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First published online 28 March 2006
doi: 10.1242/jcs.02869

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Targeted wild-type and jerker espins reveal a novel, WH2-domain-dependent way to make actin bundles in cells

Patricia A. Loomis^1,*, Alexander E. Kelly^2,, Lili Zheng¹, Benjarat Changyaleket¹, Gabriella Sekerková¹, Enrico Mugnaini¹, Adriana Ferreira¹, R. Dyche Mullins³ and James R. Bartles^1,

¹ Department of Cell and Molecular Biology, Feinberg School of Medicine, and Institute for Neuroscience, Northwestern University, Chicago, IL 60611, USA
² Graduate Group in Biophysics, University of California, San Francisco, CA 94107, USA
³ Department of Cellular and Molecular Pharmacology, University of California, San Francisco, CA 94107, USA

Author for correspondence (e-mail: j-bartles{at}northwestern.edu)

Accepted 4 January 2006

The espin actin-bundling proteins, which are the target of deafness mutations, are present in the parallel actin bundles of stereocilia and microvilli and appear to increase their steady-state length. Here, we report a new activity of the espins, one that depends on their enigmatic WH2 domain: the ability to assemble a large actin bundle when targeted to a specific subcellular location. This activity was observed for wild-type espins targeted to the centrosome in transfected neuronal cells and for jerker espins targeted to the nucleolus in a wide variety of transfected cells as a result of the frameshifted peptide introduced into the espin C-terminus by the jerker deafness mutation. This activity, which appears specific to espins, requires two espin F-actin-binding sites and the actin-monomer-binding activity of the espin WH2 domain, but can be mimicked by adding a WH2 domain to an unrelated actin-bundling protein, villin. Espins do not activate the Arp2/3 complex in vitro, and bundle assembly is not indicative of in-vitro nucleation activity. Our results suggest a novel way to build actin bundles at specific sites in cells.

Key words: Microtubule, WASP, Neuron, Nucleus, Hearing, RS domain

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