High-resolution structure of hair-cell tip links

Transduction-channel gating by hair cells apparently requires a gating spri ng, an elastic element that transmits force to the channels. To determine w hether the gating spring is the tip link, a filament interconnecting two st ereocilia along the axis of mechanical sensitivity, we examined the tip lin k's structure at high resolution by using rapid-freeze, deep-etch electron microscopy, We found that the tip link is a right-handed, coiled double fil ament that usually forks into two branches before contacting a taller stere ocilium; at the other end, several short filaments extend to the tip link f rom the shorter stereocilium. The structure of the tip link suggests that i t is either a helical polymer or a braided pair of filamentous macromolecul es and is thus likely to be relatively stiff and inextensible. Such behavio r is incompatible with the measured elasticity of the gating spring, sugges ting that the gating spring instead lies in series with the helical segment of the tip link.