Characterization of a Unique Myosin IIIa Deafness Mutation which Enhances Actin-Sliding Velocity but Abolishes Filopodia Tip Localization

Abstract

MYO3A, an unconventional myosin that localizes to the distal tips of stereocilia in the sensory epithelia of the vertebrate auditory and vestibular systems, consists of an N-terminal kinase domain, an actin-activated motor domain, and a class-specific tail domain. Previous work indicates that MYO3A tip-localization is regulated through autophosphorylation. MYO3A is thought to transport the actin regulatory protein, ESPIN1, to the stereocilia tips in inner ear hair cells and mutations in MYO3A lead to non-syndromic deafness in humans. We investigated the G488E mutation in MYO3A, a deafness associated mutation located near the conserved switch I region. The G488E mutation reduces the maximum actin-activated ATPase rate 2-fold and increases 10-fold the actin concentration required to reach one-half maximal ATPase. Interestingly, G488E MYO3A increases 2-fold the actin filament sliding velocity measured with the in vitro motility assay. We also investigated the impact of the G488E mutation on cellular localization by examining GFP tagged MYO3A dK constructs transfected into COS7 cells. The G488E mutation completely abolished MYO3A dK filopodia tip localization both in the presence and absence of ESPIN1. In addition, co-transfection of G488E MYO3A dK, ESPIN1 and wild-type (WT) MYO3A dK demonstrated that the presence of WT MYO3A does not rescue the tip localization of G488E. We hypothesize that the G488E mutation disrupts the duty ratio which prevents processive movement to the tips of parallel actin bundles. In support, chimeras of the MYO3A tail with other myosin motors (MYO1A, MYO19, MYO5A, and MYO10) demonstrate that only high duty-ratio motors are capable of filopodia tip localization. Our results demonstrate that intra-filopodia motility requires high duty ratio motor activity, which may be crucial for the formation, maintenance, and ultrastructure of parallel actin bundles.