Transduction-channel gating by hair cells apparently requires a
gating spring, an elastic element that transmits force to the channels.
To determine whether the gating spring is the tip link, a filament
interconnecting two stereocilia along the axis of mechanical
sensitivity, we examined the tip link'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 filament that usually forks
into two branches before contacting a taller stereocilium; at the other
end, several short filaments extend to the tip link from the shorter
stereocilium. The structure of the tip link suggests that it is either
a helical polymer or a braided pair of filamentous macromolecules and
is thus likely to be relatively stiff and inextensible. Such behavior
is incompatible with the measured elasticity of the gating spring,
suggesting that the gating spring instead lies in series with the
helical segment of the tip link.
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