Abstract
Myosins are actin based motors that are mechanically and kinetically tuned to function in a myriad of cellular processes. The myosin-I family member Myosin-IB (Myo1b) has very slow kinetics and a force sensitivity that is greater than any other characterized myosin, enabling it to function as a tension-sensing anchor. Because there have been no high-resolution structures of myosins that act as tension-sensitive anchors, we determined the 2.3-A resolution structure of the motor domain and first IQ-motif of Myo1b with bound calmodulin in the absence of nucleotide. The structure reveals several striking features not yet seen in other myosins, including a unique positioning of the light chain binding domain (LCBD) and converter region. This unique conformation results in a cavity that sandwiches the N-terminal region (NTR) of the heavy-chain between the motor domain and LCBD. Single molecule and ensemble biochemical analyses show that the NTR plays an important role in stabilizing the post-power stroke conformation of Myo1b and in tuning the rate of the force-sensitive transitions that limit actin detachment. The sequence of the NTR is highly variable within the myosin-I family, so we propose that together with unique LCBD and converter conformations, the NTR plays a role in tuning the mechanochemical properties of the motor.
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