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Published online 22 October 2001. doi:10.1083/jcb.200012039
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© The Rockefeller University Press, 0021-9525/2001/10/393 $5.00
The Journal of Cell Biology, Volume 155, Number 3, October 29, 2001 393-404

Article

Differentiation- and stress-dependent nuclear cytoplasmic redistribution of myopodin, a novel actin-bundling protein



Astrid Weins1, Karin Schwarz1, Christian Faul1, Laura Barisoni2,3, Wolfgang A. Linke4 and Peter Mundel1

1 Department of Medicine and Department of Anatomy and Cell Biology, Albert Einstein College of Medicine, Bronx, NY 10461
2 Department of Pathology, Johns Hopkins University, Baltimore, MD 21287
3 National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892
4 Department of Physiology, University of Heidelberg, 69120 Heidelberg, Germany

Address correspondence to Peter Mundel, Division of Nephrology, Albert Einstein College of Medicine, 1300 Morris Park Ave., Bronx, NY 10461. Tel.: (718) 430-3219. Fax: (718) 430-8963. E-mail: mundel{at}aecom.yu.edu

We report the cloning and functional characterization of myopodin, the second member of the synaptopodin gene family. Myopodin shows no significant homology to any known protein except synaptopodin. Northern blot analysis resulted in a 3.6-kb transcript for mouse skeletal and heart muscle. Western blots showed an 80-kD signal for skeletal and a 95-kD signal for heart muscle. Myopodin contains one PPXY motif and multiple PXXP motifs. Myopodin colocalizes with {alpha}-actinin and is found at the Z-disc as shown by immunogold electron microscopy. In myoblasts, myopodin shows preferential nuclear localization. During myotube differentiation, myopodin binds to stress fibers in a punctuated pattern before incorporation into the Z-disc. Myopodin can directly bind to actin and contains a novel actin binding site in the center of the protein. Myopodin has actin-bundling activity as shown by formation of latrunculin-A–sensitive cytosolic actin bundles and nuclear actin loops in transfected cells expressing green fluorescent protein–myopodin. Under stress conditions, myopodin accumulates in the nucleus and is depleted from the cytoplasm. Nuclear export of myopodin is sensitive to leptomycin B, despite the absence of a classical nuclear export sequence. We propose a dual role for myopodin as a structural protein also participating in signaling pathways between the Z-disc and the nucleus.

Key Words: actin-binding protein; muscle differentiation; nuclear-cytoplasmic translocation; synaptopodin gene family; Z-disc


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