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A calcium-dependent protein serine/threonine kinase (GnCDPK) has been detected in groundnut (Arachis hypogea) seeds that specifically phosphorylates a peptide (MLCpep) representing the phosphate-accepting domain of smooth muscle myosin light chains. GnCDPK has been purified to near homogeneity from the soluble fraction of groundnut seeds by ammonium sulfate precipitation, Q Sepharose, Blue Sepharose, and Sephacryl 300 chromatography. The molecular weight of GnCDPK is estimated to be 53,000. Enzyme activity is stimulated about 100-fold in the presence of free Ca2+ (concentration required for half-maximal activation = 0.5 microM). GnCDPK is capable of binding 45Ca2+ ions directly in an electroblot, indicating it to be a calcium-binding protein. Phosphorylation of MLCpep is found to be optimal at an alkaline pH range (pH 9-10). Unlike all other calcium-dependent protein kinases reported from higher plants, GnCDPK does not accept casein or histones as substrate. Sequences related to MLCpep (&gt; 60% homologous) that are present in myosin light chains from skeletal muscles of chicken and rabbit also fail to act as a substrate for GnCDPK. In contrast to the Ca2+/calmodulin dependence of myosin light chain kinases, GnCDPK activity is not affected by the presence of exogenous calmodulin (1-10 microM). However, enzyme activity is considerably inhibited in the presence of calmodulin antagonists like N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (concentration required for 50% inhibition [IC50] = 30 microM) and calmidazolium (IC50 = 10 microM), indicating an endogenous calmodulin structure to be present in GnCDPK. The probability of GnCDPK being a bona fide plant myosin light chain kinase is discussed.

Characterization of a Calcium-Dependent Protein Kinase from Arachis hypogea (Groundnut) Seeds