In molluscan muscle, calcium regulation is associated with myosin rather than with actin. Muscle myosin contains two types of light chains: the catalytic light chain, essential for ATPase activity, and the regulatory light chain, a calcium-binding protein responsible for Ca(2+) dependent binding and Ca(2+) dependent Mg-ATPase activity. Smooth muscle myosin from the scallop adductor muscle contains two kinds of RLC proteins, A and B. The calcium affinity of the smooth muscle RLC is approximately 10 times that of the RLC of striated muscle.