Rho-associated Kinase ROCK Activates LIM-kinase 1 by Phosphorylation at Threonine 508 within the Activation Loop

Abstract

LIM-kinase 1 (LIMK1) phosphorylates cofilin, an actin-depolymerizing factor, and regulates actin cytoskeletal reorganization. LIMK1 is activated by the small GTPase Rho and its downstream protein kinase ROCK. We now report the site of phosphorylation of LIMK1 by ROCK. In vitro kinase reaction revealed that the active forms of ROCK phosphorylated LIMK1 on the threonine residue and markedly increased its cofilin-phosphorylating activity. A LIMK1 mutant (T508A) with replacement of Thr-508 within the activation loop of the kinase domain by alanine was neither phosphorylated nor activated by ROCK. Replacement of Thr-508 by serine changed the ROCK-catalyzed phosphorylation residue from threonine to serine. A LIMK1 mutant with replacement of Thr-508 by two glutamates increased the kinase activity about 2-fold but was not further activated by ROCK. In addition, wild-type LIMK1, but not its T508A mutant, was activated by co-expression with ROCK in cultured cells. These results suggest that ROCK activates LIMK1 in vitro and in vivo by phosphorylation at Thr-508. Together with the recent finding that PAK1, a downstream effector of Rac, also activates LIMK1 by phosphorylation at Thr-508, these results suggest that activation of LIMK1 is one of the common targets for Rho and Rac to reorganize the actin cytoskeleton.Actin cytoskeletal reorganization is essential for various cell activities, including motility, morphological change, adhesion, and cytokinesis (1). Various extracellular stimuli induce changes in actin organization, but little is known about signaling pathways which link the external stimuli to the machinery controlling actin polymerization and organization. Cofilin can bind to and depolymerize actin filaments and is considered to be a potent regulator of actin filament dynamics (2, 3). The actin-binding and -depolymerizing activity of cofilin is inhibited by phosphorylation at Ser-3 (4). We and other investigators recently provided evidence that LIM-kinase 1 (LIMK1) 1 and LIM-kinase 2 (LIMK2) phosphorylate cofilin in vitro and in vivo specifically at Ser-3 and are involved in the actin-filament dynamics by phosphorylating and inactivating cofilin (5, 6). LIMK1 and LIMK2 are members of a novel subfamily of protein serine/threonine kinases with characteristic structural features composed of two LIM domains and a PDZ domain at the N terminus and a protein kinase domain at the C terminus (7-11). We and other workers also showed that LIM-kinases are activated in cultured cells by Rho family small GTPases, Rac and Rho, albeit the activation being indirect (5, 6, 12).The Rho family small GTPases, Rho, Rac, and Cdc42, play a central role in regulating actin reorganization through downstream effectors, the activities of which are controlled by interactions with active GTP-bound forms of the Rho family (13, 14). Rho-associated kinases, termed ROCK, ROK, or Rho-kinase, are serine/threonine kinases implicated in Rho-mediated actin reorganization, such as formation of stress fibers and focal adhesions and smooth muscle contr...

The cofilin pathway in breast cancer invasion and metastasis

“…LIMKs can be activated upstream by Pak 66 as well as ROCK 65,67 and MRCK (myotonic dystrophy kinase-related CDC42-binding kinase) 68 . Normal central nervous system development may depend in part on LIMK1, as its deletion has been associated with the human genetic disease Williams syndrome, which shows similarities to the neurodevelopmental abnormalities seen in Limk1 knockout mice 69 .…”

Rho kinase in the regulation of cell death and survival

“…Most of the ROCK substrates are cellular proteins associated with the regulation of actin cytoskeleton. Besides MLC and MYPT1, this important subset of ROCK substrates include CPI-17 [63], calponin [58], LIM kinases [80,99,132], ezrin/radixin/moesin (ERM) [83], adducin [37], sodium-hydrogen exchanger (NHE1) [137] and ZIP kinase [43].…”

Scaffold mediated regulation of MAPK signaling and cytoskeletal dynamics: A perspective

“…Signaling through its effectors Rho kinase (ROCK) and mDia, Rho stimulates actin stress fiber formation by stimulating the monomeric actin binding protein profilin, and stabilizes stress fibers by inhibiting the activity of the actin depolymerizing protein, cofilin [35][36][37][38][39][40]. Bundling of actin filaments with myosin to form stress fibers, itself stimulated by ROCK and myosin light chain kinase, stimulates cell contraction and stabilization of focal adhesions [41][42][43][44], points of integrin-extracellular matrix contact between the ventral surface of the cell and the underlying substrate.…”