Purification of myosin light chain kinase from bovine cardiac muscle

Myosin light chain kinase was purified > 100,000-fold to apparent homogeneity with a yield of 10% from bovine cardiac muscle. Sodium dodecyl sulfate gels of the purified kinase showed one stained band corresponding to a Mr of 94,000. The enzyme was activated > 10-fold in the presence of Ca2+ (apparent Ka = 0.6-1.2 microM) and calmodulin (apparent Ka = 3-5 nM). The purified enzyme had a specific activity of 20-30 mumol of phosphate transferred per min per mg from ATP to cardiac myosin light chain 2. One mole of phosphate was incorporated per 94,000 g of the kinase in the presence of Ca2+ and calmodulin or of cyclic AMP-dependent protein kinase or of both additions. In addition to myosin light chain kinase, a calmodulin-binding protein of unknown function was purified from bovine cardiac muscle. This protein had a Mr of 85,000, was composed of two dissimilar subunits (Mr of 61,000 and 15,000), and competed with myosin light chain kinase for calmodulin. The protein appears to be closely related to the calmodulin-binding protein I purified from brain.