Calcium-binding protein from mouse Ehrlich ascites-tumour cells is homologous to human calcyclin

Siah-1-interacting protein regulates mutated huntingtin protein aggregation in Huntington’s disease models

Background

Huntington’s disease (HD) is a neurodegenerative disorder whereby mutated huntingtin protein (mHTT) aggregates when polyglutamine repeats in the N-terminal of mHTT exceeds 36 glutamines (Q). However, the mechanism of this pathology is unknown. Siah1-interacting protein (SIP) acts as an adaptor protein in the ubiquitination complex and mediates degradation of other proteins. We hypothesized that mHTT aggregation depends on the dysregulation of SIP activity in this pathway in HD.

Results

A higher SIP dimer/monomer ratio was observed in the striatum in young YAC128 mice, which overexpress mHTT. We found that SIP interacted with HTT. In a cellular HD model, we found that wildtype SIP increased mHTT ubiquitination, attenuated mHTT protein levels, and decreased HTT aggregation. We predicted mutations that should stabilize SIP dimerization and found that SIP mutant-overexpressing cells formed more stable dimers and had lower activity in facilitating mHTT ubiquitination and preventing exon 1 mHTT aggregation compared with wildtype SIP.

Conclusions

Our data suggest that an increase in SIP dimerization in HD medium spiny neurons leads to a decrease in SIP function in the degradation of mHTT through a ubiquitin–proteasome pathway and consequently an increase in mHTT aggregation. Therefore, SIP could be considered a potential target for anti-HD therapy during the early stage of HD pathology.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00755-0.

Molecular Characteristics of RAGE and Advances in Small-Molecule Inhibitors

Receptor for advanced glycation end-products (RAGE) is a member of the immunoglobulin superfamily. RAGE binds and mediates cellular responses to a range of DAMPs (damage-associated molecular pattern molecules), such as AGEs, HMGB1, and S100/calgranulins, and as an innate immune sensor, can recognize microbial PAMPs (pathogen-associated molecular pattern molecules), including bacterial LPS, bacterial DNA, and viral and parasitic proteins. RAGE and its ligands stimulate the activations of diverse pathways, such as p38MAPK, ERK1/2, Cdc42/Rac, and JNK, and trigger cascades of diverse signaling events that are involved in a wide spectrum of diseases, including diabetes mellitus, inflammatory, vascular and neurodegenerative diseases, atherothrombosis, and cancer. Thus, the targeted inhibition of RAGE or its ligands is considered an important strategy for the treatment of cancer and chronic inflammatory diseases.

Friend or Foe: S100 Proteins in Cancer

S100 proteins are widely expressed small molecular EF-hand calcium-binding proteins of vertebrates, which are involved in numerous cellular processes, such as Ca²⁺ homeostasis, proliferation, apoptosis, differentiation, and inflammation. Although the complex network of S100 signalling is by far not fully deciphered, several S100 family members could be linked to a variety of diseases, such as inflammatory disorders, neurological diseases, and also cancer. The research of the past decades revealed that S100 proteins play a crucial role in the development and progression of many cancer types, such as breast cancer, lung cancer, and melanoma. Hence, S100 family members have also been shown to be promising diagnostic markers and possible novel targets for therapy. However, the current knowledge of S100 proteins is limited and more attention to this unique group of proteins is needed. Therefore, this review article summarises S100 proteins and their relation in different cancer types, while also providing an overview of novel therapeutic strategies for targeting S100 proteins for cancer treatment.

Lysozyme as the anti-proliferative agent to block the interaction between S100A6 and the RAGE V domain

In this report, using NMR and molecular modeling, we have studied the structure of lysozyme-S100A6 complex and the influence of tranilast [N-(3, 4-dimethoxycinnamoyl) anthranilic acid], an antiallergic drug which binds to lysozyme, on lysozyme-S100A6 and S100A6-RAGE complex formation and, finally, on cell proliferation. We have found that tranilast may block the S100A6-lysozyme interaction and enhance binding of S100A6 to RAGE. Using WST1 assay, we have found that lysozyme, most probably by blocking the interaction between S100A6 and RAGE, inhibits cell proliferation while tranilast may reverse this effect by binding to lysozyme. In conclusion, studies presented in this work, describing the protein-protein/-drug interactions, are of great importance for designing new therapies to treat diseases associated with cell proliferation such as cancers.

Binding of S100 proteins to RAGE: an update

The Receptor for Advanced Glycation Endproducts (RAGE) is a multi-ligand receptor of the immunoglobulin family. RAGE interacts with structurally different ligands probably through the oligomerization of the receptor on the cell surface. However, the exact mechanism is unknown. Among RAGE ligands are members of the S100 protein family. S100 proteins are small calcium binding proteins with high structural homology. Several members of the family have been shown to interact with RAGE in vitro or in cell-based assays. Interestingly, many RAGE ligands appear to interact with distinct domains of the extracellular portion of RAGE and to trigger various cellular effects. In this review, we summarize the modes of S100 protein-RAGE interaction with regard to their cellular functions.

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.

CacyBP/SIP, a calcyclin and Siah-1-interacting protein, binds EF-hand proteins of the S100 family

Recently, a human ortholog of mouse calcyclin (S100A6)-binding protein (CacyBP) called SIP (Siah-1-interacting protein) was shown to be a component of a novel ubiquitinylation pathway regulating beta-catenin degradation (Matsuzawa, S., and Reed, J. C. (2001) Mol. Cell 7, 915-926). In murine brain, CacyBP/SIP is expressed at a high level, but S100A6 is expressed at a very low level. Consequently we carried out experiments to determine if CacyBP/SIP binds to other S100 proteins in this tissue. Using CacyBP/SIP affinity chromatography, we found that S100B from the brain extract binds to CacyBP/SIP in a Ca2+-dependent manner. Using a nitrocellulose overlay assay with 125I-CacyBP/SIP and CacyBP/SIP affinity chromatography, we found that this protein binds purified S100A1, S100A6, S100A12, S100B, and S100P but not S100A4, calbindin D(9k), parvalbumin, and calmodulin. The interaction of S100 proteins with CacyBP/SIP occurs via its C-terminal fragment (residues 155-229). Co-immunoprecipitation of CacyBP/SIP with S100B from brain and with S100A6 from Ehrlich ascites tumor cells suggests that these interactions are physiologically relevant and that the ubiquitinylation complex involving CacyBP/SIP might be regulated by S100 proteins.

Characterization of the interaction of calcyclin (S100A6) and calcyclin-binding protein

Calcyclin (S100A6) is an S100 calcium-binding protein whose expression is up-regulated in proliferating and differentiating cells. A novel 30-kDa protein exhibiting calcium-dependent calcyclin-binding (calcyclin-binding protein, CacyBP) had been identified, purified, and cloned previously (Filipek, A., and Kuznicki, J. (1998) J. Neurochem. 70, 1793-1798). Here, we have defined the calcyclin binding region using limited proteolysis and a set of deletion mutants of CacyBP. A fragment encompassing residues 178-229 (CacyBP-(178-229)) was capable of full binding to calcyclin. CacyBP-(178-229) was expressed in Escherichia coli as a glutathione S-transferase fusion protein and purified. The protein fragment cleaved from the glutathione S-transferase fusion protein was shown by CD to contain 5% alpha-helix, 15% beta -sheet, and 81% random coil. Fluorescence spectroscopy was used to determine calcyclin dissociation constants of 0.96 and 1.2 microm for intact CacyBP and CacyBP-(178-229), respectively, indicating that the fragment can be used for characterization of calcyclin-CacyBP interactions. NMR analysis of CacyBP-(178-229) binding-induced changes in the chemical shifts of (15)N-enriched calcyclin revealed that CacyBP binding occurs at a discrete site on calcyclin with micromolar affinity.

Calcyclin (S100A6) binding protein (CacyBP) is highly expressed in brain neurons

The expression of a novel calcyclin (S100A6) binding protein (CacyBP) in different rat tissues was determined by Western and Northern blotting. Polyclonal antibodies against recombinant CacyBP purified from E. coli exhibited the highest reaction in the brain and weaker reaction in liver, spleen, and stomach. CacyBP immunoreactivity was also detected in lung and kidney. Densitometric analysis showed that the concentration of CacyBP in the soluble fractions of total brain and cerebellum is approximately 0.17 and 0. 34 ng/microg protein, respectively. Northern blotting with a specific cDNA probe confirmed the high level of CacyBP expression in the rat brain and lower levels in other tissues examined. Immunohistochemistry and in situ hybridization of rat brain sections revealed strong expression of CacyBP in neurons of the cerebellum, hippocampus, and cortex. The in situ hybridization detected CacyBP in hippocampus as early as P7 (postnatal day 7) and a peak of expression at P21, and the expression signal was preserved until adulthood. In the entorhinal cortex, the peak of expression was observed at P7, whereas in the cerebellum it was seen at P21. The results presented here show that CacyBP is predominantly a neuronal protein. (J Histochem Cytochem 48:1195-1202)

Ca(2+)-dependent association of S100A6 (Calcyclin) with the plasma membrane and the nuclear envelope

Expression of S100A6 (Calcyclin), a member of the S100 family and of Zn(2+)-binding proteins is elevated in a number of malignant tumors. In vitro the protein associates with several actin-binding proteins and annexins in a Ca(2+)-dependent manner. We have now studied the subcellular localization of S100A6 using a new, specific monoclonal antibody. Immunofluorescence microscopy of unfixed, ultrathin, frozen sections demonstrated a dual localization of S100A6 at the nuclear envelope and the plasma membrane of porcine smooth muscle only in the presence of Ca(2+). The same localization was found by immunofluorescence and immunogold electron microscopy as well as by confocal laser scanning microscopy with cultured, fixed, human CaKi-2 and porcine ST interphase cells. Upon cell division, however, S100A6 was found exclusively in the cytoplasm. Cell fractionation studies showed that S100A6 was present in the microsomal fraction in the presence of Ca(2+) and was released from this fraction by the addition of EGTA/EDTA but not by Triton X-100. The data demonstrate that S100A6 is localized both at the plasma membrane and the nuclear envelope in vivo and suggest a Ca(2+)-dependent interaction with annexins or other components of the nuclear envelope.

Identification and initial characterization of calcyclin and phospholipase A2 in equine conceptuses

For development to proceed normally, the appropriate genes must be expressed in the correct tissues and in the correct time frame. Knowledge of gene expression during development provides information about the changes taking place within the conceptus as well as possible reasons for pregnancy failure. However, little is known about gene expression during development in the equine conceptus. In this study, we examined differences in gene expression between day 12 and day 15 equine conceptuses by suppression subtractive hybridization. This technique was used to isolate transcripts that are more abundantly expressed in day 15 conceptuses compared to day 12 conceptuses. Between day 12 and 15 of pregnancy in horses, maternal recognition of pregnancy occurs, gastrulation is taking place, and mesoderm is beginning to form. Fifty cDNA clones were isolated, sequenced, and compared to known sequences in the GenBank database. Two cDNA clones identified that were of primary interest were calcyclin and phospholipase A2. Calcyclin is a calcium-binding protein of the S-100 protein family that has been found in mouse decidua and trophoblast. Calcyclin was found to be expressed in both day 12 and 15 equine conceptuses, with approximately a 30-fold increase in transcript abundance between days 12 and 15. Phospholipase A2 is an enzyme that cleaves phospholipids to release fatty acids and is involved in arachidonic acid release needed for prostaglandin, thromboxane, and leukotriene synthesis. Multiple forms of PLA2, that appear to be differentially regulated in day 12 and 15 conceptuses, were detected by northern blotting.

p30, a novel protein target of mouse calcyclin (S100A6)

A novel protein target of mouse calcyclin (S100A6) was detected by a gel overlay method with 125I-labelled calcyclin. Interaction of calcyclin with its 30 kDa target protein (p30) present in Ehrlich ascites tumour (EAT) cells depended on the presence of Ca2+ ions. The binding of p30, evidenced by the reaction with 125I-labelled calcyclin, was found to be of higher affinity than the binding between mouse calcyclin and annexin II or glyceraldehyde-3-phosphate dehydrogenase. Examination of tissue extracts by the gel overlay method has shown that p30 is present not only in the EAT cells but also in mouse brain and spleen. This novel target protein of mouse calcyclin was purified to homogeneity from EAT cells by means of Phenyl-Sepharose chromatography, affinity chromatography and CM-cellulose chromatography. Purified p30 was digested with alpha-chymotrypsin and a partial amino acid sequence of one of the resulting peptides was established. A database search analysis revealed that the sequence is unique, with a similarity of less than 55% to any other known protein sequence.

1H NMR assignments of apo calcyclin and comparative structural analysis with calbindin D9k and S100 beta

The homodimeric S100 protein calcyclin has been studied in the apo state by two-dimensional 1H NMR spectroscopy. Using a combination of scalar correlation and NOE experiments, sequence-specific 1H NMR assignments were obtained for all but one backbone and > 90% of the side-chain resonances. To our knowledge, the 2 x 90 residue (20 kDa) calcyclin dimer is the largest protein system for which such complete assignments have been made by purely homonuclear methods. Sequential and medium-range NOEs and slowly exchanging backbone amide protons identified directly the four helices and the short antiparallel beta-type interaction between the two binding loops that comprise each subunit of the dimer. Further analysis of NOEs enabled the unambiguous assignment of 556 intrasubunit distance constraints, 24 intrasubunit hydrogen bonding constraints, and 2 x 26 intersubunit distance constraints. The conformation of the monomer subunit was refined by distance geometry and restrained molecular dynamics calculations using the intrasubunit constraints only. Calculation of the dimer structure starting from this conformational ensemble has been reported elsewhere. The extent of structural homology among the apo calcyclin subunit, the monomer subunit of apo S100 beta, and monomeric apo calbindin D9k has been examined in detail by comparing 1H NMR chemical shifts and secondary structures. This analysis was extended to a comprehensive comparison of the three-dimensional structures of the calcyclin monomer subunit and calbindin D9k, which revealed greater similarity in the packing of their hydrophobic cores than was anticipated previously. Together, these results support the hypothesis that all members of the S100 family have similar core structures and similar modes of dimerization. Analysis of the amphiphilicity of Helix IV is used to explain why calbindin D9k is monomeric, but full-length S100 proteins form homodimers.

Characterization of chicken gizzard calcyclin and examination of its interaction with caldesmon

Using a procedure developed to purify calcyclin from mouse Ehrlich ascites tumor cells calcyclin was purified from smooth muscle of chicken gizzard. Chicken gizzard calcyclin bound to phenyl-Sepharose in a calcium dependent manner as did mouse EAT cells and rabbit lung calcyclin but appeared to be more acidic than its mammalian counterparts as revealed by ion exchange chromatography on Mono Q. Chicken gizzard calcyclin bound 45Ca2+ on nitrocellulose filters and exhibited a shift in electrophoretic mobility on urea-PAGE depending on Ca2+ concentration. Crosslinking experiments with BS3 showed that chicken gizzard calcyclin was able to form noncovalent dimers. As indicated by a decrease in maximum tryptophan fluorescence emission of caldesmon (about 14% at 1:1 molar ratio) and displacement of calmodulin from its complex with caldesmon, chicken gizzard calcyclin binds caldesmon. This binding was, however, much weaker than that of calmodulin and could not influence the interaction of caldesmon with actin. In consequence, calcyclin was unable to reverse the inhibitory effect of caldesmon on actin-activated Mg(2+)-ATPase activity of myosin in the presence of Ca2+.

Interaction of calcyclin and its cyanogen bromide fragments with annexin II and glyceraldehyde 3-phosphate dehydrogenase

The structural properties of calcyclin protein are quite well characterized but its function remains obscure. To help elucidate the biological role of calcyclin we have performed the in vitro studies of the Ca(2+)-dependent interaction of Ehrlich ascites tumor cells calcyclin and its cyanogen bromide fragments with two potential calcyclin targets: annexin II and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). The binding of annexin II, evidenced by the reaction with 125I-calcyclin, was found to be very weak and occurred only for intact calcyclin. On the other hand the interaction between calcyclin and GAPDH was of high affinity and could be assigned to the N-terminal region of calcyclin. Intact calcyclin and its N-terminal fragment bound to GAPDH in the gel overlay and affinity chromatography assay. When examined in the presence of a crosslinking agent the interaction resulted in the formation of 46K covalent adduct between calcyclin monomer and GAPDH subunit. Fluorescence of 5-iodoacetamido-fluorescein-labelled calcyclin was efficiently quenched by GAPDH in the presence of Ca2+. Titration experiments revealed the stoichiometry of one calcyclin monomer binding to each of GAPDH subunits with a binding constant of 10(8) M-1. The results of this work suggest that the binding between calcyclin and GAPDH may have bearing on calcyclin function.

The structure of calcyclin reveals a novel homodimeric fold for S100 Ca(2+)-binding proteins

The S100 calcium-binding proteins are implicated as effectors in calcium-mediated signal transduction pathways. The three-dimensional structure of the S100 protein calcyclin has been determined in solution in the apo state by NMR spectroscopy and a computational strategy that incorporates a systematic docking protocol. This structure reveals a symmetric homodimeric fold that is unique among calcium-binding proteins. Dimerization is mediated by hydrophobic contacts from several highly conserved residues, which suggests that the dimer fold identified for calcyclin will serve as a structural paradigm for the S100 subfamily of calcium-binding proteins.

A novel EF-hand Ca(2+)-binding protein from abdominal muscle of crustaceans with similarity to calcyphosine from dog thyroidea

The amino acid sequence of a novel EF-hand Ca(2+)-binding protein from the abdominal muscle of the crayfish, Orconectes limosus, has been elucidated by tandem mass spectrometry and automated Edman degradation. The name CCBP-23 (23-kDa crustacean Ca(2+)-binding protein) is proposed. The protein can also exist as a disulfide-linked homodimer. The sequence of the monomeric form spans 200 residues with an acetylated N-terminal Ser and reveals four EF-hand domains. The 174-mass-unit difference between the calculated average molecular mass of 22,669.6 Da deduced from the sequence and the obtained electrospray ionization mass spectroscopy (ESI-MS) mass of 22,844 Da has not yet been explained. Partial sequence analysis (137 residues) of CCBP-23 from the lobster, Homarus americanus, showed a sequence identity of 74% with the crayfish protein. Homology searches revealed a 44% sequence identity of CCBP-23 from crayfish to calcyphosine, a Ca(2+)-binding protein from dog thyroidea (Lefort et al., 1989). Although CCBP-23 also shows a 44% identity to R2D5 antigen (Nemoto et al., 1993), we believe that both proteins represent two distinct subgroups within the family of EF-hand proteins.

Gene expression and protein localisation of calcyclin, a calcium-binding protein of the S-100 family in fresh neuroblastomas

Calcyclin gene, a Ca(2+)-binding protein with homology to S-100, has been found to be expressed at different levels in leukaemic cells and in other tumour cells. We recently reported the expression of the gene in human neuroblastoma (NB) cell lines, and suggested a possible role of calcyclin in cell differentiation. To extend our findings, we investigated the expression of the gene in NB cells induced to differentiate by retinoic acid (RA), using the reverse transcriptase-polymerase chain reaction (RT-PCR) technique. Time-course experiments employing LA-N-5 cells showed that calcyclin mRNA appeared 2 h after RA treatment, long before the cells were blocked in the G1 cell-cycle phase and before the neurite-like structures outgrew from the cell bodies. This suggests the involvement of the gene in the early phase of cell differentiation. Furthermore, we investigated mRNA expression in a series of fresh neuroblastomas. NB tumours showed a heterogeneous pattern of calcyclin expression, although calcyclin seemed to be expressed more frequently in cases with a favourable Shimada histology. We also studied the expression of the protein in formalin fixed and paraffin embedded tissues, by using a specific anticalcyclin antibody. The protein was detected in stromal cells which characterise a more mature histological type, and in nerve sheaths, whereas neuroblasts were negative. The tissue that expressed calcyclin protein showed a Schwann-like differentiation and, unlike S-100 protein, calcyclin was expressed in the perineurium.

Calcyclin from mouse Ehrlich ascites tumor cells and rabbit lung form non-covalent dimers

Crosslinking treatments of fresh cytosol from mouse Ehrlich ascites tumor (EAT) cells revealed the existence of calcyclin dimers which were sensitive to SDS, but not to reducing agents, which suggests the existence of non-covalent dimers. In stored EAT cell cytosol and preparations of purified calcyclin dimers were also formed by S-S bridging (covalent dimers). The S-S dimers did not bind to organomercurial Agarose and could be separated from reduced forms of calcyclin that bound to the resin. Calcyclin eluted from the resin with DTT was a mixture of monomers and non-covalent dimers as shown by crosslinking and subsequent immunoblotting. Calcyclin from rabbit lung, lacking a cysteine residue, could also be crosslinked as a dimer. It is suggested that the ability of calcyclin to form non-covalent dimers is of physiological significance.

Characterization of calcyclin fragments obtained by CNBr-cleavage

1. Two calcyclin fragments were obtained by CNBr-cleavage. 2. One fragment represented N-terminal end of a molecule (residues 1-56), and another one a C-terminal end (residues 57-89). 3. Properties of intact calcyclin such as binding of calcium, binding to hydrophobic resins and interaction with calcyclin specific antibodies were not retained by these fragments. 4. However, both fragments were able to form dimers and higher forms of aggregates as seen for uncleaved calcyclin. 5. This indicates that both halves of the molecule contain the regions responsible for non-covalent interaction which might participate in dimer formation.

Identification of annexin II, annexin VI and glyceraldehyde-3-phosphate dehydrogenase as calcyclin-binding proteins in bovine heart

1. Matrix-immobilized calcyclin as affinity ligand in chromatography led to purification of three protein bands at 68, 36 and 35 kDa from bovine heart that required Ca2+ for binding. 2. Polyacrylamide-immobilized phosphatidylserine separated this fraction into a phospholipid-binding part (68 kDa, 35 kDa), also attaching to phospholipid vesicles even in the presence of calcyclin, and a flow-through part, constituting approx 30% of the total fraction (36 kDa). 3. Enzyme assays and electrophoretic mobility showed an at least close relationship of the 36 kDa band to glyceraldehyde-3-phosphate dehydrogenase. Interaction between enzyme and calcyclin in a solid-phase assay was inhibited by sialoglycoproteins and depended strongly on the integrity of carboxyl and hydrophobic groups of the enzyme. The interaction between the two proteins had a KD value of 110 nM. 4. Application of annexin-specific antibodies revealed an immunological relationship of the 35 and 68 kDa calcyclin-binding proteins to members of the annexin family, namely to annexin II (35 kDa) and annexin VI (68 kDa). The N-terminal amino acid sequence of a cleavage peptide of the 68 kDa protein was identical to a sequence stretch in human annexin VI, corroborating this evidence.

Calcyclin as a marker of human epithelial cells and fibroblasts

The distribution of calcyclin in human tissues was studied using polyclonal antibodies against this protein. In all organs examined (breast, heart, intestine, kidney, liver, ovary, placenta, stomach, thymus, and uterus) only epithelial cells and fibroblasts were stained. This suggests that calcyclin expression is related either to proliferation rate or secretion activity. The data show that calcyclin might be considered as a marker of some human epithelial cells and fibroblasts.

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.

Carbohydrate-binding specificity of calcyclin and its expression in human tissues and leukemic cells

Binding of biotinylated fetuin in a solid-phase assay served as activity assay for purification of calcyclin, the product of a cell growth-related cDNA with homologies to Ca(2+)-binding proteins. Asialofetuin failed to bind to calcyclin, emphasizing the importance of sialic acids. Binding of fetuin was most effectively reduced by N-glycolylneuraminic acid within a panel of mostly negatively charged sugars. Bovine submaxillary mucin and the ganglioside GM1, but not asialo-GM1, proved more effective than neoglycoproteins, carrying negatively charged carbohydrate moieties. Extension of N-acetyl-neuraminic acid to its lactosyl derivative increased its inhibitory potency. Among charge-free carbohydrate residues, only N-acetylglucosamine, lactose, and mannose, but not fucose, melibiose, or N-acetylgalactosamine affected fetuin binding, substantiating the inherent selectivity. Chemical modification with group-specific reagents revealed that lysine and arginine residues appear to be involved in ligand binding that is optimal in the presence of Ca2+, but not Zn2+ and stable up to 1 m NaCl. Biotinylation of calcyclin by modification of carboxyl groups facilitated performance of solid-phase assays with calcyclin in solution, yielding similar results with (neo)glycoproteins in relation to assays with immobilized calcyclin, thereby excluding an impact of binding to nitrocellulose on calcyclin's specificity. Subcellular fractionation disclosed the presence of fetuin-binding activity in all fractions, the specific activity decreasing from the nuclear to the particulate cytoplasmic fraction and the cytoplasmic supernatant. Affinity-purified antibodies were employed to detect high levels of calcyclin expression in acute lymphoblastic, myelogenous, and monocytic leukemia cell lines, but not in myeloma or lymphoblastoid cells. In comparison, most cells were nearly devoid of an O-acetylsialic acid-specific protein that is more abundant in various tissue types than calcyclin.

Ca(2+)-dependent regulation of vascular smooth-muscle caldesmon by S.100 and related smooth-muscle proteins

1. We have investigated the ability of bovine brain S.100, and of three related proteins from sheep aorta smooth muscle, to confer Ca(2+)-sensitivity on thin filaments reconstituted from smooth-muscle actin, tropomyosin and caldesmon. 2. At 37 degrees C in pH 7.0 buffer containing 120 mM-KCl, approximately stoichiometric amounts of S.100 reversed caldesmon's inhibition of the activation of myosin MgATPase by smooth-muscle actin-tropomyosin. The [S.100] which reversed by 50% the inhibition by caldesmon (the E.C.50) was 2.5 microM when [caldesmon] = 2-3 microM in the assay mixture. When [KCl] was decreased to 70 mM, E.C.50 = 11.5 microM; at 25 degrees C in 70 mM-KCl, up to 20 microM-S.100 had no effect. When skeletal-muscle actin rather than smooth-muscle actin was used to reconstitute thin filaments, 20 microM-S.100 did reverse inhibition by caldesmon, at 25 degrees C in buffer containing 70 mM-KCl. This dependence on conditions is also characteristic of the calmodulin-caldesmon interaction. 3. These results suggested that S.100 or a related protein might interact with caldesmon in smooth muscle. We therefore attempted to prepare such a protein from sheep aorta. Three proteins were purified: an Mr-17,000 protein (yield 16 mg/kg), an abundant Mr-11,000 protein (yield 48 mg/kg), and an Mr-9000 protein (yield 4 mg/kg). Neither of the last two low-Mr proteins had any effect on activation of myosin MgATPase by reconstituted thin filaments. The protein of Mr 17,000 had Ca(2+)-sensitizing activity, and behaved exactly like brain calmodulin in the assay system.

Characterization of the cell-cycle-regulated protein calcyclin from Ehrlich ascites tumor cells. Identification of two binding proteins obtained by Ca2(+)-dependent affinity chromatography

The nearly complete amino acid sequence obtained for murine calcyclin from Ehrlich ascites tumor cells reveals a very strong similarity with the rat and human sequences previously deduced from corresponding cDNA clones. While mouse and rat calcyclins are identical, the human protein shows at three positions a conservative amino acid replacement. Using a mouse calcyclin affinity matrix, two proteins with molecular masses of about 36 kDa have been purified from Ehrlich ascites tumor cells. The interaction between these two proteins and the immobilized calcyclin is strictly Ca2(+)-dependent. Immunological criteria and partial sequence data identify the two calcyclin-binding proteins as the phospholipid-binding protein annexin II (p36) and the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase. These observations suggest that calcyclin may exert its physiological function by a Ca2(+)-dependent interaction with cellular targets, e.g. annexin II or glyceraldehyde-3-phosphate dehydrogenase.

Calcyclin is a calcium and zinc binding protein

Calcyclin, a cell cycle regulated protein, was recently purified from Ehrlich ascites tumour (EAT) cells and shown to be a calcium binding protein. Here we show that calcyclin monomer and dimer also bind zinc ions. Zinc binding sites seem to be different from calcium binding sites since: preincubation with Ca2+ lacks effect on the binding of Zn2+, and Ca2+ (but not Zn2+) increases tyrosine fluorescence intensity. Binding of Zn2+ reduces the extent of the conformational changes induced by Ca2+, and seems to affect Ca2(+)-binding. The data suggest that Ca2+ and Zn2+ might trigger the biological activity of calcyclin.