Ca2+ and Zn2+ bind to different sites and induce different conformational changes in human calcyclin.
Calcyclin (CaCY) is a member of the S100 subfamily of helix-loop-helix (EF-hand) calcium-binding proteins. Human CaCY was overexpressed in Escherichia coli and purified with an overall yield of 40 mg/l culture. Ca2+ and Zn2+ binding properties of CaCY were examined with respect to the oxidation state of the single Cys residue at position 3. CaCY with the SH group either reduced, blocked or oxidized stays as a dimer as shown by analytical ultracentrifugation. Upon binding of Ca2+, CaCY exhibits 30% enhancement of the Tyr fluorescence, the apparent binding constant (Ka) being 2.8-5.8x10(4) M(-1). Oxidized CaCY binds Ca2+ approximately twice as weakly than its reduced form. The affinity for Ca2+ is increased in the presence of caldesmon, which could be a potential target molecule. Fully reduced CaCY binds Zn2+ with an affinity of at least 1.0x10(7) M(-1). As compared to Ca2+, Zn2+ binding results in a three times greater enhancement of the Tyr fluorescence. Saturation occurs at a Zn2+/CaCY ratio of 2:1. The reactivity of Cys3 is reduced by Zn2+ binding, although oxidized CaCY still binds Zn2+. On the basis of the effects of thiol-directed labels on the affinities for Ca2+ and Zn2+, the fluorescence changes accompanying the binding, and the CaCY reactivity with a hydrophobic probe, it was concluded that the two cations bind to CaCY at different sites: Ca2+ binds probably at the EF-hand type sites, whereas binding of at least one Zn2+ ion involves the Cys residue, and results in a different structural change.
Kordowska
J
J
Muscle Research Group, Boston Biomedical Research Institute, MA 02114, USA.
Stafford
W F
WF
Wang
C L
CL
D001483:Base Sequence; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018797:Cell Cycle Proteins; D003545:Cysteine / Q000737:chemistry; D017931:DNA Primers / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D010084:Oxidation-Reduction; D011487:Protein Conformation / Q000187:drug effects; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D000075383:S100 Calcium Binding Protein A6; D009418:S100 Proteins; D013050:Spectrometry, Fluorescence; D013438:Sulfhydryl Compounds / Q000737:chemistry; D014443:Tyrosine / Q000737:chemistry; D015032:Zinc / Q000378:metabolism* / Q000494:pharmacology
D002135:Calcium-Binding Proteins; D018797:Cell Cycle Proteins; D017931:DNA Primers; D011994:Recombinant Proteins; D000075383:S100 Calcium Binding Protein A6; D009418:S100 Proteins; D013438:Sulfhydryl Compounds; C055351:S100A6 protein, human; D014443:Tyrosine; D015032:Zinc; D003545:Cysteine; D002118:Calcium