Phosphorylation of calmodulin on threonine residue(s) by cytosol prepared from the adrenal cortex

During the course of experiments examining Ca2+/calmodulin-dependent protein phosphorylation by cytosol prepared from the outer and inner zones of the guinea pig adrenal cortex, it appeared that the calmodulin added to incubation mixtures was undergoing phosphorylation. This was subsequently confirmed by immunoprecipitation following phenyl-Sepharose purification of the phosphocalmodulin. The phosphorylation of calmodulin was maximal in the absence of calcium, and markedly increased by the presence of poly-basic amino acids. Phosphoamino acid analysis revealed that calmodulin was phosphorylated exclusively on threonine residue(s). It was of interest to note that the phosphorylation of calmodulin was always considerably more intense with the cytosol prepared from the inner adrenocortical zone than with cytosol prepared from the outer zone.