A Ca²⁺-CALMODULIN-DEPENDENT PROTEIN KINASE IN THE PARTICULATE FRACTION OF RAT BRAIN AND ENDOGENOUS PHOSPHORYLATION OF PARTICULATE-BOUND SUBSTRATES

Abstract

A Ca²⁺-calmodulin-dependent protein kinase has been solubilized by Triton X-100 from the particulate fraction of rat brain and purified about 350-fold to apparent homogeneity. The purified enzyme was similar to the cytosol enzyme previously reported (15) with respect to physicochemical and kinetic properties. The titration of the enzyme activity with calmodulin showed that 14 moles of calmodulin are required for full activation of 1 mole of the enzyme. Various endogenous substrate proteins in nuclear, synaptic membrane, and synaptic vesicle fractions were phosphorylated in a Ca²⁺-calmodulin-dependent manner. The endogenous phosphorylation of proteins in the particulate fraction reached the maximal level within 15 sec and then decreased gradually for 10 min. The addition of the purified enzyme resulted in the phosphorylation with a pattern similar to that of the endogenous phosphorylation.