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J. Biol. Chem., Vol. 280, Issue 41, 34974-34984, October 14, 2005

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Rabphilin Localizes with the Cell Actin Cytoskeleton and Stimulates Association of Granules with F-actin Cross-linked by -Actinin^*

Giovanna Baldini, Alberto M. Martelli¶, Giovanna Tabellini||, Chad Horn**, Khaled Machaca, Paola Narducci, and Giulia Baldini**1

From the Dipartimento di Morfologia Umana Normale, via Manzoni 16, Trieste, Universita' di Trieste, Trieste I-34138, Italy, the Dipartimento di Scienze Anatomiche Umane e Fisiopatologia dell'Apparato Locomotore, Sezione di Anatomia, Cell Signalling Laboratory, Universita' di Bologna, via Irnerio 48, Bologna I-40126, Italy, the ^¶Istituto per i Trapianti d'Organo e l'Immunocitologia del Consiglio Nazionale delle Ricerche, Sezione di Bologna c/o IOR, via di Barbiano 1/10, Bologna I-40136, Italy, the ^||Dipartimento di Scienze Biomediche e Biotecnologie, Sezione di Citologia e Istologia, Università di Brescia, Viale Europa 11, Brescia I-25123, Italy, and the Departments of Physiology and Biophysics and of ^**Biochemistry and Molecular Biology, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205

In endocrine cell, granules accumulate within an F-actin-rich region below the plasma membrane. The mechanisms involved in this process are largely unknown. Rabphilin is a cytosolic protein that is expressed in neurons and neuroendocrine cells and binds with high affinity to members of the Rab3 family of GTPases localized to synaptic vesicles and dense core granules. Rabphilin also interacts with -actinin, a protein that cross-links F-actin into bundles and networks and associates with the granule membrane. Here we asked whether rabphilin, in addition to its granule localization, also interacts with the cell actin cytoskeleton. Immunofluorescence and immunoelectron microscopy show that rabphilin localizes to the sub-plasmalemmal actin cytoskeleton both in neuroendocrine and unspecialized cells. By using purified components, it is found that association of rabphilin with F-actin is dependent on added -actinin. In an in vitro assay, granules, unlike endosomes or mitochondria, associate with F-actin cross-linked by -actinin. Rabphilin is shown to stimulate this process. Rabphilin enhances by 8-fold the granule ability to localize within regions of elevated concentration of cross-linked F-actin. These results suggest that rabphilin, by interacting with -actinin, organizes the cell cytoskeleton to facilitate granule localization within F-actin-rich regions.

Received for publication, March 11, 2005 , and in revised form, July 8, 2005.

^* This work was supported by National Institutes of Health Grant RO1-DK53293 and by the Arkansas Tobacco Settlement (to Giulia Baldini) and by an Associazione Italiana per la Ricerca sul Cancro grant (to A. M. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Biochemistry and Molecular Biology, University of Arkansas for Medical Sciences, 4301 West Markham St., Slot 516, Little Rock, AR 72205. Tel.: 501-526-7793; Fax: 501-686-8169; E-mail: gbaldini{at}uams.edu.

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