A model of stereocilia adaptation based on single molecule mechanical studies of myosin I

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Abstract
We have used an optical tweezers–based apparatus to perform single molecule mechanical experiments using the unconventional myosins, Myo1b and Myo1c. The single–headed nature and slow ATPase kinetics of these myosins make them ideal for detailed studies of the molecular mechanism of force generation by acto–myosin. Myo1c exhibits several features that have not been seen using fast skeletal muscle myosin II. (i) The working stroke occurs in two, distinct phases, producing an initial 3 nm and then a further 1.5 nm of movement. (ii) Two types of binding interaction were observed: short–lived ATP–independent binding events that produced no movement and longer–lived, ATP–dependent events that produced a full working stroke. The stiffness of both types of interaction was similar. (iii) In a new type of experiment, using feedback to apply controlled displacements to a single acto–myosin cross–bridge, we found abrupt changes in force during attachment of the acto–Myo1b cross–bridge, a result that is consistent with the classical ‘ T 2 ’ behaviour of single muscle fibres. Given that these myosins might exhibit the classical T 2 behaviour, we propose a new model to explain the slow phase of sensory adaptation of the hair cells of the inner ear.

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