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

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)

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.

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.