The use of dipolar couplings for determining the solution structure of rat apo-S100B(betabeta)

Deciphering S100B Allosteric Signaling: The Role of a Peptide Target, TRTK-12, as an Ensemble Modulator

Allostery is an essential biological phenomenon in which perturbation at one site in a biomolecule elicits a functional response at a distal location(s). It is integral to biological processes, such as cellular signaling, metabolism, and transcription regulation. Understanding allostery is also crucial for rational drug discovery. In this work, we focus on an allosteric S100B protein that belongs to the S100 class of EF-hand Ca2+-binding proteins. The Ca2+-binding affinity of S100B is modulated allosterically by TRTK-12 peptide binding 25 Å away from the Ca2+-binding site. We investigated S100B allostery by carrying out nuclear magnetic resonance (NMR) measurements along with microsecond-long molecular dynamics (MD) simulations on S100B/Ca2+ with/without TRTK-12 at different NaCl salt concentrations. NMR HSQC results show that TRTK-12 reorganizes how S100B/Ca2+ responds to different salt concentrations at both orthosteric and allosteric sites. The MD data suggest that TRTK-12 breaks the dynamic aromatic and hydrogen-bond interactions (not observed in X-ray crystallographic structures) between the hinge/helix and Ca2+-binding EF-hand loop of the two subunits in the homodimeric protein. This triggers rearrangement in the protein network architectures and leads to allosteric communication. Finally, computational studies of S100B at distinct ionic strengths suggest that ligand-bound species are more robust to the changing environment relative to the S100B/Ca2+ complex.

Molecular docking of SARS-COV-2 Spike epitope sequences identifies heterodimeric peptide-protein complex formation with human Zo-1, TLR8 and brain specific glial proteins

SARS-COV-2 infection causes severe respiratory tract illness leading to asphyxia and death. The onset of infection is associated with loss of smell, blurred vision, headache with bronchopulmonary symptoms. The clinical observations of neurological abnormalities lead us to address the question, does the virus enter into brain and what is the underlying mechanism of brain infection? The working hypothesis is, SARS-COV-2 Spike epitopes modify blood brain barrier and infect glial cells to induce brain inflammation in genetically diverse human population. The hypothesis is tested by determining binding or interacting ability of virus Spike epitope peptides M1Lys60 and Ala240Glu300 with human toll-like receptor 8 (TLR 8), brain targeted Vascular Cell adhesion Molecules (VCAM1) proteins, Zonula Occludens (ZO), glial cell specific protein NDRG2 and Apo- S100B. The molecular dynamic experiments are performed, and root mean square deviation (RMSD) values are determined for interactions between the Spike peptides and selected proteins. The observations demonstrate formation of heterodimeric complex between the epitope peptides and selected protein structures. The viral epitopes have ability to bind with HLA-DRB1 15:01, 07:01 or 03.01 alleles thus found immunogenic in nature. The observations altogether suggest entry of these Spike protein epitopes into human brain causes inflammation.

Computational Design of Macrocyclic Binders of S100B(ββ): Novel Peptide Theranostics

S100B(ββ) proteins are a family of multifunctional proteins that are present in several tissues and regulate a wide variety of cellular processes. Their altered expression levels have been associated with several human diseases, such as cancer, inflammatory disorders and neurodegenerative conditions, and hence are of interest as a therapeutic target and a biomarker. Small molecule inhibitors of S100B(ββ) have achieved limited success. Guided by the wealth of available experimental structures of S100B(ββ) in complex with diverse peptides from various protein interacting partners, we combine comparative structural analysis and molecular dynamics simulations to design a series of peptides and their analogues (stapled) as S100B(ββ) binders. The stapled peptides were subject to in silico mutagenesis experiments, resulting in optimized analogues that are predicted to bind to S100B(ββ) with high affinity, and were also modified with imaging agents to serve as diagnostic tools. These stapled peptides can serve as theranostics, which can be used to not only diagnose the levels of S100B(ββ) but also to disrupt the interactions of S100B(ββ) with partner proteins which drive disease progression, thus serving as novel therapeutics.

Mouse S100G protein exhibits properties characteristic of a calcium sensor

Bovine S100 G (calbindin D_9k, small Ca²⁺-binding protein of the EF-hand superfamily) is considered as a calcium buffer protein; i.e., the binding of Ca²⁺ practically does not change its general conformation. A set of experimental approaches has been used to study structural properties of apo- and Ca²⁺-loaded forms of mouse S100 G (81.4% identity in amino acid sequence with bovine S100 G). This analysis revealed that, in contrast to bovine S100 G, the removal of calcium ions increases α-helices content of mouse S100 G protein and enhances its accessibility to digestion by α-chymotrypsin. Furthermore, mouse apo-S100 G is characterized by a decreased surface hydrophobicity and reduced tendency for oligomerization. Such behavior is typical of calcium sensor proteins. Apo-state of mouse S100 G still has rather compact structure, which can be cooperatively unfolded by temperature and GdnHCl. Computational analysis of amino acid sequences of S100 G proteins shows that these proteins could be in a disordered state upon a removal of the bound calcium ions. The experimental data show that, although mouse apo-S100 G is flexible compared to the Ca²⁺-loaded state, the apo-form is not completely disordered and preserves some cooperatively meting structure. The origin of the unexpectedly high stability of mouse S100 G can be rationalized by an exceptionally strong association of its N- and C-terminal parts containing the EF-hands I and II, respectively.

The Zn2+ and Ca2+ -binding S100B and S100A1 proteins: beyond the myths

The S100 genes encode a conserved group of 21 vertebrate-specific EF-hand calcium-binding proteins. Since their discovery in 1965, S100 proteins have remained enigmatic in terms of their cellular functions. In this review, we summarize the calcium- and zinc-binding properties of the dimeric S100B and S100A1 proteins and highlight data that shed new light on the extracellular and intracellular regulation and functions of S100B. We point out that S100B and S100A1 homodimers are not functionally interchangeable and that in a S100A1/S100B heterodimer, S100A1 acts as a negative regulator for the ability of S100B to bind Zn²⁺ . The Ca²⁺ and Zn²⁺ -dependent interactions of S100B with a wide array of proteins form the basis of its activities and have led to the derivation of some initial rules for S100B recognition of protein targets. However, recent findings have strongly suggested that these rules need to be revisited. Here, we describe a new consensus S100B binding motif present in intracellular and extracellular vertebrate-specific proteins and propose a new model for stable interactions of S100B dimers with full-length target proteins. A chaperone-associated function for intracellular S100B in adaptive cellular stress responses is also discussed. This review may help guide future studies on the functions of S100 proteins in general.

A subset of calcium-binding S100 proteins show preferential heterodimerization

The assembly of proteins into dimers and oligomers is a necessary step for the proper function of transcription factors, muscle proteins, and proteases. In uncontrolled states, oligomerization can also contribute to illnesses such as Alzheimer's disease. The S100 protein family is a group of dimeric proteins that have important roles in enzyme regulation, cell membrane repair, and cell growth. Most S100 proteins have been examined in their homodimeric state, yet some of these important proteins are found in similar tissues implying that heterodimeric molecules can also be formed from the combination of two different S100 members. In this work, we have established co-expression methods in order to identify and quantify the distribution of homo- and heterodimers for four specific pairs of S100 proteins in their calcium-free states. The split GFP trap methodology was used in combination with other GFP variants to simultaneously quantify homo- and heterodimeric S100 proteins in vitro and in living cells. For the specific S100 proteins examined, NMR, mass spectrometry, and GFP trap experiments consistently show that S100A1:S100B, S100A1:S100P, and S100A11:S100B heterodimers are the predominant species formed compared to their corresponding homodimers. We expect the tools developed here will help establish the roles of S100 heterodimeric proteins and identify how heterodimerization might alter the specificity for S100 protein action in cells.

An Interplay of S-Nitrosylation and Metal Ion Binding for Astrocytic S100B Protein

Mammalian S100B protein plays multiple important roles in cellular brain processes. The protein is a clinically used marker for several pathologies including brain injury, neurodegeneration and cancer. High levels of S100B released by astrocytes in Down syndrome patients are responsible for reduced neurogenesis of neural progenitor cells and induction of cell death in neurons. Despite increasing understanding of S100B biology, there are still many questions concerning the detailed molecular mechanisms that determine specific activities of S100B. Elevated overexpression of S100B protein is often synchronized with increased nitric oxide-related activity. In this work we show S100B is a target of exogenous S-nitrosylation in rat brain protein lysate and identify endogenous S-nitrosylation of S100B in a cellular model of astrocytes. Biochemical studies are presented indicating S-nitrosylation tunes the conformation of S100B and modulates its Ca²⁺ and Zn²⁺ binding properties. Our in vitro results suggest that the possibility of endogenous S-nitrosylation should be taken into account in the further studies of in vivo S100B protein activity, especially under conditions of increased NO-related activity.

Structure of the small Dictyostelium discoideum myosin light chain MlcB provides insights into MyoB IQ motif recognition

Dictyostelium discoideum MyoB is a class I myosin involved in the formation and retraction of membrane projections, cortical tension generation, membrane recycling, and phagosome maturation. The MyoB-specific, single-lobe EF-hand light chain MlcB binds the sole IQ motif of MyoB with submicromolar affinity in the absence and presence of Ca(2+). However, the structural features of this novel myosin light chain and its interaction with its cognate IQ motif remain uncharacterized. Here, we describe the NMR-derived solution structure of apoMlcB, which displays a globular four-helix bundle. Helix 1 adopts a unique orientation when compared with the apo states of the EF-hand calcium-binding proteins calmodulin, S100B, and calbindin D9k. NMR-based chemical shift perturbation mapping identified a hydrophobic MyoB IQ binding surface that involves amino acid residues in helices I and IV and the functional N-terminal Ca(2+) binding loop, a site that appears to be maintained when MlcB adopts the holo state. Complementary mutagenesis and binding studies indicated that residues Ile-701, Phe-705, and Trp-708 of the MyoB IQ motif are critical for recognition of MlcB, which together allowed the generation of a structural model of the apoMlcB-MyoB IQ complex. We conclude that the mode of IQ motif recognition by the novel single-lobe MlcB differs considerably from that of stereotypical bilobal light chains such as calmodulin.

Post-translational S-nitrosylation is an endogenous factor fine tuning the properties of human S100A1 protein

BACKGROUND

S100A1 protein is a proposed target of molecule-guided therapy for heart failure.

RESULTS

S-Nitrosylation of S100A1 is present in cells, increases Ca(2+) binding, and tunes the overall protein conformation.

CONCLUSION

Thiol-aromatic molecular switch is responsible for NO-related modification of S100A1 properties.

SIGNIFICANCE

Post-translational S-nitrosylation may provide functional diversity and specificity to S100A1 and other S100 protein family members. S100A1 is a member of the Ca(2+)-binding S100 protein family. It is expressed in brain and heart tissue, where it plays a crucial role as a modulator of Ca(2+) homeostasis, energy metabolism, neurotransmitter release, and contractile performance. Biological effects of S100A1 have been attributed to its direct interaction with a variety of target proteins. The (patho)physiological relevance of S100A1 makes it an important molecular target for future therapeutic intervention. S-Nitrosylation is a post-translational modification of proteins, which plays a role in cellular signal transduction under physiological and pathological conditions. In this study, we confirmed that S100A1 protein is endogenously modified by Cys(85) S-nitrosylation in PC12 cells, which are a well established model system for studying S100A1 function. We used isothermal calorimetry to show that S-nitrosylation facilitates the formation of Ca(2+)-loaded S100A1 at physiological ionic strength conditions. To establish the unique influence of the S-nitroso group, our study describes high resolution three-dimensional structures of human apo-S100A1 protein with the Cys(85) thiol group in reduced and S-nitrosylated states. Solution structures of the proteins are based on NMR data obtained at physiological ionic strength. Comparative analysis shows that S-nitrosylation fine tunes the overall architecture of S100A1 protein. Although the typical S100 protein intersubunit four-helix bundle is conserved upon S-nitrosylation, the conformation of S100A1 protein is reorganized at the sites most important for target recognition (i.e. the C-terminal helix and the linker connecting two EF-hand domains). In summary, this study discloses cysteine S-nitrosylation as a new factor responsible for increasing functional diversity of S100A1 and helps explain the role of S100A1 as a Ca(2+) signal transmitter sensitive to NO/redox equilibrium within cells.

The 1.5 Å crystal structure of human receptor for advanced glycation endproducts (RAGE) ectodomains reveals unique features determining ligand binding

Interaction of the pattern recognition receptor, RAGE with key ligands such as advanced glycation end products (AGE), S100 proteins, amyloid β, and HMGB1 has been linked to diabetic complications, inflammatory and neurodegenerative disorders, and cancer. To help answer the question of how a single receptor can recognize and respond to a diverse set of ligands we have investigated the structure and binding properties of the first two extracellular domains of human RAGE, which are implicated in various ligand binding and subsequent signaling events. The 1.5-Å crystal structure reveals an elongated molecule with a large basic patch and a large hydrophobic patch, both highly conserved. Isothermal titration calorimetry (ITC) and deletion experiments indicate S100B recognition by RAGE is an entropically driven process involving hydrophobic interaction that is dependent on Ca(2+) and on residues in the C'D loop (residues 54-67) of domain 1. In contrast, competition experiments using gel shift assays suggest that RAGE interaction with AGE is driven by the recognition of negative charges on AGE-proteins. We also demonstrate that RAGE can bind to dsDNA and dsRNA. These findings reveal versatile structural features of RAGE that help explain its ability to recognize of multiple ligands.

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.

The effects of CapZ peptide (TRTK-12) binding to S100B-Ca2+ as examined by NMR and X-ray crystallography

Structure-based drug design is underway to inhibit the S100B-p53 interaction as a strategy for treating malignant melanoma. X-ray crystallography was used here to characterize an interaction between Ca(2)(+)-S100B and TRTK-12, a target that binds to the p53-binding site on S100B. The structures of Ca(2+)-S100B (1.5-A resolution) and S100B-Ca(2)(+)-TRTK-12 (2.0-A resolution) determined here indicate that the S100B-Ca(2+)-TRTK-12 complex is dominated by an interaction between Trp7 of TRTK-12 and a hydrophobic binding pocket exposed on Ca(2+)-S100B involving residues in helices 2 and 3 and loop 2. As with an S100B-Ca(2)(+)-p53 peptide complex, TRTK-12 binding to Ca(2+)-S100B was found to increase the protein's Ca(2)(+)-binding affinity. One explanation for this effect was that peptide binding introduced a structural change that increased the number of Ca(2+) ligands and/or improved the Ca(2+) coordination geometry of S100B. This possibility was ruled out when the structures of S100B-Ca(2+)-TRTK-12 and S100B-Ca(2+) were compared and calcium ion coordination by the protein was found to be nearly identical in both EF-hand calcium-binding domains (RMSD=0.19). On the other hand, B-factors for residues in EF2 of Ca(2+)-S100B were found to be significantly lowered with TRTK-12 bound. This result is consistent with NMR (15)N relaxation studies that showed that TRTK-12 binding eliminated dynamic properties observed in Ca(2+)-S100B. Such a loss of protein motion may also provide an explanation for how calcium-ion-binding affinity is increased upon binding a target. Lastly, it follows that any small-molecule inhibitor bound to Ca(2+)-S100B would also have to cause an increase in calcium-ion-binding affinity to be effective therapeutically inside a cell, so these data need to be considered in future drug design studies involving S100B.

Improvement and analysis of computational methods for prediction of residual dipolar couplings

We describe a new, computationally efficient method for computing the molecular alignment tensor based on the molecular shape. The increase in speed is achieved by re-expressing the problem as one of numerical integration, rather than a simple uniform sampling (as in the PALES method), and by using a convex hull rather than a detailed representation of the surface of a molecule. This method is applicable to bicelles, PEG/hexanol, and other alignment media that can be modeled by steric restrictions introduced by a planar barrier. This method is used to further explore and compare various representations of protein shape by an equivalent ellipsoid. We also examine the accuracy of the alignment tensor and residual dipolar couplings (RDC) prediction using various ab initio methods. We separately quantify the inaccuracy in RDC prediction caused by the inaccuracy in the orientation and in the magnitude of the alignment tensor, concluding that orientation accuracy is much more important in accurate prediction of RDCs.

Analysis of the structure of human apo-S100B at low temperature indicates a unimodal conformational distribution is adopted by calcium-free S100 proteins

S100B is one of the best-characterized members of the calcium-signaling S100 protein family. Most S100 proteins are dimeric, with each monomer containing two EF-hand calcium-binding sites (EF1, EF2). S100B and other S100 proteins respond to calcium increases in the cell by coordinating calcium and undergoing a conformational change that allows them to interact with a variety of cellular targets. Although several three dimensional structures of S100 proteins are available in the calcium-free (apo-) state it has been observed that these structures appear to adopt a wide range of conformations in the EF2 site with respect to the positioning of helix III, the helix that undergoes the most dramatic calcium-induced conformational change. In this work, we have determined the structure of human apo-S100B at 10 degrees C to examine whether temperature might be responsible for these structural differences. Further, we have used this data, and other available apo-S100 structures, to show that despite the range of interhelical angles adopted in the apo-S100 structures, normal Gaussian distributions about the mean angles found in the structure of human apo-S100B are observed. This finding, only obvious from the analysis of all available apo-S100 proteins, provides direct structural evidence that helix III is a loosely packed helix. This is likely a necessary functional property of the S100 proteins that facilitates the calcium-induced conformational change of helix III. In contrast, the calcium-bound structures of the S100 proteins show significantly smaller variability in the interhelical angles. This shows that calcium binding to the S100 proteins causes not only a conformational change but results in a tighter distribution of helices within the EF2 calcium binding site required for target protein interactions.

S100A1 and calmodulin compete for the same binding site on ryanodine receptor

In heart and skeletal muscle an S100 protein family member, S100A1, binds to the ryanodine receptor (RyR) and promotes Ca(2+) release. Using competition binding assays, we further characterized this system in skeletal muscle and showed that Ca(2+)-S100A1 competes with Ca(2+)-calmodulin (CaM) for the same binding site on RyR1. In addition, the NMR structure was determined for Ca(2+)-S100A1 bound to a peptide derived from this CaM/S100A1 binding domain, a region conserved in RyR1 and RyR2 and termed RyRP12 (residues 3616-3627 in human RyR1). Examination of the S100A1-RyRP12 complex revealed residues of the helical RyRP12 peptide (Lys-3616, Trp-3620, Lys-3622, Leu-3623, Leu-3624, and Lys-3626) that are involved in favorable hydrophobic and electrostatic interactions with Ca(2+)-S100A1. These same residues were shown previously to be important for RyR1 binding to Ca(2+)-CaM. A model for regulating muscle contraction is presented in which Ca(2+)-S100A1 and Ca(2+)-CaM compete directly for the same binding site on the ryanodine receptor.

S100-annexin complexes--structural insights

Annexins and S100 proteins represent two large, but distinct, calcium-binding protein families. Annexins are made up of a highly alpha-helical core domain that binds calcium ions, allowing them to interact with phospholipid membranes. Furthermore, some annexins, such as annexins A1 and A2, contain an N-terminal region that is expelled from the core domain on calcium binding. These events allow for the interaction of the annexin N-terminus with target proteins, such as S100. In addition, when an S100 protein binds calcium ions, it undergoes a structural reorientation of its helices, exposing a hydrophobic patch capable of interacting with its targets, including the N-terminal sequences of annexins. Structural studies of the complexes between members of these two families have revealed valuable details regarding the mechanisms of the interactions, including the binding surfaces and conformation of the annexin N-terminus. However, other S100-annexin interactions, such as those between S100A11 and annexin A6, or between dicalcin and annexins A1, A2 and A5, appear to be more complicated, involving the annexin core region, perhaps in concert with the N-terminus. The diversity of these interactions indicates that multiple forms of recognition exist between S100 proteins and annexins. S100-annexin interactions have been suggested to play a role in membrane fusion events by the bridging together of two annexin proteins, bound to phospholipid membranes, by an S100 protein. The structures and differential interactions of S100-annexin complexes may indicate that this process has several possible modes of protein-protein recognition.

Identification of a dimeric intermediate in the unfolding pathway for the calcium-binding protein S100B

The S100 proteins comprise 25 calcium-signalling members of the EF-hand protein family. Unlike typical EF-hand signalling proteins such as calmodulin and troponin-C, the S100 proteins are dimeric, forming both homo- and heterodimers in vivo. One member of this family, S100B, is a homodimeric protein shown to control the assembly of several cytoskeletal proteins and regulate phosphorylation events in a calcium-sensitive manner. Calcium binding to S100B causes a conformational change involving movement of helix III in the second calcium-binding site (EF2) that exposes a hydrophobic surface enabling interactions with other proteins such as tubulin and Ndr kinase. In several S100 proteins, calcium binding also stabilizes dimerization compared to the calcium-free states. In this work, we have examined the guanidine hydrochloride (GuHCl)-induced unfolding of dimeric calcium-free S100B. A series of tryptophan substitutions near the dimer interface and the EF2 calcium-binding site were studied by fluorescence spectroscopy and showed biphasic unfolding curves. The presence of a plateau near 1.5 M GuHCl showed the presence of an intermediate that had a greater exposed hydrophobic surface area compared to the native dimer based on increased 4,4-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid fluorescence. Furthermore, (1)H-(15)N heteronuclear single quantum coherence analyses as a function of GuHCl showed significant chemical shift changes in regions near the EF1 calcium-binding loop and between the linker and C-terminus of helix IV. Together these observations show that calcium-free S100B unfolds via a dimeric intermediate.

Solution structure of the novel dispersin protein of enteroaggregative Escherichia coli

Enteroaggregative Escherichia coli (EAEC), increasingly recognized as an important cause of infant and travelers' diarrhoea, exhibits an aggregative, stacked-brick pattern of adherence to epithelial cells. Adherence is mediated by aggregative adherence fimbriae (AAFs), which are encoded on the pAA virulence plasmid. We recently described a highly prevalent pAA plasmid-borne gene, aap, which encodes a protein (nicknamed dispersin) that is secreted to the bacterial cell surface. Dispersin-null mutants display a unique hyper-aggregating phenotype, accompanied by collapse of AAF pili onto the bacterial cell surface. To study the mechanism of this effect, we solved the structure of dispersin from EAEC strain 042 using solution NMR, revealing a stable beta-sandwich with a conserved net positive surface charge of +3 to +4 among 23 dispersin alleles. Experimental data suggest that dispersin binds non-covalently to lipopolysaccharide on the surface of the bacterium. We also show that the AAF organelles contribute positive charge to the bacterial surface, suggesting that dispersin's role in fimbrial function is to overcome electrostatic attraction between AAF and the bacterial surface.

Restriction endonuclease inhibitor IPI* of bacteriophage T4: a novel structure for a dedicated target

Phage T4 protects its DNA from the two-gene-encoded gmrS/gmrD (glucose-modified hydroxymethylcytosine restriction endonuclease) CT of pathogenic Escherichia coli, CT596, by injecting several hundred copies of the 76-amino-acid-residue nuclease inhibitor, IPI*, into the infected host. Here, the three-dimensional solution structure of mature IPI* is reported as determined by nuclear magnetic resonance techniques using 1290 experimental nuclear Overhauser effect and dipolar coupling constraints ( approximately 17 constraints per residue). Close examination of this oblate-shaped protein structure reveals a novel fold consisting of two small beta-sheets (beta1: B1 and B2; beta2: B3-B5) flanked at the N- and C-termini by alpha-helices (H1 and H2). Such a fold is very compact in shape and allows ejection of IPI* through the narrow 30-A portal and tail tube apertures of the virion without unfolding. Structural and dynamic measurements identify an exposed hydrophobic knob that is a putative gmrS/gmrD-binding site. A single gene from the uropathogenic E. coli UT189, which codes for a gmrS/gmrD-like UT fusion enzyme (with approximately 90% identity to the heterodimeric CT enzyme), has evolved IPI* inhibitor immunity. Analysis of the gmrS/gmrD restriction endonuclease enzyme family and its IPI* family phage antagonists reveals an evolutionary pathway that has elaborated a surprisingly diverse and specifically fitted set of coevolving attack and defense structures.

Structural and functional insights into RAGE activation by multimeric S100B

Nervous system development and plasticity require regulation of cell proliferation, survival, neurite outgrowth and synapse formation by specific extracellular factors. The EF-hand protein S100B is highly expressed in human brain. In the extracellular space, it promotes neurite extension and neuron survival via the receptor RAGE (receptor for advanced glycation end products). The X-ray structure of human Ca(2+)-loaded S100B was determined at 1.9 A resolution. The structure revealed an octameric architecture of four homodimeric units arranged as two tetramers in a tight array. The presence of multimeric forms in human brain extracts was confirmed by size-exclusion experiments. Recombinant tetrameric, hexameric and octameric S100B were purified from Escherichia coli and characterised. Binding studies show that tetrameric S100B binds RAGE with higher affinity than dimeric S100B. Analytical ultracentrifugation studies imply that S100B tetramer binds two RAGE molecules via the V-domain. In line with these experiments, S100B tetramer caused stronger activation of cell growth than S100B dimer and promoted cell survival. The structural and the binding data suggest that tetrameric S100B triggers RAGE activation by receptor dimerisation.

Observation of microsecond time-scale protein dynamics in the presence of Ln3+ ions: application to the N-terminal domain of cardiac troponin C

The microsecond time-scale motions in the N-terminal domain of cardiac troponin C (NcTnC) loaded with lanthanide ions have been investigated by means of a (1)H(N) off-resonance spin-lock experiment. The observed relaxation dispersion effects strongly increase along the series of NcTnC samples containing La(3+), Ce(3+), and Pr(3+) ions. This rise in dispersion effects is due to modulation of long-range pseudocontact shifts by micros time-scale dynamics. Specifically, the motion in the coordination sphere of the lanthanide ion (i.e. in the NcTnC EF-hand motif) causes modulation of the paramagnetic susceptibility tensor which, in turn, causes modulation of pseudocontact shifts. It is also probable that opening/closing dynamics, previously identified in Ca(2+)-NcTnC, contributes to some of the observed dispersions. On the other hand, it is unlikely that monomer-dimer exchange in the solution of NcTnC is directly responsible for the dispersion effects. Finally, on-off exchange of the lanthanide ion does not seem to play any significant role. The amplification of dispersion effects by Ln(3+) ions is a potentially useful tool for studies of micros-ms motions in proteins. This approach makes it possible to observe the dispersions even when the local environment of the reporting spin does not change. This happens, for example, when the motion involves a 'rigid' structural unit such as individual alpha-helix. Even more significantly, the dispersions based on pseudocontact shifts offer better chances for structural characterization of the dynamic species. This method can be generalized for a large class of applications via the use of specially designed lanthanide-binding tags.

Pathologies involving the S100 proteins and RAGE

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

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

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

Structural basis for diversity of the EF-hand calcium-binding proteins

The calcium binding proteins of the EF-hand super-family are involved in the regulation of all aspects of cell function. These proteins exhibit a great diversity of composition, structure, Ca2+-binding and target interaction properties. Here, our current understanding of the Ca2+-binding mechanism is assessed. The structures of the EF-hand motifs containing 11-14 amino acid residues in the Ca2+-binding loop are analyzed within the framework of the recently proposed two-step Ca2+-binding mechanism. A hypothesis is put forward that in all EF-hand proteins the Ca2+-binding and the resultant conformational responses are governed by the central structure connecting the Ca2+-binding loops in the two-EF-hand domain. This structure, named EFbeta-scaffold, defines the position of the bound Ca2+, and coordinates the function of the N-terminal (variable and flexible) with the C-terminal (invariable and rigid) parts of the Ca2+-binding loop. It is proposed that the nature of the first ligand of the Ca2+-binding loop is an important determinant of the conformational change. Additional factors, including the interhelical contacts, the length, structure and flexibility of the linker connecting the EF-hand motifs, and the overall energy balance provide the fine-tuning of the Ca2+-induced conformational change in the EF-hand proteins.

Weak alignment NMR: a hawk-eyed view of biomolecular structure

Imposing a very slight deviation from the isotropic random distribution of macromolecules in solution in an NMR sample tube permits the measurement of residual internuclear dipolar couplings (RDCs). Such interactions are very sensitive functions of the time-averaged orientation of the corresponding internuclear vectors and thereby offer highly precise structural information. In recent years, advances have been made both in the technology to measure RDCs and in the computational procedures that integrate this information in the structure determination process. The exceptional precision with which RDCs can be measured under weakly aligned conditions is also starting to reveal the mostly, but not universally, subtle effects of internal protein dynamics. Importantly, RDCs potentially can reveal motions taking place on a timescale slower than rotational diffusion and analysis is uniquely sensitive to the direction of motion, not just its amplitude.

Solution structure of (gamma)S-crystallin by molecular fragment replacement NMR

The solution structure of murine gammaS-crystallin (gammaS) has been determined by multidimensional triple resonance NMR spectroscopy, using restraints derived from two sets of dipolar couplings, recorded in different alignment media, and supplemented by a small number of NOE distance restraints. gammaS consists of two topologically similar domains, arranged with an approximate twofold symmetry, and each domain shows close structural homology to closely related (approximately 50% sequence identity) domains found in other members of the gamma-crystallin family. Each domain consists of two four-strand "Greek key" beta-sheets. Although the domains are tightly anchored to one another by the hydrophobic surfaces of the two inner Greek key motifs, the N-arm, the interdomain linker and several turn regions show unexpected flexibility and disorder in solution. This may contribute entropic stabilization to the protein in solution, but may also indicate nucleation sites for unfolding or other structural transitions. The method used for solving the gammaS structure relies on the recently introduced molecular fragment replacement method, which capitalizes on the large database of protein structures previously solved by X-ray crystallography and NMR.

The three-dimensional solution structure of Ca(2+)-bound S100A1 as determined by NMR spectroscopy

S100A1 is an EF-hand-containing Ca(2+)-binding protein that undergoes a conformational change upon binding calcium as is necessary to interact with protein targets and initiate a biological response. To better understand how calcium influences the structure and function of S100A1, the three-dimensional structure of calcium-bound S100A1 was determined by multidimensional NMR spectroscopy and compared to the previously determined structure of apo. In total, 3354 nuclear Overhauser effect-derived distance constraints, 240 dihedral constraints, 160 hydrogen bond constraints, and 362 residual dipolar coupling restraints derived from a series of two-dimensional, three-dimensional, and four-dimensional NMR experiments were used in its structure determination (>21 constraints per residue). As with other dimeric S100 proteins, S100A1 is a symmetric homodimer with helices 1, 1', 4, and 4' associating into an X-type four-helix bundle at the dimer interface. Within each subunit there are four alpha-helices and a short antiparallel beta-sheet typical of two helix-loop-helix EF-hand calcium-binding domains. The addition of calcium did not change the interhelical angle of helices 1 and 2 in the pseudo EF-hand significantly; however, there was a large reorientation of helix 3 in the typical EF-hand. The large conformational change exposes a hydrophobic cleft, defined by residues in the hinge region, the C terminus, and regions of helix 3, which are important for the interaction between S100A1 and a peptide (TRTK-12) derived from the actin-capping protein CapZ.

Estimating the accuracy of protein structures using residual dipolar couplings

It has been commonly recognized that residual dipolar coupling data provide a measure of quality for protein structures. To quantify this observation, a database of 100 single-domain proteins has been compiled where each protein was represented by two independently solved structures. Backbone 1H-15N dipolar couplings were simulated for the target structures and then fitted to the model structures. The fits were characterized by an R-factor which was corrected for the effects of non-uniform distribution of dipolar vectors on a unit sphere. The analyses show that favorable R values virtually guarantee high accuracy of the model structure (where accuracy is defined as the backbone coordinate rms deviation). On the other hand, unfavorable R values do not necessarily suggest low accuracy. Based on the simulated data, a simple empirical formula is proposed to estimate the accuracy of protein structures. The method is illustrated with a number of examples, including PDZ2 domain of human phosphatase hPTP1E.

Solution structure and backbone dynamics of Calsensin, an invertebrate neuronal calcium-binding protein

Calsensin is an EF-hand calcium-binding protein expressed by a subset of peripheral sensory neurons that fasciculate into a single tract in the leech central nervous system. Calsensin is a 9-kD protein with two EF-hand calcium-binding motifs. Using multidimensional NMR spectroscopy we have determined the solution structure and backbone dynamics of calcium-bound Calsensin. Calsensin consists of four helices forming a unicornate-type four-helix bundle. The residues in the third helix undergo slow conformational exchange indicating that the motion of this helix is associated with calciumbinding. The backbone dynamics of the protein as measured by (15)N relaxation rates and heteronuclear NOEs correlate well with the three-dimensional structure. Furthermore, comparison of the structure of Calsensin with other members of the EF-hand calcium-binding protein family provides insight into plausible mechanisms of calcium and target protein binding.

Residual dipolar couplings in NMR structure analysis

Residual dipolar couplings (RDCs) have recently emerged as a new tool in nuclear magnetic resonance (NMR) with which to study macromolecular structure and function in a solution environment. RDCs are complementary to the more conventional use of NOEs to provide structural information. While NOEs are local-distance restraints, RDCs provide long-range orientational information. RDCs are now widely utilized in structure calculations. Increasingly, they are being used in novel applications to address complex issues in structural biology such as the accurate determination of the global structure of oligonucleotides and the relative orientation of protein domains. This review briefly describes the theory and methods for obtaining RDCs and then describes the range of biological applications where RDCs have been used.

Protein structure prediction using sparse dipolar coupling data

Residual dipolar coupling (RDC) represents one of the most exciting emerging NMR techniques for protein structure studies. However, solving a protein structure using RDC data alone is still a highly challenging problem. We report here a computer program, RDC-PROSPECT, for protein structure prediction based on a structural homolog or analog of the target protein in the Protein Data Bank (PDB), which best aligns with the (15)N-(1)H RDC data of the protein recorded in a single ordering medium. Since RDC-PROSPECT uses only RDC data and predicted secondary structure information, its performance is virtually independent of sequence similarity between a target protein and its structural homolog/analog, making it applicable to protein targets beyond the scope of current protein threading techniques. We have tested RDC-PROSPECT on all (15)N-(1)H RDC data (representing 43 proteins) deposited in the BioMagResBank (BMRB) database. The program correctly identified structural folds for 83.7% of the target proteins, and achieved an average alignment accuracy of 98.1% residues within a four-residue shift.

Rapid protein fold determination using unassigned NMR data

Experimental structure determination by x-ray crystallography and NMR spectroscopy is slow and time-consuming compared with the rate at which new protein sequences are being identified. NMR spectroscopy has the advantage of rapidly providing the structurally relevant information in the form of unassigned chemical shifts (CSs), intensities of NOESY crosspeaks [nuclear Overhauser effects (NOEs)], and residual dipolar couplings (RDCs), but use of these data are limited by the time and effort needed to assign individual resonances to specific atoms. Here, we develop a method for generating low-resolution protein structures by using unassigned NMR data that relies on the de novo protein structure prediction algorithm, rosetta [Simons, K. T., Kooperberg, C., Huang, E. & Baker, D. (1997) J. Mol. Biol. 268, 209-225] and a Monte Carlo procedure that searches for the assignment of resonances to atoms that produces the best fit of the experimental NMR data to a candidate 3D structure. A large ensemble of models is generated from sequence information alone by using rosetta, an optimal assignment is identified for each model, and the models are then ranked based on their fit with the NMR data assuming the identified assignments. The method was tested on nine protein sequences between 56 and 140 amino acids and published CS, NOE, and RDC data. The procedure yielded models with rms deviations between 3 and 6 A, and, in four of the nine cases, the partial assignments obtained by the method could be used to refine the structures to high resolution (0.6-1.8 A) by repeated cycles of structure generation guided by the partial assignments, followed by reassignment using the newly generated models.

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.

FAST-Modelfree: a program for rapid automated analysis of solution NMR spin-relaxation data

Herein we describe the program FAST-Modelfree for the fully automated, high throughput analysis of NMR spin-relaxation data. This program interfaces with the program Modelfree 4.1 and provides an intuitive graphical user interface for configuration as well as complete standalone operation during the model selection and rotational diffusion parameter optimization processes. FAST-Modelfree is also capable of iteratively assigning models to each spin and optimizing the parameters that describe the diffusion tensor. Tests with the protein Ribonuclease A indicate that using this iterative approach even poor initial estimates of the diffusion tensor parameters will converge to the optimal value within a few iterations. In addition to improving the quality of the final fit, this represents a substantial timesaving compared to manual data analysis and minimizes the chance of human error. It is anticipated that this program will be particularly useful for the analysis and comparison of data collected under different conditions such as multiple temperatures and the presence and absence of ligands. Further, this program is intended to establish a more uniform protocol for NMR spin-relaxation data analysis, facilitating the comparison of results both between and within research laboratories. Results obtained with FAST-Modelfree are compared with previous literature results for the proteins Ribonuclease H, E. coli glutaredoxin-1 and the Ca(2+)-binding protein S100B. These proteins represent data sets collected at both single and multiple static magnetic fields and which required analysis with both isotropic and axially symmetric rotational diffusion tensors. In all cases results obtained with FAST-Modelfree compared favorably with the original literature results.

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.

Calretinin and calbindin D28k have different domain organizations

The domain organization of calretinin (CR) was predicted to involve all six EF-hand motifs (labeled I to VI) condensed into a single domain, as characterized for calbindin D28k (Calb), the closest homolog of calretinin. Unperturbed (1)H,(15)N HSQC NMR spectra of a (15)N-labeled calretinin fragment (CR III-VI, residues 100-271) in the presence of the unlabeled complimentary fragment (CR I-II, residues 1-100) show that these fragments do not interact. Size exclusion chromatography and affinity chromatography data support this conclusion. The HSQC spectrum of (15)N-labeled CR is similar to the overlaid spectra of individual (15)N-labeled CR fragments (CR I-II and CR III-VI), also suggesting that these regions do not interact within intact CR. In contrast to these observations, but in accordance with the Calb studies, we observed interactions between other CR fragments: CR I (1-60) with CR II-VI (61-271), and CR I-III (1-142) with CR IV-VI (145-271). We conclude that CR is formed from at least two independent domains consisting of CR I-II and CR III-VI. The differences in domain organization of Calb and CR may explain the specific target interaction of Calb with caspase-3. Most importantly, the comparison of CR and Calb domain organizations questions the value of homologous modeling of EF-hand proteins, and perhaps of other protein families.

Weak alignment offers new NMR opportunities to study protein structure and dynamics

Protein solution nuclear magnetic resonance (NMR) can be conducted in a slightly anisotropic environment, where the orientational distribution of the proteins is no longer random. In such an environment, the large one-bond internuclear dipolar interactions no longer average to zero and report on the average orientation of the corresponding vectors relative to the magnetic field. The desired very weak ordering, on the order of 10(-3), can be induced conveniently by the use of aqueous nematic liquid crystalline suspensions or by anisotropically compressed hydrogels. The resulting residual dipolar interactions are scaled down by three orders of magnitude relative to their static values, but nevertheless can be measured at high accuracy. They are very precise reporters on the average orientation of bonds relative to the molecular alignment frame, and they can be used in a variety of ways to enrich our understanding of protein structure and function. Applications to date have focused primarily on validation of structures, determined by NMR, X-ray crystallography, or homology modeling, and on refinement of structures determined by conventional NMR approaches. Although de novo structure determination on the basis of dipolar couplings suffers from a severe multiple minimum problem, related to the degeneracy of dipolar coupling relative to inversion of the internuclear vector, a number of approaches can address this problem and potentially can accelerate the NMR structure determination process considerably. In favorable cases, where large numbers of dipolar couplings can be measured, inconsistency between measured values can report on internal motions.

Solution NMR structure of S100B bound to the high-affinity target peptide TRTK-12

The solution NMR structure is reported for Ca(2+)-loaded S100B bound to a 12-residue peptide, TRTK-12, from the actin capping protein CapZ (alpha1 or alpha2 subunit, residues 265-276: TRTKIDWNKILS). This peptide was discovered by Dimlich and co-workers by screening a bacteriophage random peptide display library, and it matches exactly the consensus S100B binding sequence ((K/R)(L/I)XWXXIL). As with other S100B target proteins, a calcium-dependent conformational change in S100B is required for TRTK-12 binding. The TRTK-12 peptide is an amphipathic helix (residues W7 to S12) in the S100B-TRTK complex, and helix 4 of S100B is extended by three or four residues upon peptide binding. However, helical TRTK-12 in the S100B-peptide complex is uniquely oriented when compared to the three-dimensional structures of other S100-peptide complexes. The three-dimensional structure of the S100B-TRTK peptide complex illustrates that residues in the S100B binding consensus sequence (K4, I5, W7, I10, L11) are all involved in the S100B-peptide interface, which can explain its orientation in the S100B binding pocket and its relatively high binding affinity. A comparison of the S100B-TRTK peptide structure to the structures of apo- and Ca(2+)-bound S100B illustrates that the binding site of TRTK-12 is buried in apo-S100B, but is exposed in Ca(2+)-bound S100B as necessary to bind the TRTK-12 peptide.

Breaking symmetry in the structure determination of (large) symmetric protein dimers

We demonstrate a novel methodology to disrupt the symmetry in the NMR spectra of homodimers. A paramagnetic probe is introduced sub-stoichiometrically to create an asymmetric system with the paramagnetic probe residing on only one monomer within the dimer. This creates sufficient magnetic anisotropy for resolution of symmetry-related overlapped resonances and, consequently, detection of pseudocontact shifts and residual dipolar couplings specific to each monomeric component. These pseudocontact shifts can be readily incorporated into existing structure refinement calculations and enable determination of monomer orientation within the dimeric protein. This methodology can be widely used for solution structure determination of symmetric dimers.

The crystal structure of metal-free human EF-hand protein S100A3 at 1.7-A resolution

S100A3 is a unique member of the EF-hand superfamily of Ca(2+)-binding proteins. It binds Ca(2+) with poor affinity (K(d) = 4-35 mm) but Zn(2+) with exceptionally high affinity (K(d) = 4 nm). This high affinity for Zn(2+) is attributed to the unusual high Cys content of S100A3. The protein is highly expressed in fast proliferating hair root cells and astrocytoma pointing toward a function in cell cycle control. We determined the crystal structure of the protein at 1.7 A. The high resolution structure revealed a large distortion of the C-terminal canonical EF-hand, which most likely abolishes Ca(2+) binding. The crystal structure of S100A3 allows the prediction of one putative Zn(2+) binding site in the C terminus of each subunit of S100A3 involving Cys and His residues in the coordination of the metal ion. Zn(2+) binding induces a large conformational change in S100A3 perturbing the hydrophobic interface between two S100A3 subunits, as shown by size exclusion chromatography and CD spectroscopy.

A structural basis for S100 protein specificity derived from comparative analysis of apo and Ca(2+)-calcyclin

Calcyclin is a homodimeric protein belonging to the S100 subfamily of EF-hand Ca(2+)-binding proteins, which function in Ca(2+) signal transduction processes. A refined high-resolution solution structure of Ca(2+)-bound rabbit calcyclin has been determined by heteronuclear solution NMR. In order to understand the Ca(2+)-induced structural changes in S100 proteins, in-depth comparative structural analyses were used to compare the apo and Ca(2+)-bound states of calcyclin, the closely related S100B, and the prototypical Ca(2+)-sensor protein calmodulin. Upon Ca(2+) binding, the position and orientation of helix III in the second EF-hand is altered, whereas the rest of the protein, including the dimer interface, remains virtually unchanged. This Ca(2+)-induced structural change is much less drastic than the "opening" of the globular EF-hand domains that occurs in classical Ca(2+) sensors, such as calmodulin. Using homology models of calcyclin based on S100B, a binding site in calcyclin has been proposed for the N-terminal domain of annexin XI and the C-terminal domain of the neuronal calcyclin-binding protein. The structural basis for the specificity of S100 proteins is discussed in terms of the variation in sequence of critical contact residues in the common S100 target-binding site.