Dynamic Compartmentalization of Protein Tyrosine Phosphatase Receptor Q at the Proximal End of Stereocilia: Implication of Myosin VI-Based Transport Hirofumi Sakaguchi, 1,2 * Joshua Tokita, 1 Moshe Naoz, 3 Daniel Bowen-Pope, 4 Nir S. Gov, 3 and Bechara Kachar 1 1 Section on Structural Cell Biology, National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Bethesda, Maryland 2 Department of Otolaryngology-Head and Neck Surgery, Kyoto Prefectural University of Medicine, Kyoto, Japan 3 Department of Chemical Physics, Weizmann Institute of Science, Rehovot, Israel 4 Department of Pathology, University of Washington, Seattle, Washington Hair cell stereocilia are apical membrane protrusions filled with uniformly polar- ized actin filament bundles. Protein tyrosine phosphatase receptor Q (PTPRQ), a membrane protein with extracellular fibronectin repeats has been shown to local- ize at the stereocilia base and the apical hair cell surface, and to be essential for stereocilia integrity. We analyzed the distribution of PTPRQ and a possible mech- anism for its compartmentalization. Using immunofluorescence we demonstrate that PTPRQ is compartmentalized at the stereocilia base with a decaying gradient from base to apex. This distribution can be explained by a model of transport directed toward the stereocilia base, which counteracts diffusion of the molecules. By mathematical analysis, we show that this counter transport is consistent with the minus end-directed movement of myosin VI along the stereocilia actin filaments. Myosin VI is localized at the stereocilia base, and exogenously expressed myosin VI and PTPRQ colocalize in the perinuclear endosomes in COS-7 cells. In myosin VI-deficient mice, PTPRQ is distributed along the entire stereocilia. PTPRQ-defi- cient mice show a pattern of stereocilia disruption that is similar to that reported in myosin VI-deficient mice, where the predominant features are loss of tapered base, and fusion of adjacent stereocilia. Thin section and freeze-etching electron micros- copy showed that localization of PTPRQ coincides with the presence of a dense cell surface coat. Our results suggest that PTPRQ and myosin VI form a complex that dynamically maintains the organization of the cell surface coat at the stereocilia base and helps maintain the structure of the overall stereocilia bundle. Cell Motil. Cytoskeleton 65: 528–538, 2008. Published 2008 Wiley-Liss, Inc. y Key words: membrane protein; cargo transport; hair cells; cell surface coat; tapered base; myosin motors This article contains supplementary material available via the Internet at http://www.interscience.wiley.com/jpages/0886-1544/suppmat. Contract grant sponsors: NIH Intramural Program; Alvin and Gertrude Levine Career Development Chair; Contract grant sponsor: Israel Sci- ence Foundation; Contract grant number: 337/05; Contract grant spon- sor: BSF; Contract grant number: 2006285. *Correspondence to: Hirofumi Sakaguchi, Section on Structural Cell Biology, NIDCD, National Institutes of Health, 50 South Drive, Rm 4249, Bethesda, MD 20892-8027, USA. E-mail: hiro-s @koto.kpu- m.ac.jp Received 12 December 2007; Accepted 25 February 2008 Published online 15 April 2008 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/cm.20275 Published 2008 Wiley-Liss, Inc. y This article is a US government work and, as such, is in the public domain in the United States of America Cell Motility and the Cytoskeleton 65: 528–538 (2008)