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Activation of LIM Kinases by Myotonic Dystrophy Kinase-related Cdc42-binding Kinase α*

LIM kinases (LIMK1 and LIMK2) regulate actin cytoskeletal reorganization through cofilin phosphorylation downstream of distinct Rho family GTPases. Pak1 and ROCK, respectively, activate LIMK1 and LIMK2 downstream of Rac and Rho; however, an effector protein kinase for LIMKs downstream of Cdc42 remains to be defined. We now report evidence that LIMK1 and LIMK2 activities toward cofilin phosphorylation are stimulated in cells by the co-expression of myotonic dystrophy kinase-related Cdc42-binding kinase α (MRCKα), an effector protein kinase of Cdc42. In vitro, MRCKα phosphorylated the protein kinase domain of LIM kinases, and the site in LIMK2 phosphorylated by MRCKα proved to be threonine 505 within the activation segment. Expression of MRCKα induced phosphorylation of actin depolymerizing factor (ADF)/cofilin in cells, whereas MRCKα-induced ADF/cofilin phosphorylation was inhibited by the co-expression with the protein kinase-deficient form of LIM kinases. These results indicate that MRCKα phosphorylates and activates LIM kinases downstream of Cdc42, which in turn regulates the actin cytoskeletal reorganization through the phosphorylation and inactivation of ADF/cofilin.