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© The Rockefeller University Press, 0021-9525/1997//1287 $5.00
The Journal of Cell Biology, Volume 137, Number 6, , 1997 1287-1307

Article

Unconventional Myosins in Inner-Ear Sensory Epithelia



Tama Hasson*, Peter G. Gillespie{ddagger}, Jesus A. Garcia§, Richard B. MacDonald§,||, Yi-dong Zhao{ddagger}, Ann G. Yee§, Mark S. Mooseker*, and David P. Corey§,||,¶

* Department of Biology, Department of Cell Biology, Department of Pathology, Yale University, New Haven, Connecticut 06520; {ddagger} Department of Physiology, Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205; § Department of Neurobiology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114; || Program in Speech and Hearing, Joint Program in Health Sciences and Technology, Harvard Medical School and Massachusetts Institute of Technology, Cambridge, Massachusetts 02139; and Howard Hughes Medical Institute

To understand how cells differentially use the dozens of myosin isozymes present in each genome, we examined the distribution of four unconventional myosin isozymes in the inner ear, a tissue that is particularly reliant on actin-rich structures and unconventional myosin isozymes. Of the four isozymes, each from a different class, three are expressed in the hair cells of amphibia and mammals. In stereocilia, constructed of cross-linked F-actin filaments, myosin-Iβ is found mostly near stereociliary tips, myosin-VI is largely absent, and myosin-VIIa colocalizes with crosslinks that connect adjacent stereocilia. In the cuticular plate, a meshwork of actin filaments, myosin-Iβ is excluded, myosin-VI is concentrated, and modest amounts of myosin-VIIa are present. These three myosin isozymes are excluded from other actin-rich domains, including the circumferential actin belt and the cortical actin network. A member of a fourth class, myosin-V, is not expressed in hair cells but is present at high levels in afferent nerve cells that innervate hair cells. Substantial amounts of myosins-Iβ, -VI, and -VIIa are located in a pericuticular necklace that is largely free of F-actin, squeezed between (but not associated with) actin of the cuticular plate and the circumferential belt. Our localization results suggest specific functions for three hair-cell myosin isozymes. As suggested previously, myosin-Iβ probably plays a role in adaptation; concentration of myosin-VI in cuticular plates and association with stereociliary rootlets suggest that this isozyme participates in rigidly anchoring stereocilia; and finally, colocalization with cross-links between adjacent stereocilia indicates that myosin-VIIa is required for the structural integrity of hair bundles.

1. Abbreviations used in this paper: GST, glutathione-S-transferase, MBP, maltose-binding protein; PVDF, polyvinylidene difluoride.

Please address all correspondence to David P. Corey, WEL414, Massachusetts General Hospital, Boston, MA 02114. Tel.: (617) 726-6147. Fax: (617) 726-5256. e-mail: corey{at}helix.mgh.harvard.edu

All three laboratories contributed equally to this work.


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