Conformational changes induced by binding of divalent cations to calregulin

Area of Science:

• Biochemistry
• Protein-ligand interactions
• Spectroscopy

Background:

• Calregulin is a calcium-binding protein.
• Understanding metal ion interactions with calregulin is crucial for elucidating its function.
• Intrinsic fluorescence and hydrophobic probe assays can reveal conformational changes upon ligand binding.

Purpose of the Study:

• To investigate the binding characteristics of Ca2+ and Zn2+ to calregulin.
• To characterize the conformational changes induced by Ca2+ and Zn2+ binding using fluorescence spectroscopy.
• To determine the specificity of calregulin for different metal ions.

Main Methods:

• Scatchard analysis of equilibrium dialysis studies.
• Measurement of intrinsic fluorescence changes upon metal ion addition.
• Fluorescence spectroscopy using 8-anilino-1-napthalenesulfonate (ANS) as a hydrophobic probe.

Main Results:

• Calregulin exhibits one Ca2+ binding site (apparent Kd = 0.05 microM) and 14 Zn2+ binding sites (apparent Kd = 310 microM).
• Ca2+ binding causes a slight increase in intrinsic fluorescence and a blue shift, while Zn2+ binding induces a large fluorescence increase and a red shift.
• Zn2+ binding exposes hydrophobic regions of calregulin, as evidenced by ANS fluorescence enhancement and emission shift.

Conclusions:

• Calregulin possesses distinct binding sites for Ca2+ and Zn2+.
• Ca2+ binding likely involves tryptophan moving away from the solvent.
• Zn2+ binding induces significant conformational changes, including tryptophan exposure and increased hydrophobicity.