capping protein in solution. The C-terminal 28 amino acids of the alpha-subunit, the proposed tentacle, bound to S100B as a free synthetic peptide or a glutathione S-transferase fusion (K(d) approximately 0.4-1 microm). In contrast, S100B did not bind to whole native capping protein or functionally affect its capping activity. S100B does not bind, with any significant affinity, to the proposed alpha-tentacle sequence of whole native capping protein in solution. In the NMR structure of S100B complexed with the alpha-subunit-derived 12-amino acid peptide, the hydrophobic side of a short alpha-helix in the peptide, containing an important tryptophan residue, contacts S100B. In the x-ray structure of native capping protein, the corresponding sequence of the alpha-subunit C terminus, including Trp(271), interacts closely with the body of the protein. Therefore, our results suggest the alpha-subunit C terminus is not mobile as predicted by the tentacle model. Addition of non-ionic detergent allowed whole capping protein to bind weakly to S100B, indicating that the alpha-subunit C terminus can be mobilized from the surface of the capping protein molecule, presumably by weakening the hydrophobic binding at the contact site.

Functional significance of metastasis-inducing S100A4(Mts1) in tumor-stroma interplay

Causal implication of S100A4 in inducing metastases was convincingly shown previously. However, the mechanisms that associate S100A4 with tumor progression are not well understood. S100A4 protein, as a typical member of the S100 family, exhibits dual, intracellular and extracellular, functions. This work is focused on the extracellular function of S100A4, in particular its involvement in tumor-stroma interplay in VMR (mouse adenocarcinoma cell line) tumor cells, which exhibit stroma-dependent metastatic phenotype. We demonstrated the reciprocal influence of tumor and stroma cells where tumor cells stimulate S100A4 secretion from fibroblasts in culture. In turn, extracellular S100A4 modifies the cytoskeleton and focal adhesions and triggers several other events in tumor cells. We found stabilization of the tumor suppressor protein p53 and modulation of its function. In particular, extracellular S100A4 down-regulates the pro-apoptotic bax and the angiogenesis inhibitor thrombospondin-1 genes. For the first time, we demonstrate here that the S100A4 protein added to the extracellular space strongly stimulates proteolytic activity of VMR cells. This activity most probably is associated with matrix metalloproteinases and, in particular, with matrix metalloproteinase-13. Finally, the application of the recombinant S100A4 protein confers stroma-independent metastatic phenotype on VMR tumor cells. In conclusion, our results indicate that metastasis-inducing S100A4 protein plays a pivotal role in the tumor-stroma environment. S100A4 released either by tumor or stroma cells triggers pro-metastatic cascades in tumor cells.

Mts1 regulates the assembly of nonmuscle myosin-IIA

The formation of myosin-II filaments is fundamental to contractile and motile processes in nonmuscle cells, and elucidating the mechanisms controlling filament assembly is essential for understanding how myosin-II rapidly responds to changing conditions within the cell. Several proteins including KRP and a novel 38 kDa protein (1, 2) have been shown to modulate filament assembly through the stabilization of myosin-II assemblies. In contrast, we demonstrate that mts1, a member of the Ca(2+)-regulated S100 family of proteins, may regulate the monomeric, unassembled state in an isoform-specific manner. Biochemical analyses demonstrate that mts1 has a 9-fold higher affinity for myosin-IIA filaments than for myosin-IIB filaments. At stoichiometric levels, mts1 inhibits the assembly of myosin-IIA monomers into filaments and promotes the disassembly of myosin-IIA filaments into monomers; however, mts1 has little effect on the assembly properties of myosin-IIB. Using a solution based-assay, we have demonstrated that mts1 binds to residues 1909-1924 of the myosin-IIA heavy chain, which is near the C-terminal tip of the alpha-helical coiled-coil. The observation that mts1 binds a linear sequence of approximately 16 amino acids is consistent with other S100 family members, which bind linear sequences of 13-22 residues in their protein targets. In addition, mts1 increases the critical monomer concentration for myosin-IIA filament assembly by approximately 11-fold. Kinetic assembly assays indicate that the elongation rate and the extent of polymerization depend on the initial myosin-IIA concentration; however, mts1 had only a small affect on the half-time for assembly and predominately affected the extent of myosin IIA polymerization. Altogether, these observations are consistent with mts1 regulating myosin IIA assembly by monomer sequestration and suggest that mts1 regulates cell shape and motility through the modulation of myosin-IIA function.

Identification of binding domains in the sodium channel Na(V)1.8 intracellular N-terminal region and annexin II light chain p11

The interaction of p11 (annexin II light chain) with the N-terminal domain of Na(V)1.8, a tetrodotoxin-resistant sodium channel, is essential for the functional expression of the channel. Here we show that p11 binds to Na(V)1.8 but not to sodium channel isoforms Na(V)1.2, 1.5, 1.7 or Na(V)1.9. The binding of amino acids 74-103 of Na(V)1.8 to p11 residues 33-78 occurs in a random coiled region flanked by two EF hand motifs whose crystal structure has been established. As Na(V)1.8 channel expression is associated with pain pathways, drugs that disrupt the Na(V)1.8-p11 interaction and down-regulate channel expression may have analgesic activity.

S100B in Schizophrenic Psychosis

Recent findings have strengthened the hypothesis that a dysfunction of neuronal synapses and dendrites is relevant for the pathogenesis of schizophrenia. It might be present during neurodevelopment as well as in degenerative and regenerative processes of the mature brain. S1OOB, a small, Ca2+-binding, astrocytic protein, plays an important role in modulating the proliferation and differentiation of neurons and glia cells. It is involved in the regulation of cellular energy metabolism and interacts with many immunological functions of the brain. This review addresses findings from cell cultural and animal experiments potentially pertinent for the pathogenesis of schizophrenic psychoses. Morphological and functional data are analyzed and clinical studies reporting alterations of S1OOB concentrations in schizophrenic patients are reviewed. Evidence and limitations of the available studies are pointed out and promising future research strategies are outlined.

Location of the Zn2+-Binding Site on S100B As Determined by NMR Spectroscopy and Site-Directed Mutagenesis

In addition to binding Ca(2+), the S100 protein S100B binds Zn(2+) with relatively high affinity as confirmed using isothermal titration calorimetry (ITC; K(d) = 94 +/- 17 nM). The Zn(2+)-binding site on Ca(2+)-bound S100B was examined further using NMR spectroscopy and site-directed mutagenesis. Specifically, ITC measurements of S100B mutants (helix 1, H15A and H25A; helix 4, C84A, H85A, and H90A) were found to bind Zn(2+) with lower affinity than wild-type S100B (from 2- to >25-fold). Thus, His-15, His-25, Cys-84, His-85, and perhaps His-90 of S100B are involved in coordinating Zn(2+), which was confirmed by NMR spectroscopy. Previous studies indicate that the binding of Zn(2+) enhances calcium and target protein-binding affinities, which may contribute to its biological function. Thus, chemical shift perturbations observed here for residues in both EF-hand domains of S100B during Zn(2+) titrations could be detecting structural changes in the Ca(2+)-binding domains of S100B that are pertinent to its increase in Ca(2+)-binding affinity in the presence of Zn(2+). Furthermore, Zn(2+) binding causes helix 4 to extend by one full turn when compared to Ca(2+)-bound S100B. This change in secondary structure likely contributes to the increased binding affinity that S100B has for target peptides (i.e., TRTK peptide) in the presence of Zn(2+).

Trichohyalin mechanically strengthens the hair follicle: multiple cross-bridging roles in the inner root shealth

Trichohyalin is expressed in specialized epithelia that are unusually mechanically strong, such as the inner root sheath cells of the hair follicle. We have previously shown that trichohyalin is sequentially subjected to post-synthetic modifications by peptidylarginine deaminases, which convert many of its arginines to citrullines, and by transglutaminases, which introduce intra- and interprotein chain cross-links. Here we have characterized in detail the proteins to which it becomes cross-linked in vivo in the inner root sheath of the mouse hair follicle. We suggest that it has three principal roles. First, it serves as an interfilamentous matrix protein by becoming cross-linked both to itself and to the head and tail end domains of the inner root sheath keratin intermediate filament chains. A new antibody reveals that arginines of the tail domains of the keratins are modified to citrullines before cross-linking, which clarifies previous studies. Second, trichohyalin serves as a cross-bridging reinforcement protein of the cornified cell envelope of the inner root sheath cells by becoming cross-linked to several known or novel barrier proteins, including involucrin, small proline-rich proteins, repetin, and epiplakin. Third, it coordinates linkage between the keratin filaments and cell envelope to form a seamless continuum. Together, our new data document that trichohyalin is a multi-functional cross-bridging protein that functions in the inner root sheath and perhaps in other specialized epithelial tissues by conferring to and coordinating mechanical strength between their peripheral cell envelope barrier structures and their cytoplasmic keratin filament networks.

Molecular cloning and characterization of alternatively spliced mRNA isoforms from psoriatic skin encoding a novel member of the S100 family

In an effort to identify psoriasis-associated genes, we compared gene expression in normal and psoriatic skin, using differential display RT-PCR technique. Sequence analysis of a 650-bp cDNA fragment (clone 110) that was highly up-regulated in lesional skin revealed homology to a noncoding cDNA (NICE-2). By subsequent cDNA cloning, using RNA from psoriatic skin, we have identified two alternatively spliced mRNA-isoforms (0.5 and 4.4 kb), which differ in composition of their untranslated regions. By sequence comparison, we have mapped the novel gene, named S100A15, to the S100 gene cluster within the epidermal differentiation complex (chromosome 1q21). Analysis of the deduced amino acid sequence revealed a protein of 101 amino acids containing two potential EF-hand motifs with high homology to the S100A7. Northern blot hybridization and semiquantitative RT-PCR analysis confirmed the S100A15 overexpression in psoriasis, showing different levels of expression of the S100A15 mRNA isoforms. In situ hybridization of the S100A15 revealed a markedly increased staining of basal and suprabasal epidermal layers of psoriatic skin compared with healthy tissue. Our data suggest an involvement of the novel S100A15 in epidermal differentiation and inflammation and might therefore be important for the pathogenesis of psoriasis and other diseases.

Characterization of the metastasis-associated protein, S100A4. Roles of calcium binding and dimerization in cellular localization and interaction with myosin

Elevated S100A4 protein expression is associated with metastatic tumor progression and appears to be a strong molecular marker for clinical prognosis. S100A4 is a calcium-binding protein that is known to form homodimers and interacts with several proteins in a calcium-dependent manner. Here we show that S100A4 localizes to lamellipodia structures in a migrating breast cancer-derived cell line and colocalizes with a known S100A4-interacting protein, myosin heavy chain IIA, at the leading edge. We demonstrate that S100A4 mutants that are defective in either their ability to dimerize or in calcium binding are unable to interact with myosin heavy chain IIA. An S100A4 mutant that is deficient for calcium binding retains the ability to form homodimers, suggesting that S100A4 can exist as calcium-free or calcium-bound dimers in vivo. However, a calcium-bound S100A4 monomer only interacts with another calcium-bound monomer and not with an S100A4 mutant that does not bind calcium. Interestingly, despite the calcium dependence for interaction with known protein partners, calcium binding is not necessary for localization to lamellipodia. Both wild type and a mutant that is deficient for calcium binding colocalize with known markers of actively forming leading edges of lamellipodia, Arp3 and neuronal Wiskott-Aldrich syndrome protein. These data suggest that S100A4 localizes to the leading edge in a calcium-independent manner, and identification of the proteins that are involved in localizing S100A4 to the lamellipodial structures may provide novel insight into the mechanism by which S100A4 regulates metastasis.

Calcium-regulated interaction of Sgt1 with S100A6 (calcyclin) and other S100 proteins

S100A6 (calcyclin), a small calcium-binding protein from the S100 family, interacts with several target proteins in a calcium-regulated manner. One target is Calcyclin-Binding Protein/Siah-1-Interacting Protein (CacyBP/SIP), a component of a novel pathway of beta-catenin ubiquitination. A recently discovered yeast homolog of CacyBP/SIP, Sgt1, associates with Skp1 and regulates its function in the Skp1/Cullin1/F-box complex ubiquitin ligase and in kinetochore complexes. S100A6-binding domain of CacyBP/SIP is in its C-terminal region, where the homology between CacyBP/SIP and Sgt1 is the greatest. Therefore, we hypothesized that Sgt1, through its C-terminal region, interacts with S100A6. We tested this hypothesis by performing affinity chromatography and chemical cross-linking experiments. Our results showed that Sgt1 binds to S100A6 in a calcium-regulated manner and that the S100A6-binding domain in Sgt1 is comprised of 71 C-terminal residues. Moreover, S100A6 does not influence Skp1-Sgt1 binding, a result suggesting that separate Sgt1 domains are responsible for interactions with S100A6 and Skp1. Sgt1 binds not only to S100A6 but also to S100B and S100P, other members of the S100 family. The interaction between S100A6 and Sgt1 is likely to be physiologically relevant because both proteins were co-immunoprecipitated from HEp-2 cell line extract using monoclonal anti-S100A6 antibody. Phosphorylation of the S100A6-binding domain of Sgt1 by casein kinase II was inhibited by S100A6, a result suggesting that the role of S100A6 binding is to regulate the phosphorylation of Sgt1. These findings suggest that protein ubiquitination via Sgt1-dependent pathway can be regulated by S100 proteins.

Unmasking the annexin I interaction from the structure of Apo-S100A11

S100A11 is a homodimeric EF-hand calcium binding protein that undergoes a calcium-induced conformational change and interacts with the phospholipid binding protein annexin I to coordinate membrane association. In this work, the solution structure of apo-S100A11 has been determined by NMR spectroscopy to uncover the details of its calcium-induced structural change. Apo-S100A11 forms a tight globular structure having a near antiparallel orientation of helices III and IV in calcium binding site II. Further, helices I and IV, and I and I', form a more closed arrangement than observed in other apo-S100 proteins. This helix arrangement in apo-S100A11 partially buries residues in helices I (P3, E11, A15), III (V55, R58, M59), and IV (A86, C87, S90) and the linker (A45, F46), which are required for interaction with annexin I in the calcium-bound state. In apo-S100A11, this results in a "masked" binding surface that prevents annexin I binding but is uncovered upon calcium binding.

Structure of the human S100A12-copper complex: implications for host-parasite defence

S100A12 is a member of the S100 family of EF-hand calcium-modulated proteins. Together with S100A8 and S100A9, it belongs to the calgranulin subfamily, i.e. it is mainly expressed in granulocytes, although there is an increasing body of evidence of expression in keratinocytes and psoriatic lesions. As well as being linked to inflammation, allergy and neuritogenesis, S100A12 is involved in host-parasite response, as are the other two calgranulins. Recent data suggest that the function of the S100-family proteins is modulated not only by calcium, but also by other metals such as zinc and copper. Previously, the structure of human S100A12 in low-calcium and high-calcium structural forms, crystallized in space groups R3 and P2(1), respectively, has been reported. Here, the structure of S100A12 in complex with copper (space group P2(1)2(1)2; unit-cell parameters a = 70.6, b = 119.0, c = 90.2 A) refined at 2.19 A resolution is reported. Comparison of anomalous difference electron-density maps calculated with data collected with radiation of wavelengths 1.37 and 1.65 A shows that each monomer binds a single copper ion. The copper binds at an equivalent site to that at which another S100 protein, S100A7, binds zinc. The results suggest that copper binding may be essential for the functional role of S100A12 and probably the other calgranulins in the early immune response.

Catecholase activity associated with copper-S100B

This study addresses the spectroscopic properties and reactivity associated with the copper-loaded form of S100B isolated from bovine brain. Copper(II)-S100B displays EPR features typical of a type II copper center and is shown here to exhibit catecholase activity, the two-electron oxidation of catechols. The steady-state kinetics associated with the oxidation of several catecholamines has been probed in order to further characterize this activity. The evidence provided indicates that the catecholase chemistry is copper initiated. Superoxide dismutase has no effect on the rates of catecholamine oxidation catalyzed by Cu-S100B, establishing that superoxide is not produced during this reaction, ruling out an autoxidative mechanism. Addition of catalase to the Cu-S100B reaction with catechols reduces the amount of oxygen consumed by 50%, demonstrating that peroxide is released during this reaction. The release of peroxide is mechanistically distinct from the type III dinuclear copper proteins, catechol oxidase and tyrosinase.

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.

Multiple structural states of S100A12: A key to its functional diversity

S100A12 is a member of the S100 family of EF-hand calcium-binding proteins. Together with two other calgranulins, S100A8 and S100A9, it is mostly expressed in human granulocytes, although there is increasing evidence of expression in keratinocytes and psoriatic lesions. It is involved in host-parasite response, and linked to corneal autoimmune diseases connected with filarial parasite infestation. Interaction of S100A12 with a multiligand receptor for advanced glycation end products (RAGE) mediates inflammation. Human recombinant S100A12 was found to induce neuritogenesis of cultured hippocampal cells, similar to two other S100 proteins, S100B and S100A4. X-ray structure of S100A12 has been solved in two crystal forms: R3 and P2(1). In the R3 crystal form S100A12 is a dimer, and in the P2(1) crystal form the dimers are arranged as a hexamer. The hexameric form suggests its role in receptor oligomerisation. S100A12 binds copper at the predicted zinc/copper binding site, which is located close to the surface of the protein. We propose copper-mediated generation of reactive oxygen species by S100A12 as its function in host-parasite response.

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.

Frog p26olf, a molecule with two S100-like regions in a single peptide

An S100-like calcium-binding protein, p26olf, was originally isolated from the frog (Rana Catesbeiana) olfactory epithelium with four chromatographical steps. The primary structure of p26olf contains two S100-like regions aligned in tandem with four functional EF-hands. At 100 mM K(+), wild-type p26olf binds Ca(2+) with a Kd value of 22 microM and a Hill coefficient of 2.0. Each EF-hand seems to have different affinity for Ca(2+): it is high in EF-A and -B and low in EF-C and -D. In our Ca(2+)-binding model, the order of Ca(2+)-binding to p26olf is EF-B, EF-A, EF-C, and EF-D. Expression of mRNA of p26olf is detected in various frog tissues: it is high in the olfactory epithelium, lung, and spleen, moderate in brain, retina, heart, and kidney, and low in liver and muscle. Immunohistochemical studies revealed that p26olf is prominently localized in the cilia of both olfactory and lung respiratory epithelium and especially enriched in the distal segment of the olfactory cilia. Several proteins in the olfactory cilia bind to p26olf in the presence of Ca(2+), suggesting that they are possible target proteins of p26olf. One of these target proteins is immunologically identified as a beta-adrenergic receptor kinase-like protein. In the olfactory cilia, p26olf may have some roles in the olfactory transduction or adaptation through interaction with this beta-adrenergic receptor kinase-like protein.

A novel S100 target conformation is revealed by the solution structure of the Ca2+-S100B-TRTK-12 complex

The Alzheimer-linked neural protein S100B is a signaling molecule shown to control the assembly of intermediate filament proteins in a calcium-sensitive manner. Upon binding calcium, a conformational change occurs in S100B exposing a hydrophobic surface for target protein interactions. The synthetic peptide TRTK-12 (TRTKIDWNKILS), derived from random bacteriophage library screening, bears sequence similarity to several intermediate filament proteins and has the highest calcium-dependent affinity of any target molecule for S100B to date (K(d) <1 microm). In this work, the three-dimensional structure of the Ca(2+)-S100B-TRTK-12 complex has been determined by NMR spectroscopy. The structure reveals an extended, contiguous hydrophobic surface is formed on Ca(2+)-S100B for target interaction. The TRTK-12 peptide adopts a coiled structure that fits into a portion of this surface, anchored at Trp(7), and interacts with multiple hydrophobic contacts in helices III and IV of Ca(2+)-S100B. This interaction is strikingly different from the alpha-helical structures found for other S100 target peptides. By using the TRTK-12 interaction as a guide, in combination with other available S100 target structures, a recognition site on helix I is identified that may act in concert with the TRTK-12-binding site from helices III and IV. This would provide a larger, more complex site to interact with full-length target proteins and would account for the promiscuity observed for S100B target protein interactions.

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.

Human milk contains S100B protein

The present study constitutes the first finding of the calcium-binding protein S100B and of its mRNA in human milk, as revealed by a quantitative immunoluminometric assay, by Western blot analysis and by reverse transcription-polymerase chain reaction (RT-PCR) assay followed by restriction enzyme digestion. The concentration of S100B in milk is markedly higher than that observed in other biological fluids such as cord blood, peripheral blood, urine, cerebrospinal fluid and amniotic fluid. This finding could be related to a possible trophic role, which has been hypothesized for the protein.

The Janus face of glial-derived S100B: beneficial and detrimental functions in the brain

Glia play active and vital roles in the CNS, including regulation of brain development, maintenance of cellular homeostasis and reparative responses to acute and chronic neurologic insults. However, in pathological conditions, glial function may be dysregulated resulting in enhanced neuroinflammation and further neurologic injury. The fundamental concept that aberrant regulation of a normally beneficial process may instead result in pathology is exemplified by the properties of the glial-derived protein, S100B. In the developing brain and following acute glial activation in response to injury, S100B acts as a neurotrophic factor and neuronal survival protein. In contrast, overproduction of S100B by activated glia can lead to exacerbation of neuroinflammation and neuronal dysfunction. This duality of S100B actions, both reparative and degenerative, is consistent with the compelling clinical evidence of the association between increases in S100B levels and outcome following brain injury. S100B levels generally correlate with the severity of damage and may have predictive value for adverse neurological outcomes. The cumulative data support the potential of S100B as a biomarker for brain damage, implicate glial activation as a possible treatment target in acute and chronic CNS disorders, and highlight the dual role of glia in the reparative and pathologic responses to neurologic injury.

The zinc- and calcium-binding S100B interacts and co-localizes with IQGAP1 during dynamic rearrangement of cell membranes

The Zn(2+)- and Ca(2+)-binding S100B protein is implicated in multiple intracellular and extracellular regulatory events. In glial cells, a relationship exists between cytoplasmic S100B accumulation and cell morphological changes. We have identified the IQGAP1 protein as the major cytoplasmic S100B target protein in different rat and human glial cell lines in the presence of Zn(2+) and Ca(2+). Zn(2+) binding to S100B is sufficient to promote interaction with IQGAP1. IQ motifs on IQGAP1 represent the minimal interaction sites for S100B. We also provide evidence that, in human astrocytoma cell lines, S100B co-localizes with IQGAP1 at the polarized leading edge and areas of membrane ruffling and that both proteins relocate in a Ca(2+)-dependent manner within newly formed vesicle-like structures. Our data identify IQGAP1 as a potential target protein of S100B during processes of dynamic rearrangement of cell membrane morphology. They also reveal an additional cellular function for IQGAP1 associated with Zn(2+)/Ca(2+)-dependent relocation of S100B.