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First published online 28 September 2004
doi: 10.1242/jcs.01410
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Research Article |
1 Department of Medical Protein Research, Flanders Interuniversity Institute for Biotechnology (V.I.B.), Ghent University, Faculty of Medicine and Health Sciences, Albert Baertsoenkaai 3, 9000 Ghent, Belgium
2 Laboratory of Experimental Cancerology, Department of Radiotherapy and Nuclear Medicine, Ghent University Hospital (1P7), De Pintelaan 185, 9000, Ghent, Belgium
Author for correspondence (e-mail: jan.gettemans{at}ugent.be)
Accepted 13 July 2004
CapG (gCap39) is a ubiquitous gelsolin-family actin modulating protein involved in cell signalling, receptor-mediated membrane ruffling, phagocytosis and motility. CapG is the only gelsolin-related actin binding protein that localizes constitutively to both nucleus and cytoplasm. Structurally related proteins like severin and fragmin are cytoplasmic because they contain a nuclear export sequence that is absent in CapG. Increased CapG expression has been reported in some cancers but a causal role for CapG in tumour development, including invasion and metastasis, has not been explored. We show that moderate expression of green fluorescent protein-tagged CapG (CapG-EGFP) in epithelial cells induces invasion into collagen type I and precultured chick heart fragments. Nuclear export sequence-tagged CapG-EGFP fails to induce invasion, whereas point mutations in the nuclear export sequence permitting nuclear re-entry restore cellular invasion. Nuclear import of CapG is energy-dependent and requires the cytosolic receptor importin ß but not importin 
. Nuclear CapG does not possess intrinsic transactivation activity but suppresses VP16 transactivation of a luciferase reporter gene in a dose-dependent manner. Furthermore, invasion requires signalling through the Ras-phosphoinositide 3-kinase pathway and Cdc42 or RhoA, but not Rac1. We show for the first time active nuclear import of an actin binding protein, and our findings point to a role for nuclear CapG in eliciting invasion, possibly through interfering with the cellular transcription machinery.
Key words: CapG, gCap39, Importin, Nuclear import, Collagen, Invasion
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