Characterization of skeletal muscle calsequestrin by 1H NMR spectroscopy

Calsequestrin (Mr = 44,000) is a calcium-binding (KD congruent to 1 mM, congruent to 50 sites/molecule) protein found in the lumen of the sarcoplasmic reticulum of skeletal muscle. The 1H NMR spectrum of calsequestrin in the calcium-free form is presented and is characteristic of a protein largely in the random coil configuration. A number of peaks in the aromatic region have been assigned based on their chemical shifts and sensitivity to pH. The interaction of this protein with Ca2+ and K+ was studied by 1H NMR. Potassium ion binding to calsequestrin caused broadening and concomitant loss of intensity in both the aromatic and aliphatic regions of the spectrum. Calcium ion binding caused similar effects but at much lower metal ion concentrations. It was found that the binding of Ca2+ to calsequestrin was cooperative (Hill coefficient n = 2.9 +/- 0.2) with a dissociation constant of 0.25 +/- 0.06 mM in the absence of K+. In contrast, K+ showed binding to a single class of independent sites (KD = 0.20 +/- 0.04 M). Calcium binding was also studied by circular dichroism at protein concentrations similar to the NMR experiments. The binding profile and cooperativity (n = 2.0 +/- 0.1, KD = 0.19 +/- 0.04 mM) were in agreement with the 1H NMR results. Circular dichroism studies performed at low protein concentrations to reduce the possible effect of calcium binding on the concentration of free calcium gave similar values of n = 2.42 +/- 0.14 and KD = 0.21 +/- 0.005 mM. This cooperativity was also observed in the presence of 100 mM KCl although the affinity for calcium has been significantly reduced (n = 1.65 +/- 0.09, KD = 0.87 +/- 0.036 mM). In view of the large number of calcium binding sites in calsequestrin, these small Hill coefficients show that calcium binding to calsequestrin is only mildly cooperative.