Identification of TRIOBP as a novel actin bundler to generate stereocilia rootlets

Abstract

Stereocilia have “rootlets,” that penetrate the cytoplasm; these rootlets comprise bundles of actin filaments, the function of which was unknown because of the lack of a molecular description. Mutations in TRIOBP were identified from human deafness pedigrees. Immunohistochemical studies revealed that TRIOBP is the first molecule that specifically localized in the rootlets. Biochemical studies have revealed that TRIOBP bundles comprise actin filaments. Triobp-deficient mice lack stereocilia rootlets, indicating that TRIOBP is indispensable for rootlet generation. Stereocilia without rootlets show progressive degeneration, causing deafness. There are multiple TRIOBP isoforms; TRIOBP-1, TRIOBP-4, and TRIOBP-5. A comparison of Triobp-4/5 deficient mice and Triobp-5 specific deficient mice revealed a functional difference between TRIOBP isoforms.