The Ca2+-binding sequence in bovine brain S100b protein beta-subunit. A spectroscopic study

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.

S100A1: Structure, Function, and Therapeutic Potential

S100A1 is a member of the S100 family of calcium-binding proteins. As with most S100 proteins, S100A1 undergoes a large conformational change upon binding calcium as necessary to interact with numerous protein targets. Targets of S100A1 include proteins involved in calcium signaling (ryanidine receptors 1 & 2, Serca2a, phopholamban), neurotransmitter release (synapsins I & II), cytoskeletal and filament associated proteins (CapZ, microtubules, intermediate filaments, tau, mocrofilaments, desmin, tubulin, F-actin, titin, and the glial fibrillary acidic protein GFAP), transcription factors and their regulators (e.g. myoD, p53), enzymes (e.g. aldolase, phosphoglucomutase, malate dehydrogenase, glycogen phosphorylase, photoreceptor guanyl cyclases, adenylate cyclases, glyceraldehydes-3-phosphate dehydrogenase, twitchin kinase, Ndr kinase, and F1 ATP synthase), and other Ca2+-activated proteins (annexins V & VI, S100B, S100A4, S100P, and other S100 proteins). There is also a growing interest in developing inhibitors of S100A1 since they may be beneficial for treating a variety of human diseases including neurological diseases, diabetes mellitus, heart failure, and several types of cancer. The absence of significant phenotypes in S100A1 knockout mice provides some early indication that an S100A1 antagonist could have minimal side effects in normal tissues. However, development of S100A1-mediated therapies is complicated by S100A1's unusual ability to function as both an intracellular signaling molecule and as a secreted protein. Additionally, many S100A1 protein targets have only recently been identified, and so fully characterizing both these S100A1-target complexes and their resulting functions is a necessary prerequisite.

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.

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.

Role of the N-terminal helix I for dimerization and stability of the calcium-binding protein S100B

Human S100B(beta beta) is a small intracellular EF-hand calcium-binding protein that consists of two noncovalently associated 91-residue beta monomers. The three-dimensional structures of S100B reveal the dimer interface consists of four alpha-helices (I, I' and IV, IV') packed in an X-type bundle. In this study, guanidine hydrochloride denaturation and dynamic light scattering were used to assess the impact of single (L3A, L3S, M7A, I11A, F14A) and double (L3A/I11A and L3A/F14A) substitution mutations in helix I on the stability and dimerization propensity of S100B. The free energy of unfolding (Delta G(u)) of wild-type apo-S100B was determined to be 72.4 +/- 4.0 kJ mol(-1), consistent with it being the most stable calcium-binding protein to date. The order of stability of the mutants in their apo form is S100B > L3A > L3S > I11A > M7A approximately L3A/I11A > F14A > L3A/F14A. Further, there is a strong correlation between the stability and the cooperativity of unfolding. Each mutation proved to be more stable in its calcium form compared to its apo form. The calcium-bound L3S substitution proved to be significantly more stable than calcium-saturated S100B, whereas the L3A, I11A, and L3A/I11A mutants are only slightly more stable than the wild-type protein. The F14A and L3A/F14A mutants are significantly reduced in stability, even in the presence of calcium.

Calcium-binding by p26olf, an S100-like protein in the frog olfactory epithelium

Frog p26olf is a novel S100-like Ca2+-binding protein found in olfactory cilia. It consists of two S100-like domains aligned sequentially, and has a total of four Ca2+-binding sites (known as EF-hands). In this study, to elucidate the mechanism of Ca2+-binding to each EF-hand (named EF-A, -B, -C and -D from the N-terminus of p26olf), we examined Ca2+-binding in wild-type p26olf and also in its mutants in which a glutamate at the -z coordinate position within each Ca2+-binding loop was substituted for a glutamine. Flow dialysis experiments showed that the wild-type binds nearly four Ca2+ per molecule maximally, while all the mutants bind approximately three Ca2+. Although EF-B and -D are p26olf-specific EF-hands and their role in Ca2+-binding is not known, the result unequivocally showed that they actually bind Ca2+. The overall Ca2+-binding affinity decreased in the three mutants. The decrease was very large in the mutants of EF-A and -B, which suggested that the Ca2+-affinities are high in EF-A and -B in the wild-type. Assuming the presence of four steps of Ca2+-binding, we determined the dissociation constant of each step in wild-type p26olf. To assign which step takes place at which EF-hand, we measured the antagonistic effect of K+ on each step, as the effect of K+ is thought to be a function of the number of the carboxyl groups in an EF-hand. Although the actual Ca2+-binding mechanism may not be so simple, this study together with the mutation study suggested a tentative Ca2+-binding model of p26olf: the order of Ca2+-binding to p26olf is EF-B, EF-A, EF-C and EF-D. Based on these results, we speculate that similar Ca2+-binding takes place in an S100 dimer.

Oligomerization and divalent ion binding properties of the S100P protein: a Ca2+/Mg2+-switch model

S100P is a 95 amino acid residue protein which belongs to the S100 family of proteins containing two putative EF-hand Ca2+-binding motifs. In order to characterize conformational properties of S100P in the presence and absence of divalent cations (Ca2+, Mg2+ and Zn2+) in solution, we have analyzed hydrodynamic and spectroscopic characteristics of wild-type and several variants (Y18F, Y88F and C85S) of S100P using equilibrium centrifugation, gel-filtration chromatography, circular dichroism and fluorescence spectroscopies. Analysis of the experimental data shows the following. (1) In agreement with the predictions there are two Ca2+-binding sites in the S100P molecule with different affinity; the high affinity binding site has an apparent binding constant of approximately 10(7) M-1 and the low affinity binding site has an apparent binding constant of approximately 10(4) M-1. (2) The high and low affinity Ca2+-binding sites are located in the C and N-terminal parts of the S100P molecule, respectively. (3) These C and N-terminal sites can also bind other divalent ions. The C-terminal site binds Zn2+ (with relatively low affinity approximately 10(3) M-1), but not Mg2+. The N-terminal site binds Mg2+ with the apparent binding constant approximately 10(2) M-1. (4) Binding of Ca2+ to the C-terminal site and binding of Mg2+ to the N-terminal site occur in the physiological concentration range of these ions (micromolar for Ca2+ and millimolar for Mg2+). (5) Oligomerization state of the S100P molecule appears to change upon addition of Ca2+. On the basis of these observations a plausible model for S100P as a Ca2+/Mg2+ switch has been proposed.

High level expression and dimer characterization of the S100 EF-hand proteins, migration inhibitory factor-related proteins 8 and 14

The phenotypical and functional heterogeneity of different macrophage subpopulations are defined by discrete changes in the expression of two S100 calcium-binding proteins, migration inhibitory factor-related proteins (MRPs) 8 and 14. To further our understanding of MRP8 and MRP14 in the developmental stages of inflammatory responses, overexpression of the MRPs was obtained through a combination of a T7-based expression vector and the Escherichia coli BL21 (DE3) cell line. An efficient, two-step chromatographic protocol was then developed for rapid, facile purification. Extensive biophysical characterization and chemical cross-linking experiments show that MRP8 and MRP14 form oligomers with a strong preference to associate as a heterodimer. Heteronuclear NMR experiments indicate that a specific well packed dimer is formed only in equimolar mixtures of the two proteins. Our results suggest that there is a unique complementarity in the interface of the MRP8/MRP14 complex that cannot be fully reproduced in the MRP8 and MRP14 homodimers.

A novel calcium-sensitive switch revealed by the structure of human S100B in the calcium-bound form

BACKGROUND

S100B is a homodimeric member of the EF-hand calcium-binding protein superfamily. The protein has been implicated in cellular processes such as cell differentiation and growth, plays a role in cytoskeletal structure and function, and may have a role in neuropathological diseases, such as Alzheimers. The effects of S100B are mediated via its interaction with target proteins. While several studies have suggested that this interaction is propagated through a calcium-induced conformational change, leading to the exposure of a hydrophobic region of S100B, the molecular details behind this structural alteration remain unclear.

RESULTS

The solution structure of calcium-saturated human S100B (Ca(2+)-S100B) has been determined by heteronuclear NMR spectroscopy. Ca(2+)-S100B forms a well defined globular structure comprising four EF-hand calcium-binding sites and an extensive hydrophobic dimer interface. A comparison of Ca(2+)-S100B with apo S100B and Ca(2+)-calbindin D9k indicates that while calcium-binding to S100B results in little change in the site I EF-hand, it induces a backbone reorientation of the N terminus of the site II EF-hand. This reorientation leads to a dramatic change in the position of helix III relative to the other helices.

CONCLUSIONS

The calcium-induced reorientation of calcium-binding site II results in the increased exposure of several hydrophobic residues in helix IV and the linker region. While following the general mechanism of calcium modulatory proteins, whereby a hydrophobic target site is exposed, the 'calcium switch' observed in S100B appears to be unique from that of other EF-hand proteins and may provide insights into target specificity among calcium modulatory proteins.

Structural influence of cation binding to recombinant human brain S100b: evidence for calcium-induced exposure of a hydrophobic surface

The dimeric calcium-binding protein S100b is proposed to undergo a calcium-induced structural change allowing it to interact, via a hydrophobic surface, with other proteins. Previously it has been suggested that calcium binding to S100b leads to the exposure of at least one phenylalanine residue (Mani et al., 1982, 1983). This effect appears to be "reversed" at higher ionic strength, leading to a possible reburying of phenylalanine residues (Mani et al., 1982, 1983). To study these effects, we monitored calcium binding to recombinant human S100b by NMR spectroscopy under different salt (KCI) conditions. 15N-Labeled glycine residues in S100b showed calcium-induced chemical shift changes similar to those reported for the related monomeric protein calbindin D9k, suggesting similar conformational changes are occurring in the calcium-binding loops of these two proteins. Calcium binding to S100b also resulted in a shifting and broadening of several 1H resonances from the Ca-S100b form only including those from the side chains of residues F14, F70, and F73 but not those of residue Y17. This broadening was enhanced with increased ionic strength (KCI). However, small additions ( < 15% v/v) of the hydrophobic solvent trifluoroethanol relieved this phenomenon, leading to narrower line widths. These observations are consistent with the calcium-induced exposure of at least one of these hydrophobic residues, resulting in self-association of the S100b dimer. Trifluoroethanol serves to dissociate these complexes back to the dimeric calcium species. We propose that this cluster of hydrophobic residues which include F14, F73, and F88 may be important for interactions with a target protein.

Characterization of S-100b binding epitopes. Identification of a novel target, the actin capping protein, CapZ

Short amino acid sequences that interact with the Ca2+ binding protein S-100b were identified by screening a bacteriophage random peptide display library. S-100b binding bacteriophages were selected by Ca(2+)-dependent affinity chromatography, and the sequence of the random peptide insert contained in 51 clones was determined. Alignment of the sequence of 44 unique S-100b binding peptides identified a common motif of eight amino acids. A subgroup of peptides that contained sequences with the highest degree of similarity had the consensus motif (K/R)(L/I)XWXXIL, in which predominantly P, S, and N were found in position 3, and S and D were found in position 5. Analysis of sequence databanks identified a similar sequence in the COOH-terminal region of the alpha-subunit of actin capping proteins. The peptide TRTKIDWNKILS (TRTK-12), corresponding to the region of greatest homology within this region of the subunit of actin capping proteins (e.g. amino acids 265-276 in CapZ alpha 1 and CapZ alpha 2), was synthesized and shown by fluorescence spectrophotometry to bind S-100b in a Ca(2+)-dependent manner. Gel overlay and cross-linking experiments demonstrated the interaction of S-100b with CapZ to be Ca2+ dependent. Moreover, this interaction was blocked by addition of TRTK-12 peptide. These results identify Ca(2+)-dependent S-100b target sequence epitopes and designate the carboxyl terminus of the alpha-subunit of actin capping proteins, like CapZ, to be a target of S-100b activity. The high level of conservation within this region of actin capping proteins and the apparent high affinity of this interaction strongly suggest that the interaction between S-100b and the alpha-subunit of actin capping proteins is biologically significant.

The S100 protein family: history, function, and expression

The S100 family of calcium binding proteins contains approximately 16 members each of which exhibits a unique pattern of tissue/cell type specific expression. Although the distribution of these proteins is not restricted to the nervous system, the implication of several members of this family in nervous system development, function, and disease has sparked new interest in these proteins. We now know that the original two members of this family, S100A1 and S100B, can regulate a diverse group of cellular functions including cell-cell communication, cell growth, cell structure, energy metabolism, contraction and intracellular signal transduction. Although some members of the family may function extracellularly, most appear to function as intracellular calcium-modulated proteins and couple extracellular stimuli to cellular responses via interaction with other cellular proteins called target proteins. Interaction of these proteins with target proteins appear to involve cysteine residues (one in S100A1 and two in S100B), as well as a stretch of 13 amino acids, in the middle of the molecule called the linker region, which connects the two EF-hand calcium binding domains. In addition to the amino acid sequence and secondary structures of these proteins, the structures of the genes encoding these proteins are highly conserved. Studies on the expression of these proteins have demonstrated that a complex mixture of transcriptional and postranscriptional mechanisms regulate S100 expression. Further analysis of the function and expression of these proteins in both nervous and nonnervous tissues will provide important information regarding the role of altered S100 expression in nervous system development, function and disease.

Disulfide-linked S100 beta dimers and signal transduction

S100 beta is a calcium-binding protein with neurotrophic and mitogenic activities, both of which may be mediated by the protein's ability to stimulate an increase in intracellular free calcium ([Ca2+]i). These extracellular trophic activities of S100 beta require a disulfide-linked, dimeric form of the protein. In this chapter, we present a minireview on the current state of knowledge concerning extracellular functions of S100 beta, with emphasis on the potential relevance of these activities to neuropathological disorders. We also report a simplified procedure for preparation of pharmacological amounts of biologically active S100 beta dimers, based on the finding that formation of disulfide-linked S100 beta dimers can be stimulated by the presence of calcium or lipid.

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.

S100P, a novel Ca(2+)-binding protein from human placenta. cDNA cloning, recombinant protein expression and Ca2+ binding properties

A novel member of the S100 protein family, present in human placenta, has been characterized by protein sequencing, cDNA cloning, and analysis of Ca(2+)-binding properties. Since the placenta protein of 95 amino acid residues shares about 50% sequence identity with the brain S100 proteins alpha and beta, we proposed the name S100P. The cDNA was expressed in Escherichia coli and recombinant S100P was purified in high yield. S100P is a homodimer and has two functional EF hands/polypeptide chain. The low-affinity site (Kd = 800 microM), which, in analogy to S100 beta, seems to involve the N-terminal EF hand, can be followed by the Ca(2+)-dependent decrease in tyrosine fluorescence. The high-affinity site, provided by the C-terminal EF hand, influences the reactivity of the sole cysteine which is located in the C-terminal extension (Cys85). Binding to the high-affinity site (Kd = 1.6 microM) can be monitored by fluorescence spectroscopy of S100P labelled at Cys85 with 6-proprionyl-2-dimethylaminonaphthalene (Prodan). The Prodan fluorescence shows a Ca(2+)-dependent red shift of the maximum emission wavelength from 485 nm to 502 nm, which is accompanied by an approximately twofold loss in integrated fluorescence intensity. This indicates that Cys85 becomes more exposed to the solvent in Ca(2+)-bound S100P, making this region of the molecule, the so-called C-terminal extension, an ideal candidate for a putative Ca(2+)-dependent interaction with a cellular target. In p11, a different member of the S100 family, the C-terminal extension which contains a corresponding cysteine (Cys82 in p11), is involved in a Ca(2+)-independent complex formation with the protein ligand annexin II. The combined results support the hypothesis that S100 proteins interact in general with their targets after a Ca(2+)-dependent conformational change which involves hydrophobic residues of the C-terminal extension.

Spectral [corrected] studies on the cadmium-ion-binding properties of bovine brain S-100b protein

The effect of Cd2+ binding on bovine brain S-100b protein was studied using c.d. u.v. difference spectroscopy and fluorescence measurements. At pH 7.5, S-100b protein binds two Cd2+ ions per monomer with a Kd value of 3 x 10(-5) M. Addition of Cd2+ resulted in perturbing the single tyrosine residue (Tyr17) in the protein as indicated by u.v. difference spectroscopy and aromatic c.d. measurements. In the presence of Cd2+, the tyrosine residue moves to a more non-polar environment, since a red shift was observed in the u.v. difference spectrum. When the protein was excited at 278 nm, the tyrosine fluorescence emission maximum was centred at 306 nm. Cd2+ addition resulted in an increase in intrinsic fluorescence intensity. Fluorescence titration with Cd2+ indicated the protein binds Cd2+ with a Kd value of 3 x 10(-5) M. 2-p-Toluidinylnaphthalene-6-sulphonate-labelled protein, when excited at 345 nm, had a fluorescence emission maximum at 440 nm. Addition of Cd2+ to labelled protein resulted in a 5-fold increase in fluorescence intensity accompanied by a 5 nm blue shift in the emission maximum, suggesting that the probe, in the presence of Cd2+, moves to a hydrophobic domain. U.v. difference spectroscopic studies indicated a unique Cd2(+)-binding site on the protein, since Cd2+ addition yielded a large positive absorption band in the 240 nm region that is not found with either Ca2+ or Zn2- ions. Similar absorption bands have been observed in Cd-protein complexes such as Cd-metallothionein [Vasak, Kagi & Hill (1981) Biochemistry 20, 2852-2856] and also in model complexes of Cd2+ with 2-mercaptoethanol. This absorption band is believed to arise as a result of charge-transfer transitions between the thiolate and Cd2+. Of the two Cd2- -binding sites on the beta-chain, one must be located at the N-terminal end near the single tyrosine residue, since Cd2- and Zn2+ produced similar effects on the intrinsic protein fluorescence. The other Cd2+ site which is unique to Cd2+ must be Cys84, located at the C-terminal end.