Kazumasa Ohashi

The actin cytoskeleton undergoes extensive remodeling during cell morphogenesis and motility. The small guanosine triphosphatase Rho regulates such remodeling, but the underlying mechanisms of this regulation remain unclear. Cofilin exhibits actin-depolymerizing activity that is inhibited as a result of its phosphorylation by LIM-kinase. Cofilin was phosphorylated in N1E-115 neuroblastoma cells during lysophosphatidic acid-induced, Rho-mediated neurite retraction. This phosphorylation was sensitive to Y-27632, a specific inhibitor of the Rho-associated kinase ROCK. ROCK, which is a downstream effector of Rho, did not phosphorylate cofilin directly but phosphorylated LIM-kinase, which in turn was activated to phosphorylate cofilin. Overexpression of LIM-kinase in HeLa cells induced the formation of actin stress fibers in a Y-27632-sensitive manner. These results indicate that phosphorylation of LIM-kinase by ROCK and consequently increased phosphorylation of cofilin by LIM-kinase contribute to Rho-induced reorganization of the actin cytoskeleton.

Axl, Sky, and Mer, members of an Axl/Sky receptor tyrosine kinase subfamily, are typified by the cell adhesion molecule-related extracellular domain. The product of growth arrest-specific gene 6 (Gas6), structurally homologous to the anticoagulant protein S, was recently identified as the ligand for Axl and Sky, but the ligand for Mer remained unknown. We have now obtained evidence that Gas6 can also function as a ligand for Mer. Co-precipitation analysis, using soluble receptors of Axl, Sky, and Mer (Axl-Fc, Sky-Fc, and Mer-Fc) composed of the extracellular domain of receptors fused to the Fc domain of immunoglobulin G1, clearly showed that Gas6, but not protein S, specifically bound to Axl-Fc, Sky-Fc, and Mer-Fc fusion proteins. Quantitative kinetic analyses using a BIAcore biosensor instrument revealed dissociation constants (Kd) of the binding of rat Gas6 to Axl-Fc, Sky-Fc, and Mer-Fc are 0.4, 2.7, and 29 nM, respectively. We also found that Gas6 stimulated tyrosine phosphorylation of Axl, Sky, and Mer receptors ectopically expressed in Chinese hamster ovary cells. Taken together, these findings suggest that Gas6 is a common ligand for Axl, Sky, and Mer, all known members of an Axl/Sky receptor subfamily.

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.