Cryo-EM reveals a right-handed double-helix dimer architecture of PCDH15.

Tip links connect the stereocilia of mechanosensory hair cells in the inner ear and transmit force onto mechanotransduction (MET) channels. Tip links consist of protocadherin 15 (PCDH15) and cadherin 23, which assemble into an extracellular filament approximately 150 nm in length. Rare freeze-etched electron microscopy (EM) images have suggested that tip links could be right-handed double helices in vivo, but direct structural evidence has been lacking. Using cryo-EM we determined the structure of a large part of the extracellular PCDH15 domain. Two PCDH15 molecules form a parallel cis dimer stabilized by several dimerization interfaces, including two strand crossovers and two parallel contacts, yielding a right-handed double helix. Functional studies show that mutations in PCDH15 dimerization-domains impair MET. Our results establish the molecular foundation for how PCDH15 forms a right-handed double helix to enable mechanical sensing.