Modulation of tyrosine phosphorylation of p36 and other substrates by the S-100 protein

The biochemistry and regulation of S100A10: a multifunctional plasminogen receptor involved in oncogenesis

The plasminogen receptors mediate the production and localization to the cell surface of the broad spectrum proteinase, plasmin. S100A10 is a key regulator of cellular plasmin production and may account for as much as 50% of cellular plasmin generation. In parallel to plasminogen, the plasminogen-binding site on S100A10 is highly conserved from mammals to fish. S100A10 is constitutively expressed in many cells and is also induced by many diverse factors and physiological stimuli including dexamethasone, epidermal growth factor, transforming growth factor-α, interferon-γ, nerve growth factor, keratinocyte growth factor, retinoic acid, and thrombin. Therefore, S100A10 is utilized by cells to regulate plasmin proteolytic activity in response to a wide diversity of physiological stimuli. The expression of the oncogenes, PML-RARα and KRas, also stimulates the levels of S100A10, suggesting a role for S100A10 in pathophysiological processes such as in the oncogenic-mediated increases in plasmin production. The S100A10-null mouse model system has established the critical role that S100A10 plays as a regulator of fibrinolysis and oncogenesis. S100A10 plays two major roles in oncogenesis, first as a regulator of cancer cell invasion and metastasis and secondly as a regulator of the recruitment of tumor-associated cells, such as macrophages, to the tumor site.

Biological and methodological features of the measurement of S100B, a putative marker of brain injury

The S100B astroglial protein is widely used as a parameter of glial activation and/or death in several conditions of brain injury. Cerebrospinal fluid and serum S100B variations have been proposed to evaluate clinical outcomes in these situations. Here, we briefly broach some aspects, commonly not sufficiently valorized, concerning the biology and measurements of this protein. S100B has molecular targets and activities in and outside of astrocytes, and variations of intra and extracellular content are not necessarily coupled. We discuss the extracellular origin of this protein in brain tissue, as well as extracerebral sources of this protein in serum, comparing it with other available protein markers of brain damage. The superestimation of the heterodimer S100A1-B in the current clinical literature is also analyzed. We affirm that poor dualistic views that consider S100B elevation as "bad" or "good" simplify clinical practice and delay our comprehension of the role of this protein, both in physiological conditions and in brain disorders.

S100B in neuropathologic states: the CRP of the brain?

In recent years there has been a proliferation of interest in the brain-specific protein S100B, its many physiologic roles, and its behaviour in various neuropathologic conditions. Since the mid-1960s, its wide variety of intracellular and extracellular activities has been elucidated, and it has also been implicated in an increasing number of central nervous system (CNS) disorders. S100B is part of a superfamily of proteins, some of which (including S100B) have been implicated as calcium-dependent regulatory proteins that modulate the activity of effector proteins or cells. S100B is primarily an astrocytic protein. Within cells, it may have a role in signal transduction, and it is involved in calcium homeostasis. Information about the functional implication of S100B secretion by astrocytes into the extracellular space is scant but there is substantial evidence that secreted glial S100B exerts trophic or toxic effects depending on its concentration. This review summarises the historic development and current knowledge of S100B, including recent interesting findings relating S100B to a diversity of CNS pathologies such as traumatic brain injury, Alzheimer's disease, Down's syndrome, schizophrenia, and Tourette's syndrome. These broad implications have led some workers to describe S100B as 'the CRP (C-reactive protein) of the brain.' This review also examines S100B's potential role as a neurologic screening tool, or biomarker of CNS injury, analogous to the role of CRP as a marker of systemic inflammation.

Identification of intracellular target proteins of the calcium-signaling protein S100A12

In this report, we have focused our attention on identifying intracellular mammalian proteins that bind S100A12 in a Ca2+-dependent manner. Using S100A12 affinity chromatography, we have identified cytosolic NADP+-dependent isocitrate dehydrogenase (IDH), fructose-1,6-bisphosphate aldolase A (aldolase), glyceraldehyde-3-phosphate dehydrogenese (GAPDH), annexin V, S100A9, and S100A12 itself as S100A12-binding proteins. Immunoprecipitation experiments indicated the formation of stable complexes between S100A12 and IDH, aldolase, GAPDH, annexin V and S100A9 in vivo. Surface plasmon resonance analysis showed that the binding to S100A12, of S100A12, S100A9 and annexin V, was strictly Ca2+-dependent, whereas that of GAPDH and IDH was only weakly Ca2+-dependent. To localize the site of S100A12 interaction, we examined the binding of a series of C-terminal truncation mutants to the S100A12-immobilized sensor chip. The results indicated that the S100A12-binding site on S100A12 itself is located at the C-terminus (residues 87-92). However, cross-linking experiments with the truncation mutants indicated that residues 87-92 were not essential for S100A12 dimerization. Thus, the interaction between S100A12 and S100A9 or immobilized S100A12 should not be viewed as a typical S100 homo- or heterodimerization model. Ca2+-dependent affinity chromatography revealed that C-terminal residues 75-92 are not necessary for the interaction of S100A12 with IDH, aldolase, GAPDH and annexin V. To analyze the functional properties of S100A12, we studied its action in protein folding reactions in vitro. The thermal aggregation of IDH or GAPDH was facilitated by S100A12 in the absence of Ca2+, whereas in the presence of Ca2+ the protein suppressed the aggregation of aldolase to less than 50%. These results suggest that S100A12 may have a chaperone/antichaperone-like function which is Ca2+-dependent.

Calcyclin, a Ca2+ ion-binding protein, contributes to the anabolic effects of simvastatin on bone

In vitro treatment with a pharmacological dose of simvastatin, a potent pro-drug of a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor, stimulates bone formation. In our study, simvastatin stimulated differentiation of osteoblasts remarkably in a dose-dependent manner, with minimal effect on proliferation. To identify the mediators of the anabolic effects of simvastatin on osteoblasts, we tried to identify and characterize simvastatin-induced proteins by using proteomic analysis. Calcyclin was significantly up-regulated by more than 10 times, and annexin I was also up-regulated by simvastatin. However, annexin III, vimentin, and tropomyosin were down-regulated. Up-regulated calcyclin mRNA by simvastatin was validated by reverse transcription in mouse calvarial cells. In confocal microscope analysis, green fluorescence protein-calcyclin fusion protein was ubiquitously observed in the of MC3T3-E1 cells transfected with green fluorescence protein-calcyclin cDNA containing plasmid and was quickly concentrated in the nucleus 20 min after simvastatin treatment. Overexpression of calcyclin cDNA stimulated both the proliferation and expression of alkaline phosphatase mRNA significantly, without exposure to simvastatin in MC3T3-E1 cells. However, both the rate of proliferation of the osteoblasts and the expression of alkaline phosphatase mRNA were suppressed significantly 1 day after treatment with the calcyclin-specific small interference RNA, and furthermore, simvastatin did not overcome this suppression in the small interference RNA-pretreated MC3T3-E1 cells. In conclusion, calcyclin is one of the candidate proteins that plays a role in osteoblastogenesis in response to simvastatin, although the precise functions of calcyclin in osteoblast remain to be verified.

Association of hepatitis B virus polymerase with promyelocytic leukemia nuclear bodies mediated by the S100 family protein p11

Hepatitis B virus (HBV) polymerase (Pol) interacts with cellular chaperone proteins and thereby performs multiple functions necessary for viral replication. Yeast two-hybrid analysis was applied to identify additional cellular targets required for HBV Pol function. HBV Pol interacted with S100A10 (p11), a Ca(2+)-modulated protein previously shown to bind to annexin II. The interaction between HBV Pol and p11 was confirmed by co-immunoprecipitation of the two proteins synthesized either in vitro or in transfected cells and by inhibition of the DNA polymerase activity of HBV Pol by p11. Immunofluorescence analysis of transfected human cell lines revealed that, although most HBV Pol and p11 was restricted to the cytoplasm, a small proportion of each protein colocalized as nuclear speckles; HBV Pol was not detected in the nucleus in the absence of p11. The HBV Pol-p11 nuclear speckles coincided with nuclear bodies containing the promyelocytic leukemia protein PML. Furthermore, the association of HBV Pol-p11 with PML was increased by exposure of cells to EGTA and inhibited by valinomycin. These results suggest a role for p11 in modulation of HBV Pol function and implicate PML nuclear bodies and intracellular Ca(2+) in viral replication.

S100B in brain damage and neurodegeneration

S100B is a calcium-binding peptide produced mainly by astrocytes that exert paracrine and autocrine effects on neurons and glia. Some knowledge has been acquired from in vitro and in vivo animal experiments to understand S100B's roles in cellular energy metabolism, cytoskeleton modification, cell proliferation, and differentiation. Also, insights have been gained regarding the interaction between S100B and the cerebral immune system, and the regulation of S100B activity through serotonergic transmission. Secreted glial S100B exerts trophic or toxic effects depending on its concentration. At nanomolar concentrations, S100B stimulates neurite outgrowth and enhances survival of neurons during development. In contrast, micromolar levels of extracellular S100B in vitro stimulate the expression of proinflammatory cytokines and induce apoptosis. In animal studies, changes in the cerebral concentration of S100B cause behavioral disturbances and cognitive deficits. In humans, increased S100B has been detected with various clinical conditions. Brain trauma and ischemia is associated with increased S100B concentrations, probably due to the destruction of astrocytes. In neurodegenerative, inflammatory and psychiatric diseases, increased S100B levels may be caused by secreted S100B or release from damaged astrocytes. This review summarizes published findings on S100B regarding human brain damage and neurodegeneration. Findings from in vitro and in vivo animal experiments relevant for human neurodegenerative diseases and brain damage are reviewed together with the results of studies on traumatic, ischemic, and inflammatory brain damage as well as neurodegenerative and psychiatric disorders. Methodological problems are discussed and perspectives for future research are outlined.

S100: a multigenic family of calcium-modulated proteins of the EF-hand type with intracellular and extracellular functional roles

S100 is a multigenic family of non-ubiquitous Ca(2+)-modulated proteins of the EF-hand type expressed in vertebrates exclusively and implicated in intracellular and extracellular regulatory activities. Within cells, most of S100 members 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. S100 oligomers can also form, under the non-reducing conditions found in the extracellular space and/or within cells upon changes in the cell redox status. Within cells, S100 proteins have been implicated in the regulation of protein phosphorylation, some enzyme activities, the dynamics of cytoskeleton components, transcription factors, Ca(2+) homeostasis, and cell proliferation and differentiation. Certain S100 members are released into the extracellular space by an unknown mechanism. Extracellular S100 proteins stimulate neuronal survival and/or differentiation and astrocyte proliferation, cause neuronal death via apoptosis, and stimulate (in some cases) or inhibit (in other cases) the activity of inflammatory cells. A cell surface receptor, RAGE, has been identified on inflammatory cells and neurons for S100A12 and S100B, which transduces S100A12 and S100B effects. It is not known whether RAGE is a universal S100 receptor, S100 members interact with other cell surface receptors, or S100 protein interaction with other extracellular factors specifies the biological effects of a given S100 protein on a target cell. The variety of intracellular target proteins of S100 proteins and, in some cases, of a single S100 protein, and the cell specificity of expression of certain S100 members suggest that these proteins might have a role in the fine regulation of effector proteins and/or specific steps of signaling pathways/cellular functions. Future analyses should discriminate between functionally relevant S100 interactions with target proteins and in vitro observations devoid of physiological importance.

S100A1 and S100B interactions with annexins

Members of the annexin protein family interact with members of the S100 protein family thereby forming heterotetramers in which an S100 homodimer crossbridges two copies of the pertinent annexin. Previous work has shown that S100A1 and S100B bind annexin VI in a Ca(2+)-dependent manner and that annexin VI, but not annexin V, blocks the inhibitory effect of S100A1 and S100B on intermediate filament assembly. We show here that both halves of annexin VI (i.e., the N-terminal half or annexin VI-a and the C-terminal half or annexin VI-b) bind individual S100s on unique sites and that annexin VI-b, but not annexin VI-a, blocks the ability of S100A1 and S100B to inhibit intermediate filament assembly. We also show that the C-terminal extension of S100A1 (and, by analogy, S100B), that was previously demonstrated to be critical for S100A1 and S100B binding to several target proteins including intermediate filament subunits, is not part of the S100 surface implicated in the recognition of annexin VI, annexin VI-a, or annexin VI-b. Evaluation of functional properties with a liposome stability and a calcium influx assay reveals the ability of both S100 proteins to permeabilize the membrane bilayer in a similar fashion like annexins. When tested in combinations with different annexin proteins both S100 proteins mostly lead to a decrease in the calcium influx activity although not all annexin/S100 combinations behave in the same manner. Latter observation supports the hypothesis that the S100-annexin interactions differ mechanistically depending on the particular protein partners.

Functional roles of S100 proteins, calcium-binding proteins of the EF-hand type

A multigenic family of Ca2+-binding proteins of the EF-hand type known as S100 comprises 19 members that are differentially expressed in a large number of cell types. Members of this protein family have been implicated in the Ca2+-dependent (and, in some cases, Zn2+- or Cu2+-dependent) regulation of a variety of intracellular activities such as protein phosphorylation, enzyme activities, cell proliferation (including neoplastic transformation) and differentiation, the dynamics of cytoskeleton constituents, the structural organization of membranes, intracellular Ca2+ homeostasis, inflammation, and in protection from oxidative cell damage. Some S100 members are released or secreted into the extracellular space and exert trophic or toxic effects depending on their concentration, act as chemoattractants for leukocytes, modulate cell proliferation, or regulate macrophage activation. Structural data suggest that many S100 members exist within cells as dimers in which the two monomers are related by a two-fold axis of rotation and that Ca2+ binding induces in individual monomers the exposure of a binding surface with which S100 dimers are believed to interact with their target proteins. Thus, any S100 dimer is suggested to expose two binding surfaces on opposite sides, which renders homodimeric S100 proteins ideal for crossbridging two homologous or heterologous target proteins. Although in some cases different S100 proteins share their target proteins, in most cases a high degree of target specificity has been described, suggesting that individual S100 members might be implicated in the regulation of specific activities. On the other hand, the relatively large number of target proteins identified for a single S100 protein might depend on the specific role played by the individual regions that in an S100 molecule contribute to the formation of the binding surface. The pleiotropic roles played by S100 members, the identification of S100 target proteins, the analysis of functional correlates of S100-target protein interactions, and the elucidation of the three-dimensional structure of some S100 members have greatly increased the interest in S100 proteins and our knowledge of S100 protein biology in the last few years. S100 proteins probably are an example of calcium-modulated, regulatory proteins that intervene in the fine tuning of a relatively large number of specific intracellular and (in the case of some members) extracellular activities. Systems, including knock-out animal models, should be now used with the aim of defining the correspondence between the in vitro regulatory role(s) attributed to individual members of this protein family and the in vivo function(s) of each S100 protein.

Annexin VI binds S100A1 and S100B and blocks the ability of S100A1 and S100B to inhibit desmin and GFAP assemblies into intermediate filaments

Annexin VI, a member of a family of Ca(2+)-dependent phospholipid- and membrane-binding proteins, interacts with the Ca(2+)-regulated EF-hand proteins, S100A1 and S100B, and blocks the ability of these two proteins to inhibit the assembly of desmin and glial fibrillary acidic protein (GFAP) into intermediate filaments in a Ca(2+)- and dose-dependent manner. S100A1 and S100B each possess one annexin VI binding site, characterized by an affinity for annexin VI in the submicromolar range. Binding of annexin VI to either S100 protein occurs at a site that appears to differ in some parts from that recognizing desmin and GFAP. As S100A1 and S100B exist in solution as homodimers in which the two monomers are related by a 2-fold symmetry axis, each of the above S100 homodimers likely crosslinks two annexin VI molecules, a situation that appears typical of all the annexin-S100 protein complexes described thus far. However, whereas in the cases of other annexin-S100 complexes the C-terminal extension of the S100 molecule appears indispensable for annexin binding, the annexin VI binding site cannot be restricted to the S100A1 and S100B C-terminal extension. We speculate that the annexin VI site on S100A1/B may only partially overlap to the desmin/GFAP site. In contrast, no effects of annexin V on the ability of S100A1 or S100B to affect the desmin and GFAP assemblies could be documented, although binding of annexin V to S100A1 and S100B could be detected at relatively high Ca2+ concentrations. The present data suggest that annexin VI might regulate S100A1 and S100B activities and vice versa.

Identification of a novel cis-acting element for fibroblast-specific transcription of the FSP1 gene

The FSP1 gene encodes a filament-binding S100 protein with paired EF hands that is specifically expressed in fibroblasts. This led us to look for cis-acting elements in the FSP1 promoter that might engage nuclear transcription factors unique to fibroblasts. The first exon of FSP1 is noncoding, therefore, a series of luciferase reporter minigenes were created containing varying lengths of 5'-flanking sequence, the first intron, and the noncoding region of the second exon. A position and promoter-dependent proximal element between -187 and -88 bp was shown to be active in fibroblasts but not in epithelium. Sequence in the first intron from +777 to +964 had an enhancing effect that was not cell type specific. Hsv TK reporter constructs driven by this promoter/intron cassette in transgenic mice were coexpressed appropriately with FSP1 in tissue fibroblasts. Gel mobility shift competitor assays identified a novel domain, FTS-1 (fibroblast transcription site-1; TTGAT from -177 to -173 bp), that specifically interacts with nuclear extracts from fibroblasts. The necessity of this binding site was confirmed by site-specific mutagenesis. Database searches also turned up putative FTS-1 sites in the early promoter regions of other fibroblast expressed proteins, including the alpha1 and alpha2(I), and alpha1(III) collagens and the alphaSM-actin gene. We hypothesize that the selective engagement of FTS-1 elements may contribute to the mesenchymal phenotype of fibroblasts and perhaps other dedifferentiated cells.

Early role of Fsp1 in epithelial-mesenchymal transformation

A seamless plasticity exists among cells shifting between epithelial and mesenchymal phenotypes during early development and again later, in adult tissues, following wound repair or organ remodeling in response to injury. Fsp1, a gene encoding a fibroblast-specific protein associated with mesenchymal cell morphology and motility, is expressed during epithelial-mesenchymal transformations (EMT) in vivo. In the current study, we identified several cytokines that induce Fsp1 in cultured epithelial cells. A combination of these factors, however, was most efficacious at completing the process of EMT. The optimal combination identified were two of the cytokines classically associated with fibrosis, i.e., transforming growth factor-beta1 (TGF-beta1) and epidermal growth factor (EGF). To confirm that it was the induction of Fsp1 by these cytokines mediating EMT, we used antisense oligomers to block Fsp1 production and subsequently measured cell motility and markers of EMT phenotype. The antisense oligomers suppressed Fsp1 expresison and epithelial transformation; therefore, we conclude that the appearance of Fsp1 is an important early event in the pathway toward EMT.

S-100 (alpha and beta) binding peptide (TRTK-12) blocks S-100/GFAP interaction: identification of a putative S-100 target epitope within the head domain of GFAP

Alignment of previously characterized S-100 (alpha and beta)-binding peptides (J. Biol. Chem. 270, 14651-14658) has enabled the identification of a putative S-100 target epitope within the head domain of glial fibrillary acidic protein (GFAP). The capacity of a known peptide inhibitor of S-100 protein (TRTK-12), homologous to this region, to perturb the interaction of S-100 (alpha and beta) and GFAP (J. Biol. Chem 268, 12669-12674) was investigated. Fluorescence spectrophotometry and chemical cross-linking analyses determined TRTK-12 to disrupt S-100:GFAP interaction in a dose- and Ca(2+_dependent manner. TRTK-12 also inhibited S-100's ability to block GFAP assembly and to mediate disassembly of preformed glial filaments. Each of these events was strictly dependent upon the presence of calcium and inhibitory peptide, maximal inhibition occurring at a concentration of TRTK-12 equivalent to the molar amount of S-100 monomer present. Together with our recent report demonstrating TRTK-12 also blocks the interaction of S-100 protein with the actin capping protein, CapZ, these results suggest TRTK-12 functions as a pleiotropic inhibitor of S-100 function. Availability of a functional inhibitor of S-100 will assist the further characterization of S-100 protein function in vitro and in vivo. Moreover, this report provides additional evidence supportive of a role for S-100 as a multi-faceted regulator of cytoskeletal integrity.

S-100 immunoreactivity in rat brain glial cultures is associated with both astrocytes and oligodendrocytes

S-100 protein, a Ca(2+)-binding protein of the EF-hand type, is most abundant in the brain, and is involved in cell differentiation and the molecular mechanisms underlying cytoskeletal organization. We have investigated the immunocytochemical localization of S-100 protein in rat brain glial cultures prepared from the cerebral hemispheres of newborn rats. In mixed glial cultures, containing astrocytes type I and II and oligodendrocytes at various stages of differentiation, S-100 immunoreactivity was detected in all three cell types. Double immunofluorescence analysis revealed that in astrocytes, S-100 immunoreactivity was mainly colocalized with glial fibrillary acidic protein (GFAP), while in oligodendrocytes a close association with microtubular structures was observed. For immunoblot analysis, highly enriched oligodendrocytes and astrocytes were separately cultured for another week, and their extracts were analyzed by immunoblotting. The immunoblots of the cell extracts of both cell types showed a single S-100-immunoreactive polypeptide with an apparent molecular weight of approximately 12,000 daltons. Thus, the data presented here demonstrate that S-100 protein is not confined to astrocytes but occurs also in oligodendrocytes of rat brain. The close association with the oligodendroglial cytoskeleton suggests that this protein could also play a regulatory role in the organization of microtubules in oligodendrocytes and hence may be involved in the formation and maintenance of the myelin-containing membrane sheets.

The S-100: a protein family in search of a function

The S-100 is a group of low molecular weight (10-12 kD) calcium-binding proteins highly conserved among vertebrates. It is present in different tissues as dimers of homologous or different subunits (alpha, beta). In the nervous system, the S-100 exists as a mixture composed of beta beta and alpha beta dimers with the monomer beta represented more often. Its intracellular localisation is mainly restricted to the glial cytoplasmic compartment with a small fraction bound to membranes. In this compartment the S-100 acts as a potent inhibitor of phosphorylation on several substrates including the synaptosomal C-Kinase and Tau, a microtubule-associated protein. The S-100 in particular conditions, after binding with specific membrane sites (Kd = 0.2 microM; Bmax = 4.5 nM), is able to modify the activity of adenylate cyclase, probably via G-proteins. In addition, the Ca2+ homeostasis is also modulated by S-100 via an increase of specific membrane conductance and/or Ca2+ release from intracellular stores. "In vitro" and "in vivo" experiments showed that lower (nM) concentrations of extracellular S-100 beta act on glial and neuronal cells as a growth-differentiating factor. On the other hand, higher concentrations of the protein induce apoptosis of some cells such as the sympathetic-like PC12 line. Finally, data obtained from physiological (development, ageing) or pathological (dementia associated with Down's syndrome, Alzheimer's disease) conditions showed that a relationship could be established between the S-100 levels and some aspects of the statii.

Inhibition of protein kinase C- and casein kinase II-mediated phosphorylation of GAP-43 by S100 beta

The effect of the glial-derived protein, S100 beta, on the in vitro phosphorylation of the growth-associated protein GAP-43 was investigated. S100 beta inhibited in a dose dependent manner the phosphorylation of GAP-43 by protein kinase C (PKC) or by casein kinase II (CKII). S100 beta appeared to slow down the rate and the degree to which GAP-43 can be phosphorylated by either kinase. The specificity of the inhibition was demonstrated by the observation that the phosphorylation of two other CKII substrates, casein and a selective peptide substrate, was not inhibited by S100 beta. The marked inhibitory effect of S100 beta required the presence of calcium in the phosphorylation reactions. In addition, S100 beta inhibition of GAP-43 phosphorylation was seen with GAP-43 purified under a variety of conditions that alter acylation, suggesting that the acylation state of GAP-43 does not affect the ability of S100 beta to modulate CKII- or PKC-mediated phosphorylation of GAP-43.

Quantitative assay of S-100 protein in mouse brain cortex synaptosomes

1. Data on the presence of S-100 protein in synaptic endings are revised, and evidence is given in favor of its localization inside mouse brain cortex synaptosomes and on the surface of their external membrane. 2. For identification of the S-100-specific polypeptide, proteins of external synaptosomal membranes were iodinated with lactoperoxidase fixed on cyanogen bromide (CNBr)-Sepharose, and after synaptosome lysis S-100-positive material was isolated by means of affinity chromatography antibodies to S-100 protein (a-S-100)-Sepharose. The molecular weight of the polypeptide obtained corresponded to that of S-100 subunits (10 kD), and iodine incorporation pointed to its localization on the surface of synaptosomal membranes. 3. With the help of antibodies labeled with horseradish peroxidase (a-S-100-HP) or 125I (a-S-100-125I), which do not penetrate into noninjured synaptosomes, the amount of S-100 protein on synaptosomal membranes was found to be 18.5 ng/mg total protein (as assayed with a-S-100-HP) or 95.33 ng/mg (as assayed with a-S-100-125I). 4. At the same time, the total S-100 protein content in synaptosomes measured by means of radioimmune analysis after their complete lysis turned out to be 284 +/- 0.84 ng/mg, i.e., a part of S-100 seemed to be inside synaptosomes. 5. Cosedimentation of water-soluble S-100 protein with the synaptosomal fraction during isolation was insignificant. Prefixation with glutaraldehyde or paraformaldehyde decreased the amount of material reacting with antibodies, possibly due to steric effects or denaturation of active centers. This could have influenced the earlier attempts to detect S-100 protein in synapses. Treatment of nonfixed synaptosomes with a conjugate of a-S-100 with colloidal gold made it possible to detect S-100-positive material on pre- and postsynaptic membranes, which confirms the biochemical data.

S-100 protein binds to annexin II and p11, the heavy and light chains of calpactin I

S-100 protein, a dimeric, Ca(2+)-binding protein of the EF-hand type, interacts with annexin II (p36, the heavy chain of the cytoskeletal protein complex, calpactin I), with p11 (the light and regulatory chain of calpactin I) and with the hetero-tetramer annexin II2-p11(2) (calpactin I) in a Ca(2+)-regulated way, but not with annexins I, V and VI. The interaction of S-100 protein with the above proteins was investigated by fluorescence spectroscopy using acrylodan-S-100 protein and acrylodan-annexin II and by cross-linking experiments using the bifunctional cross-linker disuccinimidyl suberate (DSS). S-100 protein binds with the highest affinity to annexin II (Kd approx. 0.4 microM) and with the lowest affinity to calpactin I (Kd approx. 10 microM), with a constant stoichiometry of about 2 mol of protein/S-100 dimer. Thus, S-100 protein could substitute for p11 in regulating the activities of annexin II in cells which do not express p11 and/or act synergistically with p11 in cells expressing both p11 and S-100. The binding of S-100 protein to p11 could reflect the natural tendency of S-100 subunits and p11 to dimerize. Chimeric p11-S-100 alpha and p11-S-100-beta proteins could therefore form in a Ca(2+)-regulated way. The interaction of S-100 protein with calpactin I appears of doubtful physiological importance, because of the low binding affinity, of the small extent of fluorescence changes induced by calpactin I in acrylodan-S-100 protein and of lack of DSS-induced complex formation between the two protein species.

Binding study of metal ions to S100 protein: 43Ca, 25Mg, 67Zn and 39K n.m.r

The interactions of the S100 protein (S100) with metal cations such as Ca2+, Mg2+, Zn2+ and K+ were studied by the metal n.m.r. spectroscopy. The line widths of 43Ca, 25Mg, 67Zn and 39K n.m.r. markedly increased by adding all S100s. A broad 43Ca n.m.r. band of Ca(2+)-S100a solution was not affected by Zn2+ and K+, while it was greatly decreased by adding Mg2+. The 43Ca n.m.r. spectra of Ca(2+)-S100a0 and -S100b solutions consisted of two slow-exchangeable signals which corresponded to Ca2+ bound to two environmentally different sites of the S100a0. These two 43Ca n.m.r. signals were not affected by Zn2+ and K+. The line width of broad 25Mg n.m.r. band of the Mg(2+)-S100 solution greatly decreased by adding Ca2+, while it did not change by adding Zn2+ and K+. Further, the addition of Ca2+, Mg2+ and K+ did not affect the line width of the 67Zn n.m.r. of the Zn(2+)-S100 solutions. These findings suggest that: (1) Mg2+ binds to all S100s, and at least one of the Mg2+ binding sites of S100 molecule is the same as the Ca2+ binding site; (2) Zn2+ binds to S100s, although the binding site(s) is/are different from Ca(2+)- or Mg(2+)-binding site(s), and the environment of Zn2+ nuclei will not change even though Ca2+ binds to S100s.

Perspectives in S-100 protein biology. Review article

The S-100 protein family constitutes a subgroup of Ca(2+)-binding proteins of the EF-hand type comprising three dimeric isoforms, S-100a0, S-100a and S-100b, plus a number of structurally related proteins displaying 28-55% homology with S-100 subunits. S-100 protein was discovered in 1965; yet, its biological functions have not been fully elucidated. The present report will review the putative biological roles of S-100 protein. Both intracellular and extracellular roles have been proposed for S-100 protein. Within cells, S-100 protein has been reported to regulate protein phosphorylation, ATPase, adenylate cyclase, and aldolase activities and Ca(2+)-induced Ca2+ release. Also, cytoskeletal systems, namely microtubules and microfilaments have been reported to be regulated by the protein in the presence of Ca2+. Some molecular targets of S-100 protein within cells, have been identified. This is the case with microtubule proteins, caldesmon, and a brain aldolase. S-100 protein has been reported to be secreted; extracellular S-100 protein can stimulate neuronal differentiation, glial proliferation, and prolactin secretion. However, the mechanisms by which S-100 is secreted and stimulates the above processes are largely unknown. Future research should characterize these latter aspects of S-100 biology and find out the linkage between its intracellular effects and its extracellular activities.

Phosphorylation of bovine brain 81-kDa acidic calmodulin binding protein (ACAMP-81) in vitro

We found a novel 81-kDa acidic protein (ACAMP-81) in the bovine brain membrane fraction, which bound to calmodulin in a Ca(2+)-dependent manner. The present study reveals physicochemical properties and phosphorylation of this protein with various protein kinases in vitro. The Stokes radius and sedimentation coefficient were calculated to be 52 A and 2.05 S, respectively, suggesting that the structure of ACAMP-81 is highly elongated. Purified Ca2+/phospholipid-dependent protein kinase (protein kinase C), cAMP-dependent protein kinase, and Ca2+/calmodulin-dependent protein kinase II (Ca2+/CaM kinase II) catalyzed the incorporation of 1.46, 0.72, and 0.44 mol of phosphate/mol of ACAMP-81, respectively. The amino acid residues of ACAMP-81 phosphorylated by either protein kinase C or cAMP-dependent protein kinase were almost exclusively on serine. Sequential phosphorylation of ACAMP-81 by cAMP-dependent protein kinase and protein kinase C resulted in the additional incorporation of 1.15 mol of [32P]phosphate into ACAMP-81. Comparison of phosphopeptide maps of ACAMP-81 phosphorylated by each kinase revealed that there are two classes of phosphorylatable polypeptide, one is phosphorylatable by both protein kinases which contained two polypeptides and the others are specific sites for protein kinase C.

Examination of the calcium-modulated protein S100 alpha and its target proteins in adult and developing skeletal muscle

In this study radioimmunoassay, immunohistochemistry, Northern blot analysis, and a gel overlay technique have been used to examine the level, subcellular distribution, and potential target proteins of the S100 family of calcium-modulated proteins in adult and developing rat skeletal muscles. Adult rat muscles contained high levels of S100 proteins but the particular form present was dependent on the muscle type: cardiac muscle contained exclusively S100 alpha, slow-twitch skeletal muscle fibers contained predominantly S100 alpha, vascular smooth muscle contained both S100 alpha and S100 beta, and fast-twitch skeletal muscle fibers contained low but detectable levels of S100 alpha and S100 beta. While the distribution of S100 mRNAs paralled the protein distribution in all muscles there was no direct correlation between the mRNA and protein levels in different muscle types, suggesting that S100 protein expression is differentially regulated in different muscle types. Immunohistochemical analysis of the cellular distribution of S100 proteins in adult skeletal muscles revealed that S100 alpha staining was associated with muscle cells, while S100 beta staining was associated with nonmuscle cells. Radioimmunoassays of developing rat skeletal muscles demonstrated that all developing muscles contained low levels of S100 alpha at postnatal day 1 and that as development proceeded the S100 alpha levels increased. In contrast to adult muscle S100 alpha expression was confined to fast-twitch fibers in developing skeletal muscle until postnatal day 21. At postnatal day 1, developing contractile elements were S100 alpha positive, but no staining periodicity was detectable. At postnatal day 21, S100 alpha exhibited the same subcellular localization as seen in the adult: colocalization with the A-band and/or longitudinal sarcoplasmic reticulum. Comparison of the S100 alpha-binding protein profiles in fast- and slow-twitch fibers of various species revealed few, if any, species- or fiber type-specific S100 binding proteins. Isolated sarcoplasmic reticulum fractions and myofibrils contained multiple S100 alpha-binding proteins. The colocalization of S100 alpha and S100 alpha-binding proteins with the contractile apparatus and sarcoplasmic reticulum suggest that S100 alpha may regulate excitation and/or contraction in slow-twitch fibers.

Evolution of EF-hand calcium-modulated proteins. I. Relationships based on amino acid sequences

The relationships among 153 EF-hand (calcium-modulated) proteins of known amino acid sequence were determined using the method of maximum parsimony. These proteins can be ordered into 12 distinct subfamilies--calmodulin, troponin C, essential light chain of myosin, regulatory light chain, sarcoplasmic calcium binding protein, calpain, aequorin, Stronglyocentrotus purpuratus ectodermal protein, calbindin 28 kd, parvalbumin, alpha-actinin, and S100/intestinal calcium-binding protein. Eight individual proteins--calcineurin B from Bos, troponin C from Astacus, calcium vector protein from Branchiostoma, caltractin from Chlamydomonas, cell-division-cycle 31 gene product from Saccharomyces, 10-kd calcium-binding protein from Tetrahymena, LPS1 eight-domain protein from Lytechinus, and calcium-binding protein from Streptomyces--are tentatively identified as unique; that is, each may be the sole representative of another subfamily. We present dendrograms showing the relationships among the subfamilies and uniques as well as dendrograms showing relationships within each subfamily. The EF-hand proteins have been characterized from a broad range of organismal sources, and they have an enormous range of function. This is reflected in the complexity of the dendrograms. At this time we urge caution in assigning a simple scheme of gene duplications to account for the evolution of the 600 EF-hand domains of known sequence.

S-100a0 protein stimulates the basal (Mg2+-activated) adenylate cyclase activity associated with skeletal muscle membranes

S-100a0 protein, the alpha alpha isoform of the S-100 family, stimulates basal (Mg2+-activated) adenylate cyclase (AC) activity associated with the sarcolemma, longitudinal tubules and terminal cisternae of rat skeletal muscle cells. The stimulatory effect of S-100a0 on AC activity is maximal around 5 microM S-100a0 and half-maximal around 0.2 microM S-100a0. Also, the stimulatory effect is greatest on the AC activity associated with the terminal cisternae than on the other membrane fractions studied. These data are discussed in relation to the subcellular localization of S-100a0 in muscle cells.