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First published online April 24, 2006
doi: 10.1242/10.1242/jcs.02874

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Phosphorylation on Ser5 increases the F-actin-binding activity of L-plastin and promotes its targeting to sites of actin assembly in cells

Bassam Janji^1,*, Adeline Giganti^1,*, Veerle De Corte², Marie Catillon¹, Erik Bruyneel³, Delphine Lentz¹, Julie Plastino⁴, Jan Gettemans² and Evelyne Friederich^1,

¹ Laboratory for Molecular Biology, Genomics and Modelling, Public Research Centre for Health (CRP-Santé), 84 Val Fleuri, 1526 Luxembourg
² Department of Biochemistry, Faculty of Medicine and Health Sciences, Ghent University, Albert Baertsoenkaai 3, 9000 Ghent, Belgium and Flanders Interuniversity, Institute for Biotechnology (V.I.B.), 9052 Ghent, Belgium
³ Laboratory of Experimental Cancerology, Department of Radiotherapy and Nuclear Medicine, Ghent University Hospital (1P7), De Pintelaan 185, 9000 Ghent, Belgium
⁴ Laboratoire Physicochimie "Curie", UMR168 CNRS/Institut Curie, 11, rue Pierre et Marie Curie, 75231 Paris CEDEX 05, France

Author for correspondence (e-mail: Evelyne.Friederich{at}crp-sante.lu)

Accepted 4 January 2006

L-plastin, a malignant transformation-associated protein, is a member of a large family of actin filament cross-linkers. Here, we analysed how phosphorylation of L-plastin on Ser5 of the headpiece domain regulates its intracellular distribution and its interaction with F-actin in transfected cells and in in vitro assays. Phosphorylated wild-type L-plastin localised to the actin cytoskeleton in transfected Vero cells. Ser5Ala substitution reduced the capacity of L-plastin to localise with peripheral actin-rich membrane protrusions. Conversely, a Ser5Glu variant mimicking a constitutively phosphorylated state, accumulated in actin-rich regions and promoted the formation of F-actin microspikes in two cell lines. Similar to phosphorylated wild-type L-plastin, this variant remained associated with cellular F-actin in detergent-treated cells, whereas the Ser5Ala variant was almost completely extracted. When compared with non-phosphorylated protein, phosphorylated L-plastin and the Ser5Glu variant bound F-actin more efficiently in an in vitro assay. Importantly, expression of L-plastin elicited collagen invasion in HEK293T cells, in a manner dependent on Ser5 phosphorylation. Based on our findings, we propose that conversely to other calponin homology (CH)-domain family members, phosphorylation of L-plastin switches the protein from a low-activity to a high-activity state. Phosphorylated L-plastin might act as an integrator of signals controlling the assembly of the actin cytoskeleton and cell motility in a 3D-space.

Key words: Fimbrin, Actin bundling, CH-domain, Invasion, Motility

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