The Stereociliary Paracrystal Is a Dynamic Cytoskeletal Scaffold In Vivo

•Transgenic zebrafish reveal stereociliary protein dynamics in vivo•Actin cross-linker fascin 2b exchange within the stereocilium is rapid and unbiased•Stereocilia exchange β-actin and myosin XVa in the shafts and tips, respectively•Protein dynamism is suggested to underpin stereociliary function and longevity

SummaryPermanency of mechanosensory stereocilia may be the consequence of low protein turnover or rapid protein renewal. Here, we devise a system, using optical techniques in live zebrafish, to distinguish between these mechanisms. We demonstrate that the stereocilium’s abundant actin cross-linker fascin 2b exchanges, without bias or a phosphointermediate, orders of magnitude faster (t1/2 of 76.3 s) than any other known hair bundle protein. To establish the logic of fascin 2b’s exchange, we examine whether filamentous actin is dynamic and detect substantial β-actin exchange within the stereocilium’s paracrystal (t1/2 of 4.08 hr). We propose that fascin 2b’s behavior may enable cross-linking at fast timescales of stereocilia vibration while noninstructively facilitating the slower process of actin exchange. Furthermore, tip protein myosin XVa fully exchanges in hours (t1/2 of 11.6 hr), indicating that delivery of myosin-associated cargo occurs in mature stereocilia. These findings suggest that stereocilia permanency is underpinned by vibrant protein exchange.

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