Interaction of Calcium and Calmodulin in the Presence of Sodium Dodecyl Sulfate

Calmodulin has been purified to homogeneity using an improved procedure that allows rapid processing of several kilograms of bovine brain. A calcium-dependent change in the electrophoretic mobility of calmodulin in the presence of sodium dodecyl sulfate (SDS) has been observed. Freshly prepared calmodulin or lyophilized calmodulin, stored at --80 degrees C for 1--7 months, migrates as a single band with an apparent molecular weight of 21 000 when the sample, gel and running buffer are made 0.1 mM in EDTA. When 0.1 mM CaCl2 is substituted for EDTA, freshly isolated calmodulin migrates as a single band with an apparent molecular weight of 15 000. More slowly migrating bands, in addition to the 15 000 molecular weight band, are observed when the stored protein is electrophoresed under the same conditions. Calcium binding experiments show that freshly prepared calmodulin binds 4 mol of calcium per mol of protein in the presence of 0.1% SDS in 0.1 mM CaCl2. Skeletal muscle troponin C, carp parvalbumin, and bovine brain S-100b do not show this mobility change. The calcium-dependent mobility change can be used to identify calmodulin in crude protein preparations. Calmodulin has been identified in the sperm of the sea urchin, Strongylocentrotus purpuratus, and purified. The urchin calmodulin activates cyclic nucleotide phosphodiesterase to the same extent as does brain calmodulin. We used several criteria to determine that calmodulin is not present as a soluble protein in Escherichia coli.