Ca2+ and Zn2+ bind to different sites and induce different conformational changes in human calcyclin

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Chemistry

Background:

• Calcyclin (CaCY) is identified as a member of the S100 subfamily, characterized by helix-loop-helix (EF-hand) calcium-binding domains.
• Understanding CaCY's ion-binding properties is crucial for elucidating its cellular functions.

Purpose of the Study:

• To investigate the Ca2+ and Zn2+ binding characteristics of human Calcyclin (CaCY).
• To explore the influence of the Cys residue's oxidation state on CaCY's ion-binding properties.
• To identify potential interactions and binding sites for Ca2+ and Zn2+ within CaCY.

Main Methods:

• Overexpression and purification of human Calcyclin (CaCY) in Escherichia coli.
• Analytical ultracentrifugation to assess CaCY's dimeric state.
• Spectroscopic analysis (Tyr fluorescence) to quantify cation binding affinities (Ka).
• Examination of Cys residue reactivity and effects of thiol-directed labels.

Main Results:

• CaCY remains dimeric irrespective of the Cys residue's oxidation state (reduced, blocked, or oxidized).
• Ca2+ binding induces a 30% Tyr fluorescence enhancement, with affinity influenced by Cys oxidation state and increased by caldesmon.
• Zn2+ binding results in a greater fluorescence enhancement and involves the Cys residue, suggesting distinct binding sites for Ca2+ and Zn2+.

Conclusions:

• Human Calcyclin (CaCY) exhibits differential binding sites for Ca2+ and Zn2+, with Ca2+ likely binding to EF-hand motifs and Zn2+ involving the Cys residue.
• The oxidation state of the Cys residue significantly impacts Ca2+ binding affinity.
• Caldesmon may act as a Ca2+ binding modulator for CaCY, suggesting potential functional interactions.