Antonellis, P.J., Pollock, L.M., Chou, S.W., Hassan, A., Geng, R., Chen, X., Fuchs, E., Alagramam, K.N., Auer, M., and McDermott, B.M. (2014) ACF7 is a hair-bundle antecedent, positioned to integrate cuticular plate actin and somatic tubulin. The Journal of neuroscience : the official journal of the Society for Neuroscience. 34(1):305-312.
The precise morphology of the mechanosensitive hair bundle requires seamless integration of actin and microtubule networks.
Here, we identify Acf7a (actin crosslinking family protein 7a) as a protein positioned to bridge these distinct cytoskeletal
networks in hair cells. By imaging Acf7a–Citrine fusion protein in zebrafish and immunolabeling of vestibular and cochlear
mouse hair cells, we show that Acf7a and ACF7 circumscribe, underlie, and are interwoven into the cuticular plate (CP), and
they also encircle the basal body of the kinocilium. In cochlear hair cells, ACF7 localization is graded, with the highest
concentration near each fonticulus—an area free of F-actin in the region of the CP that contains the basal body. During hair-cell
development and regeneration, Acf7a precedes formation of the hair bundle and CP. Finally, electron tomography demonstrates
that the ends of microtubules insert into the CP and are decorated with filamentous linkers connecting microtubules to the
CP. These observations are consistent with ACF7 being a linker protein, which may shape the cytoskeleton of the hair cell
early during hair-bundle genesis.