A New Compartment at Stereocilia Tips Defined by Spatial and Temporal Patterns of Myosin IIIa Expression

Abstract

Class III myosins are motor proteins that contain an N-terminal kinase domain and a C-terminal actin-binding domain. We show that myosin IIIa, which has been implicated in nonsyndromic progressive hearing loss, is localized at stereocilia tips. Myosin IIIa progressively accumulates during stereocilia maturation in a thimble-like pattern around the stereocilia tip, distinct from the cap-like localization of myosin XVa and the shaft localization of myosin Ic. Overexpression of deletion mutants for functional domains of green fluorescent protein (GFP)-myosin IIIa shows that the motor domain, but not the actin-binding tail domain, is required for stereocilia tip localization. Deletion of the kinase domain produces stereocilia elongation and bulging of the stereocilia tips. The thimble-like localization and the influence myosin IIIa has on stereocilia shape reveal a previously unrecognized molecular compartment at the distal end of stereocilia, the site of actin polymerization as well as operation of the mechanoelectrical transduction apparatus.

Traffic jams and shocks of molecular motors inside cellular protrusions

“…The jams initiated by the inactive particles determine a finite length of the accumulation region and do not let it grow to the system size. Such localized accumulations of motors are observed near the tips of stereocilia of different lengths [4], and our model suggests that this arises from the turnover of motors through the formation of jams.…”

Myosin 3A Kinase Activity Is Regulated by Phosphorylation of the Kinase Domain Activation Loop

“…Both isoforms are localized to actin bundle-based structures in sensory cells, including the calycal process of photoreceptors (2,6,7) and the stereocilia of inner ear hair cells (8). MYO3A and -3B are proposed to play a role in maintaining the length of the actin-bundled structures (9,10).…”

Class III myosins shape the auditory hair bundles by limiting microvilli and stereocilia growth

“…Any perturbation of this activity would thus have profound effects on hair bundle shape. The ringlike distribution of these myosins (Schneider et al, 2006), enclosing the stereocilia actin cores just below (∼0.2 µm) the tips while being excluded from the tips (Fig. 1, E and F), is highly suggestive of their association with the membrane of stereocilia.…”

“…For myosin IIIa, we used the amino acid sequence reported in a previous study (Schneider et al, 2006). For myosin IIIb, we used the sequence SPE DTMYY NQLNG TLEYQ GSQRK PRKLG QIKVL DGEDQ YYKCL SPGAC APE ET HSVHP FFFSS SPRED PFAQH (Agro-Bio), and for espin-1, we used the sequence LDA LPVHH AARSG KLHCLR (Proteogenix; Salles et al, 2009).…”