Purification and characterization of myosin light-chain kinase from the rat pancreas

We have partially purified a protein kinase from rat pancreas that phosphorylates two light-chain subunits of pancreatic myosin, a doublet with components of 18 and 20 kDa. This protein kinase was purified approx. 1000-fold by sequential (NH4)2SO4 fractionation, gel filtration, ion-exchange and affinity chromatography on calmodulin-Sepharose 4B. The resultant enzyme preparation is free of cyclic AMP-dependent protein kinase, protein kinase C and calmodulin-dependent type I or II kinase activities. The purified protein kinase is completely dependent on Ca2+ and calmodulin, and phosphorylates a 20 kDa light-chain subunit of intact gizzard myosin, suggesting that it belongs to a class of enzymes known as myosin light-chain kinase (MLCK). The apparent Km values of the putative pancreatic MLCK for ATP (73 microM), gizzard myosin light chains (18 microM) and calmodulin (2 nM) are similar to those reported for MLCKs isolated from smooth muscle, platelet and other sources. The enzyme is half-maximally activated at a free Ca2+ concentration of 2.5 microM. A single component of the affinity-purified kinase reacts with antibodies to turkey gizzard MLCK. The apparent molecular mass of this component is 138 kDa. Immunoprecipitation of a pancreatic homogenate with these antibodies decreases calmodulin-dependent kinase activity for pancreatic myosin by over 85%. The immunoprecipitate contains a single electrophoretic band of 138 kDa. Tryptic phosphopeptide analyses of pancreatic myosin, phosphorylated by either gizzard or pancreatic MLCK, are identical. Thus the enzyme that we have purified from rat pancreas is a MLCK, as judged by (1) absolute dependence on Ca2+ and calmodulin, (2) high affinity for calmodulin, (3) narrow substrate specificity for the light-chain subunit of myosin, and (4) reactivity with antibodies to turkey gizzard MLCK. These studies establish the existence of a pancreatic MLCK which may be responsible for regulating myosin phosphorylation and enzyme secretion in situ.