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* Department of Biology, Department of Cell Biology, Department of Pathology, Yale University, New Haven, Connecticut
06520; 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-I
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
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-I
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-I
, -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-I
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.
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