1. Friedrich Miescher Institute, Maulbeerstrasse 66, CH-4058 Basel, Switzerland
2. Walter and Eliza Hall Institute of Medical Research, Victoria 3050, Australia
3. Present address: HHMI, Department of Biochemistry and Molecular Biophysics, Hammer Health Sciences, Columbia University, New York, New York 10032, USA.

Correspondence to: Pico Caroni¹ Correspondence and requests for materials should be addressed to P.C. (e-mail: Email: caroni@fmi.ch).

Cell division, cell motility and the formation and maintenance of specialized structures in differentiated cells depend directly on the regulated dynamics of the actin cytoskeleton¹,². To understand the mechanisms of these basic cellular processes, the signalling pathways that link external signals to the regulation of the actin cytoskeleton need to be characterized²,³. Here we identify a pathway for the regulation of cofilin, a ubiquitous actin-binding protein that is essential for effective depolymerization of actin filaments^{4, 5, 6, 7, 8}. LIM-kinase 1, also known as KIZ, is a protein kinase with two amino-terminal LIM motifs^{9, 10, 11} that induces stabilization of F-actin structures in transfected cells. Dominant-negative LIM-kinase1 inhibits the accumulation of the F-actin. Phosphorylation experiments in vivo and in vitro provide evidence that cofilin is a physiological substrate of LIM-kinase 1. Phosphorylation by LIM-kinase 1 inactivates cofilin, leading to accumulation of actin filaments. Constitutively active Rac augmented cofilin phosphorylation and LIM-kinase 1 autophosphorylation whereas phorbol ester inhibited these processes. Our results define a mechanism for the regulation of cofilin and hence of actin dynamics in vivo. By modulating the stability of actin cytoskeletal structures, this pathway should play a central role in regulating cell motility and morphogenesis.