Ca 2+ /calmodulin-dependent protein kinase kinase β (CaMKKβ) acts as a regulatory kinase that phosphorylates and activates multiple downstream kinases including CaMKI, CaMKIV, 5'AMP-activated protein kinase (AMPK) and protein kinase B (PKB), resulting in regulation of wide variety of Ca 2+ -dependent physiological responses under normal and pathological conditions. CaMKKβ is regulated by Ca 2+ /calmodulin-binding, autophosphorylation, and transphosphorylation by multiple protein kinases including cAMP-dependent protein kinase (PKA). In this report, we found that phosphorylation of CaMKKβ is dynamically regulated by protein phosphatase/kinase system in HeLa cells. Global phosphoproteomic analysis revealed the constitutive phosphorylation at 8 Ser residues including Ser128, 132, and 136 in the N-terminal regulatory domain of rat CaMKKβ in unstimulated HeLa cells as well as inducible phosphorylation of Thr144 in the cells treated with a phosphatase inhibitor, okadaic acid (OA).Thr144 phosphorylation in CaMKKβ has shown to be rapidly induced by OA treatment in a time-and dose-dependent manner in transfected HeLa cells, indicating that Thr144 in CaMKKβ is maintained unphosphorylated state by protein phosphatase(s). We confirmed that in vitro dephosphorylation of pThr144 in CaMKKβ by protein phosphatase 2A and 1. We also found that the pharmacological inhibition of protein phosphatase(s) significantly induces CaMKKβ-phosphorylating activity (at Thr144) in HeLa cell lysates as well as in intact cells; however, it was unlikely that this activity was catalyzed by previously identified Thr144-kinases, such as AMPK and PKA. Taken together, these results suggest that the phosphorylation and dephosphorylation of Thr144 in CaMKKβ is dynamically regulated by multiple kinases/phosphatases signaling resulting in fine-tuning of the enzymatic property.