Calcium sensitivity of the cross-bridge cycle of Myo1c, the adaptation motor in the inner ear

Abstract

The class I myosin Myo1c is a mediator of adaptation of mechanoelectrical transduction in the stereocilia of the inner ear. Adaptation, which is strongly affected by Ca 2؉ , permits hair cells under prolonged stimuli to remain sensitive to new stimuli. Using a Myo1c fragment (motor domain and one IQ domain with associated calmodulin), with biochemical and kinetic properties similar to those of the native molecule, we have performed a thorough analysis of the biochemical cross-bridge cycle. We show that, although the steady-state ATPase activity shows little calcium sensitivity, individual molecular events of the cross-bridge cycle are calcium-sensitive. Of significance is a 7-fold inhibition of the ATP hydrolysis step and a 10-fold acceleration of ADP release in calcium. These changes result in an acceleration of detachment of the cross-bridge and a lengthening of the lifetime of the detached M-ATP state. These data support a model in which slipping adaptation, which reduces tip-link tension and allows the transduction channels to close after an excitatory stimulus, is mediated by Myo1c and modulated by the calcium transient. molecular motor ͉ myosin M yosins are actin-based molecular motors that support a variety of cellular functions including muscle contraction and cell migration. There are at least two dozen classes in the myosin superfamily based on analysis of sequences in the catalytic domain (1). Myo1c is a class I myosin that is widely expressed in vertebrate tissues (2, 3). It consists of a motor domain, a neck or lever arm domain (three or four IQ repeats each of which binds a calmodulin), and a cargo-binding domain (4-6). In adipocytes Myo1c facilitates glucose transporter recycling in response to insulin (7,8), and in amphibian oocytes Myo1c mediates exocytosis (9). In the specialized cells of the inner ear, there is considerable evidence that Myo1c acts as a mediator of adaptation of mechanoelectrical transduction in stereocilia (10, 11).Neighboring stereocilia on the hair cells of the inner ear are connected by extracellular tip links that are attached to transduction channels, which in turn are attached to an adaptation-motor complex consisting of Myo1c molecules. The prevailing model for mechanotransduction is that deflection of the stereocilia by sound or motion affects tip-link tension and causes opening or closing of transduction channels through which potassium and calcium ions pass (5, 12, 13). Myo1c sets the transducer sensitivity by moving along the core bundle of actin filaments in the stereocilia until the resting tension in the tip link is at a point where the channels are poised just below the threshold tension required to open the channels (Fig. 1). During an excitatory stimulus, movement of the stereocilia increases tip-link tension, opening the channels. Fig. 1 illustrates how in adaptation the motor/transducer complex responds to increased tension by slipping down the actin cytoskeleton thereby reducing tip-link tension. Alternatively, reduced tension causes Myo1c to ascend the ac...

Myosin IC generates power over a range of loads via a new tension-sensing mechanism

“…The rate constants that define the myo1c actin-activated ATPase pathway in solution are very similar to myo1b (10,12,16,19), a closely related myosin-I isoform that has actin-detachment kinetics that are exquisitely sensitive to tension (20). Myo1b is suited to function as a tension-sensitive anchor, transforming from a low duty-ratio motor to a high duty ratio-motor with forces resisting its working stroke >0.5 pN.…”

“…[3][4][5], membrane ruffling (6), transcription of DNA in the nucleus (7,8), and mechanosensing in sensory hair cells (9)(10)(11)(12)(13). Although it is known that myo1c links cell membranes to the actin cytoskeleton (14,15), its molecular role in these cellular processes has not been determined.…”

Structural Analysis of the Myo1c and Neph1 Complex Provides Insight into the Intracellular Movement of Neph1

“…Loss of Myo1c binding reduces the turnover rate of Neph1 at the podocyte cell membrane. Myo1c is known to function as an adaptation motor by regulating the dynamic movement of stereociliary tip links-structures analogous to the slit diaphragm (48). This led us to believe that Myo1c may affect the dynamic movement of Neph1 at the membrane.…”

Regulation and function of the fission yeast myosins

“…This results in an increase in the structural rigidity of the IQ domain region, which allows it to act as a lever arm to amplify the magnitude of the motor power stroke. In addition, this Ca 2+ sensitivity can affect the ATPase cycle of mammalian myosin I, altering the affinity of this myosin to modulate its function within the cell (Adamek et al, 2008). However, although the fission yeast calmodulin Cam1 associates with Myo1, and is required for its localisation (Toya et al, 2001), the biophysical consequences of this interaction have yet to be explored.…”