Comparison of Ca(II), Cd(II), and Mg(II) titrations of tyrosine-99 spin-labeled bovine calmodulin.
Bovine calmodulin, spin-labeled at tyrosine-99, has been utilized in electron paramagnetic resonance (EPR) studies to investigate calmodulin interactions with Ca(II), Cd(II), and Mg(II). The addition of either Ca(II) or Cd(II) to apo-calmodulin results in a complex capable of activating target enzymes, such as 3', 5'-cyclic nucleotide phosphodiesterase (J. M. Buccigross, C. L. O'Donnell, and D. J. Nelson, Biochem. J. 235 677 [1986]), while Mg(II) is known to be incapable of activating calmodulin toward any of its target enzymes. Additions of Ca(II) and Cd(II) to spin-labeled apo-calmodulin gave rise to very similar changes in the EPR spectrum of the bound label, consistent with a dramatic decrease in the mobility of the nitroxide spin-label covalently attached to tyrosine-99. Addition of Mg(II) to spin-labeled apo-calmodulin caused no change in the EPR spectrum of the bound label. Thus, the conformational changes induced by Ca(II) and Cd(II) ion binding to calmodulin, which lead to decreased tyrosine-99 spin label mobility, are clearly not occurring when Mg(II) ion binds. These results are consistent with the results of other spectroscopic studies, which indicate that "activating" metal ions, such as Ca(II) and Cd(II), produce calmodulin conformers that are different from those produced by "inactivating" metal ions, such as Mg(II).
You
G F
GF
Department of Chemistry, Clark University, Worcester, Massachusetts 01610.
Buccigross
J M
JM
Nelson
D J
DJ
D000818:Animals; D002104:Cadmium; D002118:Calcium; D002147:Calmodulin; D002417:Cattle; D004578:Electron Spin Resonance Spectroscopy; D008274:Magnesium; D011487:Protein Conformation; D014443:Tyrosine
D002147:Calmodulin; D002104:Cadmium; D014443:Tyrosine; D008274:Magnesium; D002118:Calcium