Myosins IIIa and IIIb regulate stereocilia length by transporting espin 1 to the polymerizing end of actin filaments

Abstract

Mutations in the myosin IIIa gene are linked to DFNB30 late onset deafness, in which those affected individuals can hear for the first twenty years of life. Mutations in the espin gene are linked to DFNB36 congenital deafness. Both myosin IIIa and espin 1 are expressed in the inner ear hair cells and colocalize at stereocilia tips, the site of actin polymerization. Overexpression of these proteins in hair cells produce an increase in stereocilia length, and when both are co-expressed together they produce a length increase greater than when each one are overexpressed individually. These results suggest that these proteins interact and influence stereocilia length regulation. We confirmed the interaction of these two proteins in heterologous COS-7 cells. We observed that when co-expressed, these proteins promote elongation of filopodial actin protrusions in a synergistic manner. Mutational analyses show that myosin IIIa3THDI binds to the ankyrin repeats of espin 1. Live and fixed cell imaging shows that myosin IIIa transports espin 1 to the filopodia tips where espin 1 promotes actin polymerizations through its WH2 domain. Because of the late onset of deafness associated with myosin IIIa, it has been speculated that the lack of myosin IIIa function is partially compensated by the paralogous protein, myosin IIIb. Myosin IIIb, encoded by a distinct gene, lacks a C-terminal actin-binding domain shown to be essential for myosin IIIa filopodia tip localization. We observed that myosin IIIb localizes at stereocilia tips and is expressed at an earlier stage than myosin IIIa. We confirmed that myosin IIIb transports espin 1 to stereocilia tips and promotes actin polymerization, consistent with the hypothesis that it partially compensates for myosin IIIa. We found that binding to espin 1 is required for myosin IIIb movement and localization. We observed that myosin IIIb can downregulate the myosin IIIa localization in vestibular, but not in cochlear, hair cells. The interplay of myosins IIIa, IIIb, and espin 1 and their influence on stereocilia length unravels a novel molecular complex at the polymerizing end of F-actin and a framework to understand the cause of DFNB30 and DFNB36 deafnesses.