ACTIVATION OF CHICKEN GIZZARD MYOSIN LIGHT-CHAIN KINASE BY CA2(+) CALMODULIN IS INHIBITED BY AUTOPHOSPHORYLATION/

Myosin light chain kinase (MLCK) is a calmodulin-dependent protein kin ase which phosphorylates the 20,000 dalton regulatory light chain of m yosin II. Here we show that activation of chicken gizzard MLCK by Ca2/calmodulin is inhibited by autophosphorylation at 2 sites in the abse nce of Ca2+/calmodulin. Two phosphorylation sites are located in the f unctional domain of the kinase, the threonine site toward the actin bi nding domain near the N-terminus of MLCK and the serine site in immedi ate proximity to the calmodulin binding site. The autophosphorylation was significantly inhibited by the binding of calmodulin to MLCK in th e presence of Ca2+.