Isolation and spectral studies on the calcium binding properties of bovine brain S-100a protein

A Novel Regulator of Telomerase

Recently we reported a differentiation-dependent inhibition of telomerase activity in human epidermis. Consistent with this observation we found that in keratinocyte cultures calcium-induced differentiation correlates with a decline in telomerase activity. To get further support for a role of calcium in the regulation of telomerase and to elucidate the underlying molecular mechanisms we investigated the effect of calcium on telomerase in the human epidermal keratinocyte line HaCaT. Treatment with thapsigargin, which increases intracellular calcium concentrations, inhibited telomerase activity without down-regulating the expression of hTERT (human telomerase reverse transcriptase). This observation together with the fact that increasing calcium reduced telomerase activity in cell-free extracts suggests that calcium directly interacts with the telomerase complex. This interaction could be mediated by the calcium-binding protein S100A8 as indicated by its ability to mimic the inhibitory effect of calcium. S100A8-induced reduction in telomerase activity was abrogated by S100A9. The ratio of both proteins remained constant in cells treated with thapsigargin, but their interactions were altered similarly in intact cells after thapsigargin treatment and in cell-free extracts in response to calcium. We hypothesize that calcium binds to S100A8/S100A9 complexes and alters their composition, thus enabling S100A8 to interact with the telomerase complex and inhibit its activity.

Time-resolved fluorescence of S-100a protein: effect of Ca2+, Mg2+ and unilamellar vesicles of egg phosphatidylcholine

Phase-modulation fluorescence lifetime measurements were used to study the single Trp residue of the Ca(2+)-binding protein S-100a both in the absence and in the presence of Ca2+ and/or Mg2+. Trp fluorescence decay for the protein was satisfactorily described by Lorentzian lifetime distributions centered around two components (approximately 4 ns and 0.5 ns). Lifetime values were unchanged by 2 mM Ca2+, but the fractional intensity associated with longer lifetime increased up to 75%. In the presence of Mg2+, the Ca2+ induced increase of the fractional intensity associated with longer lifetime was only 57%. For the protein in buffer, about the 85% of the recovered anisotropy was associated to a rotational correlation time of 6.7 ns. After the addition of Ca2+, this value was increased to 16.08 ns. In the presence of Mg2+, Ca+2 increased the rotational correlation time to 33.75 ns. Similar studies were performed with S-100a interacting with egg phosphatidylcholine vesicles (SUV). Our data suggest that the conformation of the protein may be influenced by structural features of the lipidic membrane. Moreover, data obtained in the presence of Mg2+ indicate some interaction between lipids and S-100, likely mediated by this ion.

Human S100b protein: formation of a tetramer from synthetic calcium-binding site peptides

Human brain S100b protein is a unique calcium-binding protein comprised of two identical 91-amino acid polypeptide chains that each contain two proposed helix-loop-helix (EF-hand) calcium-binding sites. In order to probe the assembly of the four calcium-binding sites in S100b, a peptide comprised of the N-terminal 46 residues of S100b protein was synthesized and studied by CD and 1H NMR spectroscopies as a function of concentration and temperature. At relatively high peptide concentrations and in the absence of calcium, the peptide exhibited a significant proportion of alpha-helix (45%). Decreasing the peptide concentration led to a loss of alpha-helix as monitored by CD spectroscopy and coincident changes in the 1H NMR spectrum. These changes were also observed by 1H NMR spectroscopy as a function of temperature where it was observed that the Tm of the peptide was lowered approximately 14 degrees C with a 17-fold decrease in peptide concentration. Sedimentation equilibrium studies were used to determine that the peptide formed a tetramer in solution in the absence of calcium. It is proposed that this tetrameric fold also occurs in S100b and is a result of the interaction of portions of all four calcium-binding sites.

Characterization of two distinct calcium-binding sites in the amino-terminus of human profilaggrin

Profilaggrin is a large phosphorylated protein (approximately 400 kDa in humans) that is expressed in the granular cells of epidermis where it forms a major component of keratohyalin. It consists of multiple copies of similar filaggrin units plus amino- and carboxy-terminal domains that differ from filaggrin. Proteolytic processing of profilaggrin during terminal differentiation results in the removal of these domains and generation of monomeric filaggrin units, which associate with keratin intermediate filaments to form macrofibrils in the stratum corneum. The amino-terminal domain contains two calcium-binding motifs similar to the EF-hands found in the S-100 family of calcium-binding proteins. In this report, we expressed the 293-residue amino-terminal pro-domain of human profilaggrin as a polyhistidine fusion protein in Escherichia coli, and characterized calcium binding by a 45Ca++ binding assay and fluorescence emission spectroscopy. Fluorescence measurements indicated that the profilaggrin polypeptide undergoes conformational changes upon the removal of Ca++ with ethylenediamine tetraacetic acid, demonstrating the presence of two calcium-binding sites with affinities for calcium that differ ninefold (1.4 x 10(-4) M and 1.2 x 10(-3) M). We suggest that this functional calcium-binding domain at the amino-terminus of human profilaggrin plays a role in profilaggrin processing and in other calcium-dependent processes during terminal differentiation of the epidermis.

The S100 protein family: history, function, and expression

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

Calcium-dependent regulation of the caldesmon-heavy meromyosin interaction by caltropin

The binding of chicken gizzard caldesmon to smooth muscle heavy meromyosin (HMM) was studied using caldesmon-Sepharose 4B affinity chromatography, far-ultraviolet circular dichroism (CD), and the fluorescent probe acrylodan. When HMM was applied to a caldesmon-Sepharose column in the presence of 40 mM NaCl, most of the protein was retained on the column, and HMM could be eluted by increasing the NaCl level to 0.5 M; this interaction was not Ca(2+)-dependent. Far-UV CD studies indicated an interaction between caldesmon and HMM since the experimentally observed ellipticity values at 222 and 207 nm deviated from the theoretical values for the complex, and this interaction was also not Ca(2+)-sensitive. Addition of HMM to a caldesmon-caltropin complex induced a conformational change suggesting the formation of a ternary complex for which Ca2+ was essential. Acrylodan-labeled caldesmon, when excited at 375 nm, had an emission maximum at 515 +/- 2 nm. Addition of HMM resulted in a nearly 20% decrease in fluorescence intensity with little or no shift in the emission maximum. Titration of HMM with labeled caldesmon indicated a strong affinity for HMM [K(a) was on the order of (4.5 +/- 0.5) x 10(7) M-1], and this interaction was observed both in the presence and in the absence of calcium. When HMM was titrated with labeled caldesmon in the presence of caltropin in a 0.2 mM Ca2+ medium, its affinity for caldesmon was lowered nearly 3-fold [K(a) approximately (1.50 +/- 0.5) x 10(7) M-1].(ABSTRACT TRUNCATED AT 250 WORDS)

Time-resolved fluorescence of S-100a protein in the absence and presence of calcium and phospholipids

We have used phase-modulation fluorescence lifetime measurements to study the single Trp residue of the Ca(2+)-binding protein S-100a. Trp fluorescence decay was not exponential for the protein irrespective of the absence or presence of Ca2+. Fluorescence decay was best described by Lorentzian lifetime distributions centered around two components (approx. 3 and 0.7 ns) for protein in absence of Ca2+ and one component (approx. 2.9 ns) for the protein in presence of 2 mM Ca2+. Similar studies were performed with S-100a interacting with cardiolipin, phosphatidylserine or egg phosphatidylcholine, both in absence and in presence of 2 mM Ca2+. Our data suggest that the conformation of the protein and its Ca(2+)-binding properties vary depending on the characteristics of charge and structure of phospholipids.

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

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

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

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

Isolation and characterization of a novel molecular weight 11,000 Ca2(+)-binding protein from smooth muscle

A new low molecular weight calcium-binding protein, designated as SMCaBP-11, has been isolated from chicken gizzard using a phenyl-Sepharose affinity column followed by ion-exchange and gel filtration chromatographies. The isolated protein was homogeneous by the criteria of gel electrophoresis in the absence and presence of sodium dodecyl sulfate (NaDodSO4). Molecular weight studies by both sedimentation equilibrium in 6 M guanidine hydrochloride and 15% polyacrylamide-SDS gels indicated the subunit molecular weight to be 11,000, and since a molecular weight of 21,000 was obtained in native solvents, the protein exists as a dimer in benign medium. The amino acid composition of this protein is similar but distinct from other known low molecular weight Ca2(+)-binding proteins. Ca2(+)-binding assays using Arsenazo III (Sigma) indicated the protein to bind 2 mol of Ca2+/subunit. In non-SDS gels, the protein moved faster in the presence of EDTA, suggesting that Ca2+ binding affects its mobility in a manner similar to other smooth muscle calcium-binding proteins such as calmodulin and 67-kDa calcimedin. Upon binding calcium, the protein underwent a conformational change as revealed by UV difference spectroscopy and circular dichroism studies in the aromatic and far-ultraviolet range. When the protein was excited at 280 nm, the tyrosine fluorescence emission maximum was centered at 306 nm. Ca2+ addition resulted in a nearly 15% decrease in intrinsic fluorescence intensity. Fluorescence titration with Ca2+ exhibited two classes of calcium-binding sites with Kd values of 0.2 and 80 microM, in agreement with UV difference spectral data.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of glutathione S-transferase, placental type in T9 glioma cells by dibutyryladenosine 3',5'-cyclic monophosphate and modification of its expression by naturally occurring isothiocyanates

The effect of dibutyryladenosine 3',5'-cyclic monophosphate (dibutyryl cAMP) on the expression of glutathione S-transferase placental type (GST-P) was examined in rat glioma cell line using an immunohistochemical technique. Cultured T9 glioma cells were negative for GST-P activity under normal conditions. However, treatment with 1 mM dibutyryl cAMP produced GST-P expression in about 50% of the cells, as well as some morphological changes. The expression of GST-P was increased with addition of dibutyryl cAMP together with 1 microgram/ml allyl isothiocyanate (AITC) or 0.1 microgram/ml benzyl isothiocyanate (BITC). With these combinated treatments, almost all cultured cells showed a strong positive reaction for GST-P, although AITC or BITC alone elicited GST-P in only 5% of the cultured cells. The results of the present study indicate that dibutyryl cAMP causes functional as well as morphological differentiation of T9 glioma cells.

Effects of Ca2+ and Zn2+ on trifluoperazine-S100 proteins interactions: induced circular dichroism and fluorescence spectra

Interactions of trifluoperazine (TFP) with S100 proteins, EF-hand type Ca2+-binding proteins, in the presence of Ca2+ and Zn2+ were studied with induced circular dichroism (CD) and fluorescence spectra. The positive CD bands of TFP were induced at around 265 nm by adding either S100a or S100a0 protein in the presence of Ca2+. No CD band of TFP was, however, induced by adding S100b protein in the presence of Ca2+. Addition of Zn2+ to the TFP/S100 protein solutions did not induce any CD band at all. The fluorescence intensity of 2-p-toluidinylnaphthalene 6-sulfonate (TNS) bound to S100a or S100a0 protein decreased by adding TFP in the presence of Ca2+, while that bound to S100b protein decreased by adding TFP in the presence of Zn2+, indicating that TFP binds to S100a protein and S100a0 protein in a Ca2+-dependent manner and to S100b protein in a Zn2+-dependent manner. From these results together with other experimental findings it was suggested that (1) TFP binds to S100a protein and S100a0 protein in the presence of Ca2+, with half-saturation points of 18 and 3 microM, respectively, (2) TFP binds to S100b protein only in the presence of Zn2+, (3) alpha-subunit of S100 protein binds to TFP specifically in a Ca2+-dependent manner and beta-subunit in a Zn2+-dependent manner.

Purification and spectral studies on the Ca2+-binding properties of 67 kDa calcimedin

The 67 kDa calcimedin, isolated by using a phenyl-Sepharose affinity column followed by DEAE-cellulose and gel-filtration chromatographies, was homogeneous by the criterion of SDS/polyacrylamide-gel electrophoresis. In non-SDS gels, the protein moved faster in the presence of EDTA, suggesting that Ca2+ binding affects its mobility in a manner similar to other Ca2+-binding proteins such as calmodulin and S-100 proteins. The 67 kDa protein underwent a conformational change upon binding Ca2+, as revealed by u.v. difference spectroscopy and near-u.v. c.d. measurements. Tryptophan and tyrosine residues were perturbed upon Ca2+ binding, moving to a more non-polar environment in the presence of Ca2+. Upon excitation of the protein at 280 nm, the fluorescence emission maximum was centered around 325 nm, suggesting that the tryptophan residues are located in a fairly hydrophobic region. Ca2+ addition did not induce a significant change in the intrinsic protein fluorescence intensity at 325 nm. Addition of Ca2+ to the 67 kDa protein labelled with 2-p-toluidinylnaphthalene-6-sulphone (TNS) resulted in a 25% increase in fluorescence intensity, accompanied by a blue shift of the emission maximum from 442 to 432 nm. Hence, the probe in the presence of Ca2+ moves to a more non-polar microenvironment, like calmodulin and other Ca2+-binding proteins. Fluorescence titration with Ca2+ using TNS-labelled protein revealed one class of binding site, with a Kd value of 2 x 10(-5) M.

Isolation of a new member of the S100 protein family: amino acid sequence, tissue, and subcellular distribution

A low molecular mass protein which we term S100L was isolated from bovine lung. S100L possesses many of the properties of brain S100 such as self association, Ca++-binding (2 sites per subunit) with moderate affinity, and exposure of a hydrophobic site upon Ca++-saturation. Antibodies to brain S100 proteins, however, do not cross react with S100L. Tryptic peptides derived from S100L were sequenced revealing similarity to other members of the S100 family. Oligonucleotide probes based on these sequences were used to screen a cDNA library derived from a bovine kidney cell line (MDBK). A 562-nucleotide cDNA was sequenced and found to contain the complete coding region of S100L. The predicted amino acid sequence displays striking similarity, yet is clearly distinct from other members of the S100 protein family. Polyclonal and monoclonal antibodies were raised against S100L and used to determine the tissue and subcellular distribution of this molecule. The S100L protein is expressed at high levels in bovine kidney and lung tissue, low levels in brain and intestine, with intermediate levels in muscle. The MDBK cell line was found to contain both S100L and the calpactin light chain, another member of this protein family. S100L was not found associated with a higher molecular mass subunit in MDBK cells while the calpactin light chain was tightly bound to the calpactin heavy chain. Double label immunofluorescence microscopy confirmed the observation that the calpactin light chain and S100L have a different distribution in these cells.

Spectroscopic studies on Tb3+ binding to S-100a protein

Direct binding assay and fluorescence studies revealed that S-100a protein binds 2 mol of Tb3+/mol of protein at pH 6.6. The protein binds Tb3+ much more tightly than Ca2+, and the upper limit of the observed Kd value for Tb3+ is 3.5 x 10(-6) M. The Tb3+-binding site on the protein must be close to a tyrosine residue, as indicated by fluorescence excitation and emission spectra, where energy transfer from tyrosine is noted. Addition of Tb3+ resulted in a conformational change in the protein, as revealed by u.v.-difference spectroscopy and c.d. studies. Far-u.v. c.d. studies indicated the helical content to decrease from approx. 39% to 35% in the presence of Tb3+. From u.v.-difference-spectroscopy results the single tryptophan and the tyrosine chromophores in S-100a protein are blue-shifted (i.e. exposed to the solvent) in the presence of Tb3+ and the observed conformational changes are similar to those induced by Ca2+, suggesting that Tb3+ can be employed as a Ca2+ analogue in spectral studies with S-100a protein.

Fluorescence studies on the Ca2+ and Zn2+ binding properties of the alpha-subunit of bovine brain S-100a protein

The single cysteine on the alpha-subunit of bovine brain S-100a protein has been modified with the thiol specific probe, Acrylodan. When the labelled apoprotein was excited at 380 nm the fluorescence emission maximum was centered at 484 +/- 2 nm, suggesting that the probe is in a fairly hydrophobic environment. Addition of Ca2+ to the protein caused the emission maximum to undergo a red shift to 504 +/- 2 nm, implying that the fluorophore is now more exposed to the solvent. Zn2+, when added to the protein, induced only a small perturbation and the emission maximum shifted to 481 +/- 2 nm. Ca2+ was able to perturb the fluorophore in the presence of Zn2+. 2-p-Toluidinylnaphthalene-6-sulfonate (TNS)-labelled alpha-subunit when excited at 345 nm exhibited very little fluorescence in the absence of Ca2+. Addition of Ca2+ resulted in an increase in TNS fluorescence accompanied by a blue shift of the emission maximum to 445 +/- 1 nm indicating that the probe in the presence of Ca2+ moves to a hydrophobic domain. The fact that Ca2+ and Zn2+ can perturb the labelled sulfhydryl group in the presence of each other clearly demonstrates that the binding sites for the two metal ions must be different on the alpha-subunit as well as on the S-100a protein.

Increase in S-100b protein content in thyroid carcinoma

S-100b protein was detectable in the soluble fraction of thyroid tissue. The concentration of S-100b protein in thyroid carcinoma tissue was three to five times higher than in normal thyroid tissue and thyroid adenoma. It is, however, not higher in the thyroid tissue of Graves' disease. The increase of S-100b protein concentration was not remarkable in carcinomatous tissue of the stomach and other digestive organs. The calmodulin content in the thyroid carcinoma tissue increased but the increment was low compared to that of S-100b protein. These data suggest that S-100b protein may play a significant role in cell maturation or differentiation.

Immunocytochemical localization of S-100 protein in astrocytes and Müller cells in the rabbit retina

The localization of S-100 protein was studied in histological sections of retinae from adult rabbits. By use of double-immunolabeling techniques it was shown that most but not all radially oriented vimentin-positive Müller cells were co-labeled by an antiserum to S-100 protein. Glial fibrillary acidic protein-positive astrocytes, which in the rabbit retina are restricted to the medullary rays formed by myelinated optic nerve fibers, consistently showed S-100 protein immunoreactivity. The present report shows that, with respect to S-100 protein staining, Müller cells represent a heterogeneous population of glial elements.

Ca2+ and Zn2+-binding properties of nitrated S-100b protein from bovine brain

The single tyrosine residue in S-100b protein was nitrated by treatment with tetranitromethane in 0.1 M-Tris/HCl buffer, pH 8.0, containing 2 mM-EDTA. The nitrated protein did not differ significantly in secondary structure from its native unmodified counterpart, as revealed by far-u.v. c.d. measurements. The effect of Ca2+ on the modified protein was different from that on the native protein, e.g. addition of Ca2+ resulted in a loss of helical content from 55 to 47% with the native protein whereas Ca2+ had no significant effect on the gross conformation of the nitrated derivative. Near-u.v. c.d. studies also indicated a very minimal effect on the tyrosine residue and this was also reflected in the u.v.-absorption difference spectrum. Polyacrylamide-gel electrophoresis in the absence of SDS showed the nitrated S-100b to move faster in the presence of EDTA compared with the calcium-bound state, suggesting that the modified protein does bind Ca2+ although it does not undergo a major conformational change in response to Ca2+ addition. In contradistinction, Zn2+ binding was not influenced by nitration, as demonstrated by aromatic c.d. and u.v.-difference spectroscopy. It is clear from this study that the single tyrosine residue in S-100b is critical to sense the Ca2+-induced conformational changes in the protein.

Isolation, characterization and metal-ion-binding properties of the alpha-subunit from S-100a protein

The brain-specific S-100 protein is a mixture of two predominant components, S-100a and S-100b, with subunit compositions of alpha beta and beta beta respectively. In the present study, the alpha-subunit, isolated from S-100a by using anion-exchange chromatography in the presence of 8 M-urea, was homogeneous by the criteria of SDS/polyacrylamide-gel, urea/SDS/polyacrylamide-gel and non-SDS/polyacrylamide-gel electrophoresis. The alpha-subunit underwent a conformational change upon binding Ca2+ and Zn2+ at pH 7.5, as revealed by u.v. difference spectroscopy, c.d. and fluorescence measurements. Far-u.v. c.d. studies indicated the apparent alpha-helical content to fall when the protein bound either Ca2+ or Zn2+. Addition of Ca2+ to the alpha-subunit resulted in exposing to the solvent the single tryptophan residue and one or more tyrosine and phenylalanine residues. Zn2+ induced only a small conformational change, and among the aromatic chromophores only tyrosine residues were affected to a small extent. Ca2+ was able to bind to the alpha-subunit in the presence of Zn2+, and the two metal-ion-binding sites appeared to be different. When the apoprotein was excited at 280 nm, the fluorescence emission maximum was located at 337 nm. In the presence of Ca2+, the emission maximum occurred at 340 nm and was accompanied by a nearly 25% increase in fluorescence intensity. Fluorescence titration with Ca2+ at pH 7.5 revealed only one class of binding site, with a Kd value of 1.26 X 10(-4) M. The effect of K+ on the protein was slightly antagonistic to that of Ca2+, as indicated by u.v. difference spectroscopy and fluorescence titration.

Characterization of the Ca2+-induced conformational changes in gelsolin and identification of interaction regions between actin and gelsolin

Serum gelsolin, a Ca2+-dependent protein regulating the length of actin filaments, undergoes conformational changes upon binding Ca2+. These were detected and analyzed by several approaches including ultraviolet difference spectroscopy, circular dichroism studies, analytical ultracentrifugation, thiol group titration, and limited proteolytic digestions. The effect of Ca2+ binding on the UV absorption difference spectrum and the near-UV circular dichroism spectrum was consistent with changes in the environments of tyrosine and phenylalanine residues. In the presence of Ca2+, the S0(20),w value decreased from 5.3 to 4.7. This latter result implies a transformation to a more asymmetric molecular shape. Gelsolin contained only two accessible thiol groups per mole of protein, one of which was titratable in the native protein; it was more accessible to 5,5'-dithiobis(2-nitrobenzoic acid) in the absence than in the presence of Ca2+. The limited digestion of gelsolin from serum and bovine aorta smooth muscle by two different proteases, chymotrypsin and trypsin, proceeded much faster in the presence of Ca2+ than in its absence with the production of three main fragments of about 40K, 32K, and 21K. This fragment mixture was found still able to shorten F-actin in a Ca2+-dependent manner; this severing activity was expressed by the isolated 40K peptide. Gelsolin was cross-linked to F- and G-actin by the zero-length cross-linker 1-ethyl-3-[3-(dimethylamino)propyl]-carbodiimide (EDC), generating a covalent 130K binary complex (actin1-gelsolin1) followed by a covalent 180K ternary complex (actin2-gelsolin1).(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical co-localization of S-100b and the glial fibrillary acidic protein in rat brain

A rabbit antiserum against purified bovine brain S-100b protein was produced and characterized by immunoassay and immunoblot analysis of electrophoretically resolved soluble brain proteins. Fluorescent immunohistochemistry was conducted in order to determine the cellular localization of the S-100b immunoreactivity. Double immunohistofluorescent experiments on adult rat brain tissue sections with the rabbit antiserum to S-100b and a rat monoclonal antibody to the glial fibrillary acidic protein resulted in immunolabelling of the same cells. This finding determines a strict astroglial localization of the S-100b immunoreactivity. In addition, the immunolabelling of astrocyte perikarya and processes by the S-100b immunohistochemistry is consistent with a cytoplasmic location of S-100b. In contrast, the glial fibrillary acidic protein immunohistochemistry predominantly labeled the fine fibrillary processes of the cells. The present report suggests that S-100b immunohistochemistry is of use for the specific identification and morphological description of astrocytes in the rat brain.

Evidence that S100 proteins regulate microtubule assembly and stability in rat brain extracts

Microtubule re-assembly in rat brain extracts was inhibited by antibodies to S100 proteins. Anti-S100 antibodies caused an increase in the cold-stability of microtubules and this effect was abolished by the presence of short lengths of microtubules formed under control conditions. Anti-S100 antibodies had no effect on the stimulation of assembly or the increase in microtubule stability caused by low zinc concentrations. Addition of exogenous S100a and S100b to brain extracts had different effects on assembly; S100a caused an inhibition of assembly while S100b stimulated the early phase of assembly. The data suggest that endogenous S100b is involved in the regulation of microtubule assembly in brain extracts.

Physicochemical properties of a novel Mr-21 000 Ca2+-binding protein of bovine brain

The physicochemical properties of a novel Mr-21 000 Ca2+-binding protein isolated from bovine brain were investigated. The protein exhibited a partial specific volume of 0.724 ml/g, a degree of hydration of 0.47 g of water/g of protein and a mean residue weight of 119. Sedimentation equilibrium analysis revealed Mr = 22 600 in the absence of Ca2+; Ca2+ binding appeared to induce dimerization of the molecule. Size-exclusion chromatography indicated a compacting of the molecule on binding of Ca2+: the Stokes radius decreased from 2.75 nm in the absence of Ca2+ to 2.56 nm in its presence. Far-u.v.c.d. spectroscopy showed the apoprotein to be composed of 44% alpha-helix, 18% beta-pleated sheet and 38% random coil. Addition of either KCl (0.1 M) plus Mg2+ (1 mM), or Ca2+ (2 mM), changed the conformation to 49% alpha-helix, 18% beta-pleated sheet and 33% random coil. Near-u.v.c.d. and u.v. difference spectroscopy both indicated perturbations in the environments of all three types of aromatic amino acids on binding of Ca2+. Ca2+ binding also resulted in a 30% enhancement in the tryptophan fluorescence emission intensity. Ca2+ titration of the far-u.v.c.d. and fluorescence enhancement provided KD values of 9.91 microM and 4.68 microM respectively. Finally, the protein was shown to bind Zn2+ with KD = 1.44 microM (no Mg2+) and 1.82 microM (+ Mg2+). These observations strongly support the possibility that this novel Ca2+-binding protein resembles calmodulin and related Ca2+-binding proteins and undergoes a conformational change on binding of Ca2+ which reflects a physiological role in Ca2+-mediated regulation of brain function.

Purification, characterization and ion binding properties of human brain S100b protein

Human brain S100b (beta beta) protein was purified using zinc-dependent affinity chromatography on phenyl-Sepharose. The calcium- and zinc-binding properties of the protein were studied by flow dialysis technique and the protein conformation both in the metal-free form and in the presence of Ca2+ or Zn2+ was investigated with ultraviolet spectroscopy, sulfhydryl reactivity and interaction with a hydrophobic fluorescence probe 6-(p-toluidino)naphthalene-2-sulfonic acid (TNS). Flow dialysis measurements of Ca2+ binding to human brain S100b (beta beta) protein revealed six Ca2+-binding sites which we assumed to represent three for each beta monomer, characterized by the macroscopic association constants K1 = 0.44 X 10(5) M-1; K2 = 0.1 X 10(5) M-1 and K3 = 0.08 X 10(5) M-1. In the presence of 120 mM KCl, the affinity of the protein for calcium is drastically reduced. Zinc-binding studies on human S100b protein showed that the protein bound two zinc ions per beta monomer, with macroscopic constants K1 = 4.47 X 10(7) M-1 and K2 = 0.1 X 10(7) M-1. Most of the Zn2+-induced conformational changes occurred after the binding of two zinc ions per mole of S100b protein. These results differ significantly from those for bovine protein and cast doubt on the conservation of the S100 structure during evolution. When calcium binding was studied in the presence of zinc, we noted an increase in the affinity of the protein for calcium, K1 = 4.4 X 10(5) M-1; K2 = 0.57 X 10(5) M-1; K3 = 0.023 X 10(5) M-1. These results indicated that the Ca2+- and Zn2+-binding sites on S100b protein are different and suggest that Zn2+ may regulate Ca2+ binding by increasing the affinity of the protein for calcium.

Hydrodynamic properties of bovine brain S-100 proteins

The size and shape of S-100a and S-100b proteins in solution have been examined by gel filtration and ultracentrifugation in the presence and absence of Ca2+. S-100a and S-100b proteins, in the absence of Ca2+, have an intrinsic sedimentation coefficient, so20,w of 2.20 and 2.15 S, respectively and in 1 mM Ca2+ their so20,w values were decreased to 2.05 and 1.95 S, respectively, indicating an unfolding of the protein molecules. The Stokes radii of S-100a and S-100b (-Ca2+) were 23.4 A and 24.0 A and they decreased to 22.2 A and 22.3 A in the presence of Ca2+. The Ca2+ effect on S-100b greater than S-100a was in agreement with our earlier CD observations. Among the monovalent cations tested (K+, Na+ and Li+) K+ had the maximum effect on the Stokes radii and so20,w values of S-100 proteins. Since certain functions of the nervous system are accompanied by local changes in ionic concentrations of Ca2+, Na+ and K+, it is conceivable that these respective conformational changes induced in S-100 proteins by these metals may be related to their function in the brain.