title: Reinvestigation of the sulfhydryl reactivity in bovine brain S100b (beta beta) protein and the microtubule-associated tau proteins. Ca2+ stimulates disulfide cross-linking between the S100b beta-subunit and the microtubule-associated tau(2) protein.
abstract: Zn2+ and Ca2+ affect the conformation of bovine brain S100b (beta beta) protein and the exposure of its Cys-84 beta. Zn2+ binding to high-affinity sites of native S100b protected the sulfhydryl groups against the thiol-specific reagent 5,5'-dithiobis(2-nitrobenzoate) and antagonized the Ca2+-stimulated reactivity of Cys-84 beta toward the reagent. Spectroscopic studies on the fluorescence properties of labeled S100b with the fluorescent probes bimane and acrylodan at Cys-84 beta confirmed the antagonistic effect of Ca2+ and Zn2+ with respect to the conformational properties of the protein. Measurements of fluorescence dynamics on bimane-labeled S100b indicated that the slow monomer-dimer equilibrium that characterizes the apoprotein at micromolar concentrations was shifted to the monomer form in the presence of Zn2+, a fact that could explain the previously reported Zn2+-dependent increase of S100b protein affinity for calcium. The difference in the effects of Ca2+ and Zn2+ on the reactivity of Cys-84 beta in S100b was confirmed when we observed that Ca2+ and Zn2+ have opposite actions on the formation of disulfide bridges between Cys-84 beta of the S100b beta-subunit and sulfhydryl groups on the microtubule-associated tau(2) protein. Ca2+ stimulated the covalent complex formation whereas Zn2+ inhibited it. We suggest that Zn2+ may have a modulatory function on Cys-84 beta reactivity in the S100b beta-subunit in vivo. Two types of divalent complexes between tau(2) and beta-subunit were formed in the presence of Ca2+, an equimolar complex tau(2)-beta 1 and a complex of one molecule of tau(2) with two beta-subunits, tau(2)-beta 2.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biochemistry, University of California, Berkeley 94720.
authors.forename: J
authors.initials: J
authors.lastname: Baudier
authors.forename: R D
authors.initials: RD
authors.lastname: Cole
mesh_terms: D000818:Animals; D001921:Brain / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D002417:Cattle; D004228:Dithionitrobenzoic Acid / Q000494:pharmacology; D007700:Kinetics; D008869:Microtubule-Associated Proteins / Q000302:isolation & purification / Q000378:metabolism*; D008970:Molecular Weight; D009414:Nerve Growth Factors; D064568:S100 Calcium Binding Protein beta Subunit; D009418:S100 Proteins / Q000302:isolation & purification / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D013438:Sulfhydryl Compounds / Q000378:metabolism; D013439:Sulfhydryl Reagents / Q000494:pharmacology; D015032:Zinc / Q000494:pharmacology
chemical_list: D008869:Microtubule-Associated Proteins; D009414:Nerve Growth Factors; D064568:S100 Calcium Binding Protein beta Subunit; D009418:S100 Proteins; D013438:Sulfhydryl Compounds; D013439:Sulfhydryl Reagents; D004228:Dithionitrobenzoic Acid; D015032:Zinc; D002118:Calcium