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First published online 15 May 2007
doi: 10.1242/jcs.004366

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Dynamic cofilin phosphorylation in the control of lamellipodial actin homeostasis

¹ Ludwig Institute for Cancer Research, UCL Branch, London, W1W 7BS, UK
² Department of Biochemistry and Molecular Biology, University of Southern Denmark, DK-5230 Odense, Denmark
³ Translational Research Laboratory, Institute of Women's Health, University College London, London, WC1E 6DH, UK

^* Author for correspondence (e-mail: b.baum{at}ucl.ac.uk)

Accepted 3 April 2007

During animal cell chemotaxis, signalling at the plasma membrane induces actin polymerisation to drive forward cell movement. Since the cellular pool of actin is limited, efficient protrusion formation also requires the coordinated disassembly of pre-existing actin filaments. To search for proteins that can monitor filamentous and globular actin levels to maintain the balance of polymerisation and disassembly, we followed changes in the proteome induced by RNA interference (RNAi)-mediated alterations in actin signalling. This unbiased approach revealed an increase in the levels of an inactive, phosphorylated form of the actin-severing protein cofilin in cells unable to generate actin-based lamellipodia. Conversely, an increase in F-actin levels induced the dephosphorylation and activation of cofilin via activation of the Ssh phosphatase. Similarly, in the context of acute phosphoinositide 3-kinase (PI3K) signalling, dynamic changes in cofilin phosphorylation were found to depend on the Ssh phosphatase and on changes in lamellipodial F-actin. These results indicate that changes in the extent of cofilin phosphorylation are regulated by Ssh in response to changes in the levels and/or organisation of F-actin. Together with the recent finding that Ssh phosphatase activity is augmented by F-actin binding, these results identify Ssh-dependent regulation of phosphorylated cofilin levels as an important feedback control mechanism that maintains actin filament homeostasis during actin signalling.

Key words: Actin Cytoskeleton, Proteomics, Cofilin, Phospho-regulation

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