Single-molecule fluorescence microscopy reveals regulatory mechanisms of MYO7A-driven cargo transport in stereocilia of live inner ear hair cells.

Stereocilia are F-actin-based cylindrical protrusions on the apical surface of inner ear hair cells that function as biological mechanosensors of sound and acceleration. During stereocilia development, specific unconventional myosins transport proteins and phospholipids as cargo and mediate elongation, differentiation and acquisition of the mechanoelectrical transduction (MET). How unconventional myosins localize themselves and cargo in stereocilia using energy from ATP hydrolysis is only partially understood. Here, we developed STELLA-SPIM microscopy to visualize movement of single myosin molecules in live hair cell stereocilia. STELLA-SPIM demonstrated that MYO7A, a component of MET machinery, shows processive movement toward stereocilia tips when chemically dimerized or constitutively activated by missense mutations disabling tail-mediated autoinhibition. Conversely, MYO7A shows step-wise but not processive movement in stereocilia when its tail is tethered to the plasma membrane or F-actin in the presence of MYO7A interacting partners. We posit that MYO7A dimerizes and moves processively in stereocilia when unleashed from autoinhibition.